Benzocyclobutenes in polymer chemistry

Pergamon Prog. PO&f& M, Vol. 21,505-555,19% C0pyrisht 0 19% Elsevier Science Ltd Printed in Great Britain. All rights reserved. 0079-67W96 $32.00 0079-6700(95)00026-7 BENZOCYCLOBUTENES IN POLYMER CHEMISTRY MICHAEL F. FARONA Department of Chemistry, University of North Carolina at Greensboro, Greensboro, NC 27412, USA CONTENTS 1. Introduction 2. Synthesis of benzocyclobutene monomers 2.1. Catalytic synthesis of benxocyclobutene monomers 2.2. Preparation of monomers containing one BCB group 2.2.1. Alkynylbenzocyclobutenes 2.2.2. 3,6-Disubstituted benxycyclobutenes 2.2.3. Monomers prepared from 4-benzocyclobutene 2.2.4. Other mono-BCB monomers 2.3. Preparation of monomers containing two BCB groups 3. Studies on the reactions of benxocyclobutenes 3.1. Kinetics studies 3.2. Acid stability 3.3. Thermal studies 3.4. Liquid chromatography-mass spectral studies 4. Grafts, branches, extensions and crosslinks 4.1. Grafts 4.2. Chain extensions and branches 4.3. Crosslinking 5. Applications 5.1. Biocompatible polymers containing BCB 5.2. Fibers 5.3. Coatings and 6lms for microelectronics 5.4. High performance polymers 54.1. Polycarbonates 5.4.2. Poly(diketones) and poly(ether ketones) 5.4.3. Poly(maleimides) 5.4.4. Benxocyclobutenes in rigid-rod molecular composites 6. Reactions of BCB and potential application to polymers 6.1. Photoresponsive polymers 6.2. Reactions of o-quinodimethanes with multiple bonds containing heteroatoms 6.3. Regio- and stereoselective processes 6.4. Buckminsterfullerene-BCB chemistry Acknowledgements References 506 507 507 510 510 511 513 516 516 520 520 520 521 521 523 523 525 527 528 528 529 530 534 534 535 538 540 541 541 542 547 552 553 553 505 506 MICHAEL F. FARONA 1. INTRODUCTION In 1993, Kirchhoff and Bruza published a review of benzocyclobutenes in polymer synthesis.’ This report covered an historical overview and the literature into 1991. It is not the intention of this review to duplicate the data in the fine chapter by those authors; rather, this report reviews the literature on benzocyclobutenes in polymer chemistry since 1991 with an emphasis on the applications of this highly reactive functional group in polymers. The reader is referred to the review by Kirchhoff and Bruza for information regarding the history of the development of benzocyclobutenes, the synthesis of a variety of benzocyclobutene monomers, oligomers and polymers, and the properties and uses of the end products. Several new syntheses of benzocyclobutenes have been reported recently, including catalytic methods of forming the four-membered ring. Many new macromonomers and polymers have also been reported utilizing the benzocyclobutene functionality. Attesting to the importance of benzocyclobutenes is the fact that nearly one-third of the reports appear- ing on this subject are patents. Applications include polymerizations, primarily by a Diels-Aider process, to produce a variety of useful materials such as high-temperature composites, and the preparation of benzocyclobutene-containing monomers, oligomers, polymers and copolymers for crosslinking and grafting to achieve certain desirable proper- ties, including bio-compatibility, among others. This review uses the common name, benzocyclobutene, for structure (1) shown in Fig. 1 with the numbering system. The CAS name of the compound is bicyclo[4.2.0]octa-1,3,5- triene, and it is also known as 1,2-dihydrobenzocyclobutene, cyclobutabenzene, and gen- erically as cyclobutarene. The common abbreviation for benzocyclobutene is BCB, which will be used in this article. The BCB four-membered ring opens thermally around 200°C to produce o-quinodi- methane (o-QDM), also known as o-xylylene. This very reactive species readily undergoes Diels-Alder reactions with available dienophiles or, in the absence of a dienophile, reacts with itself to give a dimer, 1,2,5,6-dibenzocyclooctadiene. Whereas the dimerization reac- tion is thermodynamically preferred over a Die&Alder reaction, the latter is kinetically favored, so that in molecules where both Die&Alder reactions and dimerizations can occur, the Diels-Alder reaction preferentially takes place.’ A third reaction is possible for o-QDM, and that is a polymerization similar to that of a 1,3-diene to give poly(o-xylylene). These possible reactions of o-quinodimethane are shown in Fig. 2. The reader is referred to the review by Kirchhoff and Bruza for mechanisms concerning the dimerization and polymerization processes, as well as thermodynamic and kinetic data. Fig. 1. Structure of benzocyclobutene with numbering system. BENZOCYCLOBUTENES IN POLYMER CHEMISTRY 507 Fig. 2. Reactions of o-quinodimethane. 2. SYNTHESIS OF BENZOCYCLOBUTENE MONOMERS 2.1. Catalytic synthesis of benzocyclobutene monomers Reports by Dandliker, and particularly Masuda et al., showed that NbCls and TaC& were efficient catalysts for the trimerization of terminal alkynes into benzenes. 2-4 Thus, phenyl- acetylene trimerizes in the presence of these catalysts to give a mixture of 1,3,5- and 1,2,4- triphenylbenzene in high yield, the 1,2,4-isomer being the major product. Internal alkynes polymerize to disubstituted linear polyalkynes with these same catalysts.5’6 Farona and coworkers applied the Nb(V) and Ta(V) catalysts toward the trimerization of terminal diynes, with the hope of making polymers of benzene rings separated by several methylene groups. In fact, polymers are produced, but only where the number of methylene groups between the terminal alkynes is four or greater. In the case of 1,5-hexadiyne, three molecules trimerize to give 1,2-bis(benzocyclobutenyl)ethane (BCBE) (2).7 It can be envisaged that the trimerization occurs as shown in Fig. 3, although the actual mechanism is probably similar to that shown in Fig. 4. The use of SnPh, or Sn(n-C4H& as a cocatalyst (1:l with NbC14) increased conversion to lOO%, and a yield of BCBE of around 93%. Table 1 shows some pertinent data for the catalytic synthesis of BCBE (2). In a typical preparation, 0.3 mmole of catalyst and cocatalyst are activated at 75°C and 1,5-hexadiyne is added dropwise by means of a syringe. The reaction is highly exothermic, so the reaction mixture is cooled to 35°C and stirred for 5 h. Pure BCBE shows a melting point of 82-83”C and the proton NMR spectrum exhibits resonances as follows: ‘H NMR: 6 = 7.05-7.2 (m, 6H, aromatic), 3.3 (s, 4H, CH2 of ring), 3.05 (s, 4H, benzylic). 13C NMR: 6, ppm 145.7, 143.1, 140.6, 126.9, 122.6, 122.3 (aromatic); 39.3 (benzylic); 29.3, 29.2 (ring). Most BCBs show a four-member ring proton resonance singlet between 6 = 3.0 and 3.5; 508 MICHAEL F. FARONA Fig. 3. Cyclotrimerization of 1,5-hexadiyne. M=NbClX Fig. 4. Probable mechanism for cyclotrimerization. Table 1. Cyclotrimerization of lJ-hexadiyne Catalyst Temp. (“C) Conv. (%) BCBE (%) NbCls 75 85 70.8 NbCl&P~ 35 100 93.3 TaC& 75 62 53.0 NbBrS 75 95 35.0 NbCldSn (n-Bu), 35 100 91.4 NbCldSnMe&l 35 100 91.1 NbCl&Gn (n-Bu)&l 35 96 88.4 NbCl_JSr& 35 92 82.5 Solvent = toluene; NbCl+zocatalyst = 1:l; 1,5-hexadiyne:NbQ = 5O:l BENZOCYCLOBUTENES IN POLYMER CHBMISTRY 509 c +II- / co, \ I R (3 a, b, c) R = SiMe3 (3a) R=Ph (3b) R = (CH2)3C1 (3~) Fig. 5. Cross-trimerization of 1,5-hexadiyne with alkynes. variations depend on the substituents on the benzene ring. This singlet becomes a simple way to identify the presence or absence of the BCB functionality. Cross-trimerizations were also reported, wherein lJ-hexadiyne reacted with trimethyl- silylacetylene, 5-chloro-l-propyne and phenylacetylene to give BCBs substituted at the 4- position. Thu.s, reaction of 1,5hexadiyne with trimethylsilylacetylene gave 4-trimethyl- silylbenzocyclobutene in up to 60% yields. Whereas the conversions of these reactions are in the 85-100% range, competing reactions lower the yields of the desired products. For example, 1,5-hexadiyne reacts with itself to produce BCBE, and trimethylsilylacetylene trimerizes to the 1,2,4- and 1,3,5-trimethylsilylbenzene isomers. Figure 5 shows the cross- trimerization reaction while Table 2 gives some preparatory data on these reactions. These 4-substituted BCBs can serve as starting materials for easy conversion to other functionalized BCBs. For example, the trimethylsilyl group can be replaced by a variety of substituents: the TMS-BCB (3a) can be treated with Brz to give 4-bromo-BCB, which has been used as a starting material in many preparations of functionalized BCB molecules. The 4-(3-chloropropyl)-BCB (3~) can be quatemized with ammonia to give 4-(3-amino- propyl)-BCB; a dehydrohalogenation reaction using t-butoxide has been reported to give allyl-BCB (4) quantitatively. * This is apparently the first-reported preparation of allyl-BCB since methods of synthesis employed so far, such as Pd-catalyzed alkenylation, Wittig, and Grignard reactions do not allow for a three-carbon fragment to be attached to the benzene ring. Thus vinyl-BCB and 4-(4-benzocyclobutenyl)-1-butene are the only alkenyl-BCBs to be reported until recently. Figure 6 gives the preparation of allyl-BCB (4). Vinyl-BCB can also be prepared from 1,5-hexadiyne using NbQ as the catalyst. Whereas 4-chloro-1-butyne is not commercially available, the corresponding alcohol can 1-Alkyne Table 2. Cross-trimerization of lJ-hexadiyne with 1-alkynes Molar ratio lJ-HD:l-alkyne T(“C) Yield (%) H@CSiMe3 HCC=C(CH2),C1 HC=CC6HS 1:l 2s 52 1:2 25 52 1:5 25 53 2:l 25 59 51 25 59 1:l 80 38 1:l 80 18 Catalyst = NW&; solvent = toluene; substrate:catalyst = SO:1 510 MICHAEL F. FARONA HzCH =CH2 (4) Fig. 6. Preparation of allyl-BCB. be purchased and converted to the chloride. Cross-trimerization of l,S-hexadiyne with 4-chloro-1-butyne produces 4-(2-chloroethyl)BCB, which can be dehydrohalogented to give vinyl-BCB.’ The dehydrohalogenation reaction is quantitative. It is clear that, unlike other methods used to prepare alkenyl-BCB derivatives, there is no limitation on the methylene chain length separating the alkenyl group from the BCB entity when the four-membered ring is produced catalytically from 1,5hexadiyne. The synthetic chemist is only hampered by the availability of the starting chloroalkyne. Benzocyclobutenyl methanol (5) has also been prepared and used as a capping group for po1yurethanes.r’ It should be pointed out that NbC15 is not an effective catalyst in the presence of donor heteroatoms. Thus, alkynes containing 0, S, N, P, etc., poison the catalyst and render it inactive. In those cases, such as the preparation of BCB-methanol, the use of CpCo(CO)z is recommended; this catalyst is effective in the presence of hetero- atoms, and also trimerizes internal alkynes, but it is much more costly than NbC&, and generally produces lower yields of products than NbC& in reactions where both are active. In the preparation of BCB-methanol, ally1 alcohol was treated with dihydropyran (DHP) to protect the alcohol functionality, and the acetylenic hydrogen was replaced with a trimethylsilyl group. The resulting molecule was cross-trimerized with l,S-hexadiyne using COCOS as the catalyst, according to the method of Saward and Vollhardt.” The resulting product, 4-hydroxymethyl-S-trimethylsilylbenzocyclobutene could be desi- lylated with HF to produce BCB-methanol. The preparative procedure is shown in Fig. 7. Clearly, catalytic methods, particularly by NbC15, provide simple routes, generally in one or two steps, to a variety of BCB derivatives. These methods, in general, are much more convenient for laboratory preparations of BCB molecules than standard procedures. The disadvantage in using these methods to prepare large quantities of BCBs is in the cost of 1,5hexadiyne which is a sine qua non in the formation of the four-membered ring of BCB. However, some savings are realized in shorter preparation times and, often, higher yields of products. 2.2. Preparation of monomers containing one BCB group 2.2.~. Alkynylbenzocyclobutenes Several alkynyl-BCB monomers were prepared and used in copolymerization reactions BENZOCYCLOBUTENES IN POLYMER CHEMISTRY 511 Z -\ 1, PPTS, DHP H 2. n-B& 3. Me$iCI Wo(COh Fig. 7. Preparation of BCB-methanol. with alkynylbenzenes as closely related structurally to the alkynyl-BCB monomers as possible. Thus, ethynyl-BCB was copolymerized with phenylacetylene; 1-chloro-Zbenzo- cyclobutenylethyne was copolymerized with 1-chloro-2-phenylethyne, etc. ‘* The method of Kirchhoff13 was used to prepare l-trimethylsilyl-2-(4-benzocyclo- butenyl)acetylene from 4-bromo-BCB. The compound was desilylated with K2CO3 in methanol to give 4-ethynylbenzocyclobutene (6). Compound (6) was then treated with p-toluenesulfonyl chloride to yield 1-chloro-2-(4-benzocyclobutenyl)acetylene (7). A third monomer, l-(4-benzocyclobutenyl)-1-propyne (8) was prepared in 20% yield from 4- bromo-BCB and propyne in the presence of dichlorobis(triphenylphosphine)palladium and cuprous iodide. These ethynyl-BCB compounds are shown in Fig. 8. 2.2.2. 3,6-Disubstituted benzocyclobutenes A high yielding synthesis of a variety of 3,6-disubstituted benzocyclobutenes was (6) (7) H&C= C w (8) Fig. 8. Some ethynyl-BCBs. 512 MICHAEL F. FARONA reported. 14,15 Benzocyclobutene was prepared in the usual way by pyrolysis of o-chloro- methyltoluene, then converted to 3,6-bis(trimethylsilyl)-3,6-dihydrobenzoutene (9) by treating it with Li followed by clorotrimethylsilane. Compound (9) was rearomatized by bubbling 02 through the solution to give 3,6-bis(trimethylsilyl)BCB (10) (88% based on BCB). Treatment of (10) with Brz yielded 3,6-dibromo-BCB (11) (90%). When compound (11) was placed under a slight positive pressure of CO gas in a solution containing Pd(I1) acetate, triphenylphosphine, triethylamine and n-butanol, compound (12), dibutyl-BCB- 3,6-dicarboxylate, was formed in 95% yield. Saponification of (12) with NaOH and acid- ification to pH 7 gave benzocyclobutene-3,6-dicarboxylic acid (13). Compound (13) is generally known as XTA. XTA can be converted to the diacid chloride with thionyl chloride, giving compound (14), BCB-3,6-diacid chloride, in 96% yield. This compound (14) now serves as a starting material to prepare a variety of other 3,6- disubstituted BCBs. For example, treatment of (14) with aniline gives N,N’-diphenyl-3,6- dicarboxamidebenzocyclobutene (15) in 97% yield. When (14) is allowed to react with 2-aminophenol in the presence of polyphosphoric acid (PPA), an 80% yield of 3,6- bis@enzoxazol-2-yl)BCB (16) is obtained. Reaction of (14) with fluorobenzene in the presence of AK& yields 3,6-bis(4-fluorobenzoyl)benzocyclobutene (17) (90%). Treatment of (14) with sodium azide gives BCB-3,6-bis(acy1 azide) (18) in 92% yield, and (18) is converted to the corresponding bis(isocyanate) (19) in 93% yield. The bis(isocyanate) (19) is converted in 93% yield to 3,6-diaminobenzocyclobutene (20) by treatment with HCl. Compound (lo), 3,6-bis(trimethylsilyl)BCB, when treated with boron tribromide and water, gives BCB-3,6-diboronic acid (21) in 83% yield, and (21) reacts with 30% hydrogen peroxide to afford 3,6-dihydroxybenzocyclobutene (22) (65%). Figure 9 shows the pre- parations of the various 3,6-disubstituted BCBs. Moore and coworkers extended the preparation of 3,6-disubstituted benzocyclobutenes, starting from (14), XTA-Cl.16 Treatment of (14) with phenol yielded the diphenyl BCB- dicarboxylate (23), while a Pd-catalyzed reaction with phenyltrimethylstamrane gave (24), 3,6-dibenzoyl-BCB. When XTACl(l4) reacted with phenylacetylene in the presence of a base and Cu(I1) and Pd(I1) compounds, the interesting 3,6-bis[@henylethynyl)carbonyl]- benzocyclobutene (25) was isolated in 65% yield. Conversion of XTA-Cl to the bis(iso- cyanate) (19), then acidifying, gave the diamine hydrochloride, which was converted to the dibenzamido derivatives (26), (27), (28) and (29) by addition of the appropriate benzoyl chloride. Figure 10 shows the synthetic schemes for these compounds. These same authors converted 3,6-dibromobenzocyclobutene (11) to a variety of inter- esting monomers. Reaction of (11) with phenylboronic acid in the presence of a Pd(0) catalyst gave 3,6-diphenyl-BCB (30) in 79% yield. Treatment of (11) with phenylacetylene in a manner analogous to the preparation of (25) led to 3,6_bis(phenylethynyl)-BCB (31). Compound (31) was reduced to the 3,6_bis(phenylethyl)-BCB derivative (32). When (11) was treated with butyllithium, 3-bromobenzocyclobutene was obtained in 61% yield. Figure 11 shows the reaction of (11). The authors also obtained crystal structures for (ll), (15) and (27). There seems to be a wider variation in the four-membered ring proton signal in the ‘H NMR spectra of 3,6-disubstituted BCB molecules than those with substituents at the 4-position. For example, whereas most of the signals of the above compounds occur BENZOCYCLOBUTENES IN POLYMER CHEMISTRY 513 (10) (21) (16) (22) Fig. 9. Preparation of some 3,6-disubstituted BCBs. between 6 = 3.0 and 3.5, that of (23) occurs at 6 = 3.56, and (25) appears at 3.67. Interestingly, whereas the amides (26) and (28) show the strained ring resonance at 6 = 3.15, the N-methyl derivatives (27) and (29) occur around 6 = 2.56, and (32) appears at 6 = 2.82. 2.2.3. Monomers prepared from 4-bromobenzocyclobutene A new synthesis of 4-hydroxy-BCB (33) has been reported.r7 Starting with 4-bromo- BCB, conve:rsion to the corresponding hydroxy compound takes place by reaction with 514 MICHAEL F. FARONA -Ph O=k---_-Ph OOPh ‘“(‘I) PhC--_ CH Pd(W I F OCl 0= -Ph F i7OOPh COCl O=&Ph (23) (14) (24) NC0 RNCOR NC0 R=H,R’=Ph (26) R=CH3,R’=Ph (27) (19) R = I-I, R’ = pt-BuPh (28) Fig. 10. Some 3,6_disubstituted BCB molecules synthesized from XTA-Cl. sodium or potassium hydroxide in the presence of a Cu(I) or Cu(I1) catalyst. Examples of the copper catalysts include CuBr, CuS04 - 5Hz0, Cu(OAc)z, and CuBr + pyridine. Yields of the product are in the 59-72% range when the reactions are carried out at temperatures above 160°C in methanol/water or ethanol/water solvent. Figure 12 shows the conversion of 4-Br-BCB to 4-OH-BCB. An improved synthesis of the important monomer 4-amino-BCB (34) and N-substituted derivatives was developed by Bruza et ~1.‘~ The synthesis is carried out at relatively low temperatures, where side reactions such as dimerization and oligomerization are mini- mixed, thus increasing the yield of 4-amino-BCB. Starting with 4-bromo-BCB, reaction BENZOCYCLOBUTENES IN POLYMER CHEMISTRY 515 BuL.i \ (30) PhC 5 CH PhEC C--_CPh reduce (32) Fig. 11. Some BCB compounds synthesized from 3,6-dibromo-BCB. with NI&OH in aqueous solution at 15O”C, using Cu20 as the catalyst, produces 4-amino- BCB in 8896 yield. Similar reactions with other nitrogen-containing compounds have also been accom- plished. Thus, 4-bromo-BCB reacts with phthalimide to give 4+phthalimido) BCB, which can be converted to 4-amino-BCB by reaction with hydrazine hydrate. Other deri- vatives such as 4-methylamino- and 4-butylamino-BCB were synthesized using methyla- mine and butylamine, respectively, in place of ammonia. If difunctional amines are used in this reaction, such as 1,4_diaminobutane, then bis(BCB)s are obtained. Figure 13 shows the preparation of 4-amino-BCB. 516 MICHAELF.FARONA Fig. 12. Conversion of 4-bromo-BCB to 4-hydroxy-BCB. + NH‘+OH cu20 ) / Hz0 m \ I B NH2 (34) Fig. 13. Synthesis of 4-amino-BCB. 2.2.4. Other mono-BCB monomers A BCB analogue of benzoyl chloride was prepared: 4-benzocyclobutenoyl chloride (35). 19p20 The starting material, 3-bromo-4-(2chloroethyl)benzoic acid, was converted to 4-benzocyclobutene carboxylic acid (36) by treatment with t-butyllithium and working up the reaction mixture. Reaction with thionyl chloride then gave 4-chloroformyl-BCB (39, which was used in an end-capping reaction (vide infra). Figure 14 shows the synthesis of (35). 2.3. Preparation of monomers containing two BCB groups A new ethylidene-bridged bis(BCB) was synthesized. 21722 For example, 1,3-bis(4-benzo- cyclobutenoyl)benzene can be treated with methylmagnesium bromide, followed by a dehydration with a strong acid to give the corresponding ethylidene derivative. Alterna- tively, the diketone can react directly in a Wittig reaction with an ylide, e.g., methylene- triphenylphosphorane, in which the carbonyl oxygen atom is replaced by a methylene group. Figure 15 shows the preparation of the bridging bis(ethylidene) compound (36). There seems to be a paucity of hydrophobic polymers with low dielectric constants and ‘CH2CH$l Fig. 14. Synthesis of 4-chloroformyl-BCB. BENZOCYCLOBUTENES IN POLYMER CHEMISTRY 517 (36) Fig. 15. Preparation of an ethylidene-bridged monomer. low dissipative properties that can be used as thin film coatings for multichip modules and integrated circuits. To produce a polymer with these properties, Kirchhoff et al. synthesized the stereoisomers of 1,3-bis(phenyl-4-benzocyclobutenylmethyl)benzene [trityl-bis(BCB)] monomer. 23 ’ The 1,3-bis(4-benzocyclobutenoyl)benzene~’z (37) was treated with phenyl- magnesium bromide to form the diol, and this compound was reduced with triethylsilane and trifluoroacetic acid to give the final product in 55% overall yield with 98% purity. Figure 16 shows the synthesis of trityl-bis(BCB) (38). Tan and Arnold% prepared N-ethyl-l,6-bis(4-benzocyclobutenyloxy)benzylamine as a potential thermosetting attachment to a rigid-rod polymer in an effort to overcome phase separation in physically blended molecular composites. Reaction of 2,6-difluorobenzo- nitrile with 4-hydroxybenzocyclobutene (33) gave 2,6-bis(4-benzocyclobutenyloxy)- benzonitrile (39), which, when reduced, yielded the corresponding benzylamine (40). Acetylation of (40) gave the amide (41), and (41) was reduced to the final compound, 2,6-bis(4-benzocyclobutenyloxy)-N-ethylbenzylamine (42). Figure 17 shows the synthesis of (42). For certain applications of polymers, notably in the aerospace industry, materials are required that are not only thermally stable, but also stable to oxidation at high temperatures. (38) Fig. 16. Preparation of trityl-bis(BCB). 518 MICHAEL F. FARONA (33) (39) CHzN% 1. LAH 2.NaOH (42) Fig. 17. Synthesis of 2,6-bis(4-benzocyclobutenyloxy)-N-ethylbenzylamine. Kirchhoff and Schrock reported the synthesis of a bis(BCB) monomer and oligomer, the polymer from which exhibits oxidatively stable properties.27 These molecules contain two BCB groups separated by at least one azo linkage. The monomer 4-aminophenyl-4- benzocyclobutenyl ketone (43), dimerizes in the presence of cuprous chloride catalyst while sparging with air to give (44). Multiple azo linkages can be incorporated into an oligomeric bis(BCB) by using the extender oxydianiline to give (45). Compound (44) is a Fig. 18. Synthesis of bis(BCB) compounds separated by at least one azo linkage. BENZOCYCLOBUTENES IN POLYMER CHEMISTRY (46) (33) 4-BCB-OH 2 CBCB-OH (33) + HFT + n .0-j-w ?=+ (47) * = 1,2,4 Fig. 19. Preparation of a poly(ether) bicapped with BCB. red crystal1in.e material with a melting point of 21&219”C, while (45) is a black solid. The preparative reactions are shown in Fig. 18. Benzocyclobutene compounds have found an application as electronic coating materials. However, the monomer must have a low enough melting temperature for efficient processing. Parker and Regulski have reported the synthesis of some low-melting mono- mers and oligomers of BCB-capped fluoroaryl ethers.28 Monomer (33), 4-hydroxy-BCB (two equivalents) reacts with a,cr,a,2,3,5,6-heptafluor- otoluene (HFT) in the presence of K2C03 to give 70% 2,6- and 26% 2,5isomers of bis(benzocyclobutenyloxy)pentafluorotoluene (46). Also obtained was the 2,3,6-trisubsti- tuted compound. Oligomeric bis(BCB) compounds of a similar nature can be obtained by adding varying amounts of 2,2-di(4-hy~oxyphenyl)-l,l,l,3,3,3-hexafluoropropane. The resulting oligomer (47) is shown in Fig. 19. In addition, further substitution of the fluoro substituents by 4-hydroxy-BCB in (47) can also take place, and multiple BCB groups can be accommodated on oligomer (47). A paper by So reports a convenient method to produce benzocyclobutenes on a large scale.29 Starting with methyl-3-(chloromethyl)-4_methylbenzoate, pyrolysis of this mater- ial gives BCBs at around 730°C in the presence of o-xylene as a diluent (diluent:substrate = 4:1, optimally). The pressure in the pyrolysis reactor gave the best results at 25 mmHg, although higher pressures, up to 150 mm Hg also gave reasonable results. Unreacted starting material can easily be recovered and recycled. Under optimal conditions, conver- sions of starting material are around 55-60% with selectivities to the product, 4-(carbo- 520 MICHAEL F. FARONA methoxy)benzocyclobutene, around 70%. Similar results were obtained for the pyrolysis of cr-chloro-o-xylene to give the parent benzocyclobutene. 3. STUDIES ON THE REACTIONS OF BENZOCYCLOBUTENES 3.1. Kinetics studies Deeter and Moore carried out kinetic studies on some derivatives of XTA (13) as model compounds with various dienophiles, but did not report on the nature of the crosslirik.3o However, they confirmed earlier studies that the reaction of BCB is first order, and the rate determining step involves the opening of the four-membered ring to o-quinodimethane, followed by a much faster Die&Alder or dimerization reaction. The activation energy for opening the ring was found to be 38.02 kcal/mole, which is consistent with earlier studies. Hahn et al. studied the kinetics of the polymerization of l-(4-tolyl)-2-(4-benzocyclo- butenyl)ethene, a stilbene-like molecule with two functionalities. 31 According to the struc- tures shown in Fig. 2, the BCB group can undergo a Die&Alder reaction with the double bond from another molecule, and/or dime&e to give the cyclooctadiene species, and/or polymerize to give the poly(o-xylylene) crosslink. The kinetics showed a first-order reaction with the rate-determining step being the opening of the four-membered ring. The specific rate constant, k, was found to be 4.2 x lOA min-1 at 170.6”C, 1.1 x lo- 3 min-’ at 18O”C, and 2.3 x 10V3 min-’ at 190°C. The activation energy was calculated to be 35.4 kcal/mole, consistent with other calculations for BCB reactions. These authors concluded that, except for minor side reactions that retarded molecular weight growth, the o-quinodimethane reaction followed the Die&Alder process. 3.2. Acid stability As was mentioned previously, a variety of 3,6-disubstituted BCB molecules based on XTA (13) were synthesized and studied as model compounds of crosslinkable high- performance polymers. l6 In one study, the stability of the BCB group was studied in sulfuric acid, since certain fibers are often spun from concentrated sulfuric acid solutions. The parent hydrocarbon and other BCB molecules with electron-donating substituents are notoriously unstable in strong acid media. Compounds (15) and (26), which differ only in the mode of attachment of the amide group to the aromatic ring of the BCB group, were placed in DzS04 and studied by NMR spectroscopy. Compound (15) showed no change in the proton spectrum in the aromatic region and, more importantly, the resonance at 6 = 3.31, representing the protons of the strained four-membered ring, also remained constant throughout the course of the experiment (95”C, monitored over a 24 h period). In contrast to this behavior, compound (26) showed dramatic changes in the aromatic region, and the BCB methylene singlet at 6 = 3.28 decreased in intensity while two triplets at 4.10 and 2.65 ppm appeared. These observations are consistent with opening of the BCB four- membered ring, yielding a B-ethyl sulfonate ester. Therefore, it appears that when electron- withdrawing substituents are present on the aromatic ring of a BCB molecule, it is stable to the presence of strong acids. BENZOCYCLOBUTENES IN POLYMER CHEMISTRY 521 sx+ (y-q- \ Fig. 20. Postulated formation of 3.3. Them1 studies The XTA model compounds were studied by differential scanning calorimetry (DSC) to characterize melting transitions and exothermic reactions. In general, the melting points were reversible, so long as the sample remained below the reaction exotherm. However, the reaction exotherm was found to be about 35O”C, around 100°C higher than that normally required for the reaction of o-QDM with dienophiles. Interestingly, when a dienophile was placed in the presence of the model compounds, the reaction exotherm dropped into the temperature range normally observed for reactions of o-quinodimethane. It was concluded that [4 + 21 cycloaddition reactions occur readily, but in the absence of a dienophile, further reaction of a-QDM is inhibited. This was attributed primarily to steric effects which are reduced in reactions with dienophiles, but are significant in dimerization reactions of o-quinodimethane because dimerization proceeds through the spiro intermediate (see Fig. 20). 3.4. Liquid chromatography-mass spectral studies Four papers by Marks and coworkers examined the hydrolysis products of a crosslinked polymer composed of BCB-terminated bisphenol A polycarbonates. 32-35 These studies were carried out in an attempt to elucidate the mechanism of the formation of dibenzo- cyclooctadiene and poly(o-xylylene) in reactions of the o-quinodimethane intermediate with itself. :It has been postulated that two o-quinodimethane intermediates react in a MICHAEL F. FARONA HO OH (52) Fig. 21. Products from hydrolysis of BCB-crosslinked bisphenol A polycarbonate. Diels-Alder reaction to form a spirodimer, which in turn dissociates into a diradical that couples to form poly(o-xylylene) and dibenzocyclooctadiene.36’37 Whereas some evidence has been found for the existence of the thermally unstable spirodimer, the postulation of the diradical species is purely speculative. The proposed process is shown in Fig. 20. After hydrolysis of the crosslinked polymer, only those fragments from the BCB reac- tions remained, and these were isolated and identified by liquid chromatography interfaced with a mass spectrometer. In addition to unreacted BCB-OH, the only products detected were dimers, trimers and tetramers that arose solely through the reaction of BCB with itself. The structures of the products from the hydrolysis are shown in Fig. 21. It is clear that (51) arises from reaction of BCB-OH with (49), and the tetramer (52) is produced from a reaction of BCB-OH with (50). The primary BCB reaction products were found to be the dimers (48) and (49), and the trimer (50). In lesser concentrations were BENZOCYCLOBU’IENES IN POLYMER CHEMISTRY 523 obtained the trimer (51) and the tetramer (52). The absence of 3,4-dimethylphenol, which would arise from hydrogen abstraction of a ring-opened BCB, was confumed. This negative evidence would seem to rule out the possibility that the the o-quinodimethane intermediate can be regarded as a diradical. The diradical mechanism proposed for the conversion of the spirodimer is inconsistent with the selectivity of the hydrolysis products. Only the dimers, trimers and tetramers shown in Fig.. 21 were observed, and several diradical reaction products expected from such intermediates did not appear. Products derived from chain transfer and radical addition reactions were absent. Therefore, the diradical mechanism for BCB crosslinking is not likely. The authors explained the crosslinking products as arising from a series of pericyclic reactions. Their proposed mechanism, which starts with the spirodimer, involves Diels- Alder cycloadditions, a suprafacial [ 1,3] carbon-carbon sigmatropic rearrangement, an antarafacial [ 1,7] retro-ene reaction, a suprafacial [ 1,9] retro-ene process, suprafacial [1,5] hydrogen sigmatropic rearrangements, and a [1,3] hydrogen shift. These processes are all thermally allowed, and the proposed mechanism fully accounts for the types and range of products observed. It further explains why additional products are not formed. 4. GRAFTS, BRANCHES, EXTENSIONS AND CROSSLINKS 4.1. Graft Whereas several methods have been employed to produce graft copolymers, a more recent approach has been to use a BCB-monocapped polymer to react in a Diels-Alder reaction with an available dienophile on another polymer. In one stt~dy,~~ 4-(3chloropropyl)benzocyclobutene (3~) was prepared by a cross-tri- merization as shown in Fig. 5, and chloride was exchanged for iodide in a Finkelstein reaction. This molecule was used to terminate an anionic polymerization of styrene to give polystyrene end-capped with BCB. To show that the polystyrene contained the BCB end- cap, the polymer was refluxed in 1,3,5-triisopropylbenzene (bp = 236°C) and reaction of the BCB molecule to give the cyclooctadiene links occurred to give an approximate doubling of the molecular weight (from M, = 24 850 to M, = 46 990). The presence of BCB on the polystyrene was also indicated by the proton signal at 6 = 3.1, but because of the minor concentration of BCB in the polymer, quantitative data were difficult to obtain. A copolymer of l-hexene (80%) and 7-methyl-1,6-octadiene (20%) was prepared by Ziegler-Natta polymerization, with the pendant double bonds intended as the grafting sites. The grafting reaction of the BCB-terminated polystyrene onto the copolymer was carried out in refluxing 1,3,5triisopropylbenzene, and the extent of grafting (branches per back- bone) was determined by ‘H NMR spectroscopy and molecular weight measurements. Depending on the level of polystyrene in the grafting reaction, 0.8-2.7 branches per back- bone were obtained, with grafting efficiencies in the 8495% range. Figure 22 shows the proposed grafting reaction. In a study by Dean, 39 a thermoplastic polyarylate was grafted onto an EPDM rubber. The polyarylate was an AB polymer from the reaction of phenyl-ar-(p-hydroxyphenyl)cumate, 524 MICHAEL F. FARONA + * PS* CH3 CH3 Fig. 22. Proposed grafting reaction. such that phenylester and phenolic hydroxy were the end groups. Compound (35), benzo- cyclobutenoyl chloride, was used to cap the hydroxy ends of the chains. A quantitative determination of the extent of BCB capping was carried out by allowing a Die&Alder reaction with N-naphthacenemaleimide, and the tag was measured by visible spectroscopy. It was confirmed that virtually all polyarylate chains contained a BCB head group. The BCB-capped polyarylate (PAR) was grafted onto an EPDM (diene = 1,4-hexadiene) rubber by mixing at 265”C, followed by compression molding. The grafting reaction is shown in Fig. 23. EPDM +7- Fig. 23. Grafting reaction of a polyarylate on an EPDM rubber . BENZOCYCLOBUTENES IN POLYMER CHEMISTRY 525 Table 3. Mechanical properties of EPDM-g-polyarylates Polyarylate (wt%) A (IV = 0.39 dl/g) SW = 0.60 dl/g) EPDM (wt%) MFI (3OO”C, 1P) Tensile strength (psi) Tensile modulus (kpsi) Elongation at break (%) 15 35 50 - - - - - - 15 35 50 85 65 50 85 65 50 47.1 34.2 30.1 27.2 20.1 14.7 2010 3400 4800 2580 4700 5910 18.1 32.4 54.3 20.2 40.6 62.3 162 225 470 272 422 520 Several polyarylates were prepared with varying inherent viscosities and grafted onto the EPM. Table 3 shows some mechanical properties of the various graft copolymers which are thermoplastic elastomers. The dynamic mechanical thermal analysis showed two clear transitions, one for the EPDM (- 55°C) and one for the polyarylate (201-205 “C). The clear separation of Tg values for these samples shows good phase separation between the hard-segment graft and soft-segment backbone. An EPDM-g-polyethersulfone was also prepared using BCB as the grafting agent through a Die&Alder reaction on the pendant double bonds of the EPDM (diene = 1,4- hexadiene).40 The polyethersulfone was prepared from 4-chloro-4’-hydroxydiphenylsul- fone, to give an AB polymer with 4chlorophenyl and phenolic hydroxy end groups. The hydroxyl end was capped with (35), benzocyclobutenoyl chloride, and the grafting reaction of the BCB-c:apped polyethersulfone to the EPDM rubber was carried out by mixing the two components at 28O”C, followed by compression molding. The grafting reaction is similar to that shown in Fig. 23. Dynamic mechanical thermal analysis showed two clean transitions, one for the T of the EPM at - 4O”C, and the other for the polyethersulfone at 220°C. The tensile strength was 5200 psi, and the modulus was 74 000 psi for the graft copolymer. Vinylbenzocyclobutene (6) was incorporated into linear low density polyethylene by means of a free radical reaction, using benzoyl peroxide as the initiator.41 The grafting of the BCB into the LLDPE sample was carried out in the molten state (no diluent). Whereas no further chemistry of the BCB-PE polymers was mentioned, the BCB groups can be used for grafting other polymers containing chain or pendant double bonds onto the backbone polymer. 4.2. Chain extensions and branches Chain extension is possible if a BCB functionality is on one or both ends of a chain; thus, in the former case, a doubling of the molecular weight is achieved through the benzocyclo- octadiene linkage, and in the latter, these same linkages can result in a chain of indefinite length. Of course, crosslinking is also possible through the reaction to give the poly(o-xylylene) structure. Branching occurs where a BCB functional group is incor- porated within the chain, and reacts with another BCB group situated either on the end or pendant to the chain. Continued reaction of pendant and end-situated BCBs leads to crosslinking and an incipient network. Benzocyclobutenoyl peroxide radicals, like benzoyl peroxide in the initiation of the polymerization of styrene, are so reactive that they add not only to the head or tail position, 526 MICHAEL F. FARONA E + HC-_ WH2) n = 4,6, 8 Fig. 24. Formation of BCB-capped oligomers. but also to the phenyl ring (about 15%). Therefore, using benzocyclobutenyl peroxide radicals as initiators in the polymerization of styrene leads to BCB groups situated ran- domly on the chain, as well as the ends. Thermolysis of polystyrene prepared in this manner leads to chain extension and branching.42 This approach to preparing branched polystyrene should allow for the manufacture of this material in a continuous free-radical bulk poly- merization without gel formation or reactor fouling. Gentsy and Farona studied the cyclotrimerization of long-chain c~, w-diynes, e.g., 1,9- decadiyne, l,ll-dodecadiyne and 1,13_tetradecadiyne, catalyzed by NbClS.43 The resulting polymer would be composed of trisubstituted benzene rings separated by 6-10 methylene groups. These growing chains were capped very early in the reaction by 1,5-hexadiyne, giving BCB-terminated oligomers, whose molecular weights were in the 31OO-WOO range. These BCB-capped oligomers were heated at 25O”C, and oligomeric chain extension occurred through reaction of o-quinodimethane with residual triple bonds and/or through BCB-BCB interactions to give benzocycooctadiene or poly(o-xylylene) linkages. The final network polymers are hard at room temperature, but soften near 250°C. Figure 24 depicts the formation of the BCB-capped oligomers. BENZOCYCLQBWFENBS IN POLYMER CHBMISTRY 527 Table 4. Mechanical properties of SBR gum crosslinked with BCBE Sample Elongation at break (%) Tensile strength (psi) Stress at 100% strain (psi) Stress at 300% strain (Psi) S-cured 332 343 143 306 4.8% BCBE 327 472 158 428 7.5% BCBE 233 410 176 506 9.0% BCBE 192 377 202 - 43. Crosslinking Whereas a BCB-monocapped polymer can be grafted onto another macromolecule, provided a means of attachment exists, a BCB-bicapped polymer will form crosslinks with the same macromolecule. Two studies were carried out on styrene-butadiene rubber, where a BCB-bicapped species was used to crosslink the material. In the first study, 1,2-bis(benzocyclobutenyl)ethane (BCBE) (2) at various loadings was heated with SBR at 210°C under 670 psi pressure.44 For a comparison of mechanical properties, a sample of SBR was sulfur cured, using 2-mercaptobenzothiazole and tetramethylthiuram disulfide as accelerators. A linear relationship was found between the 4.8,7.5 and 9.0% weight percent BCBE and the 100% modulus of the materials, and swelling data revealed a decreasing amount of solvent uptake with an increasing amount of BCBE, as expected. A plot of percent weight increase upon swelling vs the amount of BCBE in the SBR sample also showed a linear relationship. These data can be attributed solely to the number of crosslinks associated with increasing amounts of BCBE in the polymer blend. The mechanical properties of the SBR gum crosslinked with 4.8% BCBE are superior to those of the sulfur-cured material in terms of tensile strength and modulus. Unfortunately, the same reaction of BCBE with SBR was inhibited somewhat in the presence of carbon black. Table 4 gives some mechanical properties of SBR gum crosslinked with BCBE. Deeken and Farona prepared a polyurethane from 1,9nonanediol and toluene-2,4-di- isocyanate, and allowed reaction on both ends with BCB-methanol (S).l’ Thus, the poly- urethane was bicapped with BCB, and this difunctional polymer of molecular weight 11800 was used to crosslink SBR. Some mechanical properties were compared to S-cured SBR, such as tensile strength, and modulus at 300%. For example, the tensile strength of the crosslinked copolymer containing 10% polyurethane was 236 psi, while its 300% modulus was 131 psi and elongation at break was 564%. At 20% polyurethane loading, these data were 236 psi, 144 psi, and 353%, respectively. The S-cured SBR sample showed a tensile strength of 405 psi, 300% modulus of 212 psi, and elongation of 317%. Some substituted polyalkynes were also subjected to crosslinking using randomly situ- ated BCB on one polymer to react with a double bond on another. In this study, copolymers of a mono- o:r disubstituted acetylene were copolymerized with an alkyne containing the same substituents except that BCB was used in lieu of phenyl. For example, copolymers of phenylacetylene and ethynyl-BCB (6), l-chloro-2-phenylacetylene and 1-chloro-ZBCB- acetylene (7): and 1-phenylpropyne and 1-(CBCB)-1-propyne (8) were prepared, all con- 528 MICHAEL F. FARONA taining various loadings of the BCB monomer. Pressure-thermal cures were attempted on the copolymers, and considerable decomposition occurred under curing conditions, except for the Cl-substituted acetylenes. The cured copolymer of (7) with 1-chloro-2-phenyl- acetylene was a brittle, deep-red film, whose DSC plot of the post-cured sample showed no exotherm until 325”C, and the TGA plot showed no weight loss below 230°C and only about 8% weight loss at 325”C.12 Syndiotactic polystyrene can be made using Cp*Ti(OPh)3 (where Cp* = penta- methylcyclopentadienyl) with methyl aluminoxane as the catalyst system, or with ICp *TiMed PGMd a one 1 (no cocatalyst). The former catalyst system gives around 90% syndiotactic polymer. A homopolymer of 4-vinyl-BCB and copolymers of 4-vinyl- BCB and styrene were prepared and crosslinked, by heating them in an oven at 185°C. The polymers were crosslinked quantitatively, and the crosslink was most likely made through benzocyclooctadiene linkages.45 Copolymers of poly@-phenylene terephthalamide) and XTA (13) were prepared with XTA compositions of 1,5, 10, 25 and 50 mole percent. The copolymers were crosslinked by heat treatment at 400°C. Swelling tests in 98% H2SO4 showed the extent of crosslinking could be controlled by the amount of BCB in the copolymer.46 5. APPLICATIONS 5.1. Biocompatible polymers containing BCB Hexsyn (tradename), a copolymer of 1-hexene and 5-methyl-1,4-hexadiene (MI-ID; 97:3 molar ratio, respectively), is biocompatible and chemically stable, and has excellent fatigue properties. Vulcanized Hexsyn is used to construct the blood-pumping diaphragm in left ventricular assist devices and total artificial hearts.47 The same copolymer has also been used as the flexural component in artificial finger joints and hip prostheses. Because of its excellent fatigue properties and high tensile strength and modulus, Hexsyn has also been evaluated for use in artificial spinal discs. The cytotoxicity of the accelerators 2-mercaptobenzothiazole and tetramethylthiuram disulfide used in the sulfur-cured system require extraction of the vulcanizates prior to in vivo use. These accelerators are suspected of being weakly carcinogenic.48 A study was taken up by Fishback and Farona to replace the sulfur-cured system with BCB crosslinks in Hexsyn, with the hope that the favorable mechanical properties would be retained and that the system would be non-toxic. It was also required that the BCB cross- links be carried out in the presence of carbon black. A model study showed that BCBE crosslinked SBR rubber efficiently, giving mechan- ical properties similar to those of the sulfur-cured system. However, when the same reaction was carried out in the presence of carbon black, crosslinking was retarded.44 To overcome this problem, allyl-BCB (4) was synthesized, and incorporated as a comonomer in Hexsyn.8 The polymer was subjected to crosslinking conditions in the presence of carbon black. Crosslinking occurred readily, and it was determined that both Diels-Alder and BCB-BCB linkages were present in the cured sample. Table 5 gives some mechanical properties of the cured terpolymers. BENZOCYCLOBUTENES IN POLYMER CHEMISTRY 529 Table 5. Mechanical properties of hexsyn/allyl-BCB terpolymer Sample Ultimate elongation (%) Tensile strength (psi) 100% modulus (psi) A 400 1531 309 B 273 1354 364 C 551 1192 176 Polymers cured at 21O”C, 670 psi. A = 1-hexene, 3.2% MHD, sulfur cured. B = 1-hexene, 3% MI-ID, 3% allyl- BCB. C = 1-hexene, 2% MI-ID, 2% allyl-BCB, 60 min cure. As can be determined from Table 5, the mechanical properties of the BCB- and sulfur- cured system are somewhat comparable. When 7-methyl-1,6-octadiene was used in place of MHD, inferior mechanical properties were obtained for both the sulfur-cured and the BCB crosslinked materials. Cytotoxicity tests were carried out on the BCB crosslinked materials using positive and negative controls. Carbon black-loaded samples were laid on mouse fibroblast cell cultures, and their effects on the cells were visually monitored. The results showed that the samples exhibited no effect on the cell cultures. Thus, the BCB-cured samples were deemed to be non-toxic for in vivo use, and possess the mechanical properties that render the BCB- modified Hexsyn as a viable biocompatible material. 4g75o 5.2. Fibers Two papers by Martin, Moore et ~1.~~~‘~ point out that fibers made from poly@-pheny- leneterephthalamide) (PPTA) and other rigid-rod polymers, while stiff and of high tensile strength, are limited in use because of poor compressive strength, as well as relatively poor adhesion in composite matrices, environmentally accelerated creep, and low abrasion resistance. The weakness in compressive strength, at least in part, is due to the ease in buckling of individual molecules or microfibrils because of a lack of strong lateral inter- chain interaciions. One way to increase these lateral interactions is to incorporate a potential crosslinking group in the polymer backbone. However, the crosslinking group must not diminish the favorable mechanical properties stated above, and ideally, crosslinking should be possible after fiber spinning (solid state reaction). This means that the crosslinking temperature must be above that of processing but below the degradation temperature. Benzocyclobutene seems to be eminently well suited for increasing interchain bonds, since it is chemically dormant until heated, and reactions of BCB can occur in the solid state after fiber spinning. The preparation of PPTA involves reaction of terephthaloyl chloride with p-phenylene- diamine. By incorporating XTA-Cl(l4) randomly into the polymer in varying amounts, the number of crosslinks can be controlled. Furthermore, XTA-Cl is of the same structure as terephthaloyl dichloride, except that it contains the strained four-membered ring. Therefore, th.e favorable mechanical properties of the polymer should not be affected significantly. The fibers were subjected to heat treatment, and the crosslinking reaction occurred at 530 MICHAEL F. FARONA temperatures above 35O”C, some 100°C above the usual crosslinking temperatures of BCBs, but significantly above the solution processing temperature ( - 80°C) and well below the degradation temperature ( - 500°C). The final crosslinked products were insol- uble in concentrated sulfuric acid, showing crosslinking occurred, regardless of the amount of BCB incorporated into the polymer (l-100% with respect to replacement of terephthaloyl dichloride with XTA-Cl). Whereas it was not clear whether replacement of terephthaloyl dichloride by XTA-Cl improved the compressive strength of the fibers, another study on polybenzobisthiazoles, (PBT) also using XTA-Cl in place of terephthaloyl dichloride in varying amounts, showed a significant improvement in compressive strength when the polymer was crosslinked with BCB, provided substitution with XTA-Cl was 100% (no terephthaloyl dichloride). The axial compressive strength of the crosslinked polymer was about twice that of the uncrosslinked rigid-rod PBT polymer. However, when 25-50% XTA-Cl was used in place of terephthaloyl dichloride, the compressive properties were lower than those of the uncrosslinked mater- ial. Once again, the DSC scan showed a large exotherm, which represents the cross- linking reaction of BCB in the polymer, starting at 320°C and reaching a maximum around 421°C. This is considerably higher in temperature for BCB reactions than in most cases. The morphology of the PPTA-co-XTA fibers was investigated by wide angle X-ray diffraction (WAXD).54T55 At all levels of XTA incorporation, highly-oriented fibers were observed. At low XTA contents, the WAXD patterns were similar to those of neat PPTA, but with an increasing incorporation of XTA in the polymer, expansion in the distance between hydrogen-bonded planes occurs. At very high XTA contents, an apparent trans- formation in crystal structure occurs. Treatment at crosslinking temperatures does not hinder or disrupt the order. This suggests that the crosslinking reaction may be confined to the less-ordered grain boundary areas between crystallites, and perhaps does not occur to any large degree within the crystalline phase. Significant improvements in the orientation and crystallization of PPTA-co-XTA can be induced during heat treatment, and improvements in the microstructure were also accom- plished at sub-crosslinking temperatures. This provides considerable flexibility in control- ling the macroscopic properties of the fibers by post-spinning heat treatments. The authors envision a process in which spatial distribution of crosslinking can be controlled, thus allowing for a crosslinked skin girding an uncrosslinked core. 5.3. Coatings and jlms for microelectronics Whereas the ideal material for coating electronic modules remains elusive, it is generally agreed that the ideal polymeric thin film would possess the properties given on the follow- ing “wish list”: low dielectric constant, good thermal stability, low water absorption, good adhesion (metal and itself), low coefficient of thermal expansion, low dissipation factor, good planarization, spin or spray coatability, and dry etching capability. Several BCB- containing polymers have been investigated for their use in coating applications, the monomers of which are the tram+stilbene analogue 1,2-bis(4-benzocyclobutenyl)ethylene (53), (E-BCB), b @ is enzocyclobutenyl)-m-divinylbenzene (54), (DVB-BCB), and bis(benzocyclobutenyl)divinyltetramethylsiloxane (55), DVS-BCB. The structures of these monomers are shown in Fig. 25. BENZOCYCLOBUTENES IN POLYMER CHEMISTRY Fig. 25. Structure of BCB monomers. By far, the most investigated BCB polymer for microelectronics applications is that derived from DVS-BCB (55), because it gives a thermosetting polymer with a high glass transition temperature ( > 35O”C), low dielectric constant (2.7 at 1 MHz), low dissipation factor (8 x lob3 at 1 MHz), low water absorption (0.25% in 24 h water boil) and good adhesive properties. The thermal polymerization of monomers (53), (54) and (55) was studied prior to the gel point to determine the nature of the polymerization sites.56 A relationship between mole- cular weight growth and functional group conversion was found for the polymerization of (54) and (55:), but (53) deviated significantly from the model. This was believed to be related to the fact that (53) contains two BCB groups, but only one alkene function. Thus, the gel point occurred earlier than with the other two monomers, and a broadening of the molecular weight occurred after the double bonds were spent because of residual BCB-BCB interactions. Kinetics measurements confirmed a first-order reaction with re- spect to o-quinodimethane, and it was concluded that the principal mode of polymerization for those monomers containing two BCB and two alkene groups was through Diels-Alder reactions. The structure of poly(DVS-BCB) is shown in Fig. 26. Some physical and mechanical properties of poly(DVS-BCB) and poly(E-BCB) were determined and compared with other commercially available interlayer dielectrics, such as low-stress polyimides and alkynyl-terminated polyimides. Whereas the BCB and poly- imides can be processed at temperatures below their respective Tg values, poly@VS- BCB) shows various other properties that are superior to those of the polyimides. For example, absorption of water is very low for poly@VS-BCB), about 0.25% after 24 h in boiling wate:r, while some polyimides are nearly an order of magnitude higher in this property. The coefficient of thermal expansion (5.2 x lo- 5oC-1), Young’s modulus 2.3 x 10” dynes - cm- 2 and dielectric constant for poly(DVS-BCB) are comparable to those exhibited by the highly fluorinated polyimides. However, the dissipation factor is an order of magnitude lower (8 x lOA, 1 MHz) than that of the polyimides.57P58 The chemical 532 MICHAEL F. FARONA Fig. 26. Structure of poly@VS-BCB). stability of poly(DVS-BCB) is excellent, and it is resistant to typical metal etching processes, as well as to photoresist stripper, xylene, trichloroethane and alcohols.59160 One of the key features that makes an organic polymeric film attractive as a dielectric material for microchip module fabrication is planarization (smoothing out) of the irregular surfaces of the conductor, particularly those with four or more conductor levels. Several studies have been carried out on planarization, where application of partially polymerized DVS-BCB has been achieved by spin or spray coating. 57-59Y6161,62 Coatings of 5-10 ,um were achieved with up to 95% degree of planarization in a single coat over metal surfaces containing lines up to 100 pm in width. Coating uniformity of the MS-BCB resin was very good, with a standard deviation of 1.4%. One of the disadvantages to using a largely hydrocarbon polymer for microelectronics coatings is the difficulty of adhesion of the polymer to the substrate surface. The adhesion problem is exacerbated by differences in coefficients of thermal expansion between the substrate and poly@VS-BCB). This is also typical of polyimides or any polymer with a CTE mismatch to substrate. Therefore, additional steps must be taken to improve adhesion. Coating the surface with an adhesion promoter, such as triethoxyvinylsilane or 3-amino- propyltriethoxysilane, followed by spin-coating of oligomeric MS-BCB, and curing, improves adhesion of the polymer. 58T63@ Another technique used to improve adhesion is surface modification, using, e.g., reactive ion etching treatments.65 Cure management of the polymer used for electronics coating applications is very im- portant, since it is during the cure process that the performance properties of the polymer are developed. The thermal polymerization of DVS-BCB an a single surface can be monitored by FT-IR spectroscopy. 66 In the case of laminate substances, attenuated reflec- tion IR microscopy is useful as a probe to determine the extent of curing.67 The salient BENZOCYCLOBUTENBS IN POLYMER CHEMISTRY 533 infrared absorptions are the CH rocking mode of the strained four-membered ring at 1194 cm-’ and the vinyl out-of-plane bending mode at 985 cm-‘, both of which decrease in intensity during the cure process. Concomitant to the disappearance of the BCB and vinyl bands is the appearance of ring bending modes associated with the tetrahydro- naphthalene structure being formed in the reaction (1498 and 1472 cm-‘). The glass transition temperature of the polymer increases with increasing extent of cure. At 2OO”C, the polymerization reaction proceeds at a controllable rate, so that a range of final properties is accessible. At 250°C and above, the curing is so rapid that Tg attains the cure temperature quickly. The ultimate Tg for the polymer is about 390”C.68-71 By using an infrared belt furnace, it was possible to reduce the cure time for DVS-BCB thin films from 5 h (conventional) to 5 min. The observation that the properties of the final polymer subjected to rapid thermal curing did not differ significantly from those of poly- mers obtained from the conventional cure schedules was very important.72~73 The curing of DVS-BCB must be carried out in the absence of oxygen, since the polymer is susceptible to oxidation, particularly at elevated temperatures. Oxidation occurs primar- ily at the tetrahydronaphthalene group formed during the Die&Alder reaction of BCB with the vinyl groups. The oxidation reaction was monitored by IR spectroscopy, which showed the disappearance of the bands associated with the tetrahydronaphthalene group accom- panied by the appearance of carbonyl absorptions. A variety of antioxidants have been investigated as additives in the preparation of poly(DVS-BCB) with varying success. Antioxidants such as 2,2,4-trimethyl-1,2-dihydro- quinoline and tetrakis[methylene(3,5-ditert-butyl-4-hydrocinnamate)]methane proved par- ticularly efficacious. Studies showed that an effective antioxidant can extend the oxidation time constant at 85°C to around 40 years.74s75 Kirchhoff c:t al. have reported a significant improvement in the moisture resistance of polyimides that can be used for coatings for microelectronics.76 Copolymers of l,l’- [methylene4,1-phenylenelbismaleimide and 3-[2-(4-methylphenyl)ethenyl]-BCB, for example, using an excess of the BCB monomer, retain a large portion of their storage moduli after exposure to water for seven days. The weight gain of water was only 0.5% after 168 h att 95°C when the polymer was composed of 30:70 mole ratio of the bis- maleimide to the BCB monomers. As more BCB monomer is added, both the dry and wet shear moldulus improve significantly. Monomer (38), trityl-bisBCB, which contains hydrophobic constituents and when poly- merized unde:r pressure at temperatures below the advancing Tg, gives a product potentially suitable for electronics application. The final polymer exhibits a dielectric constant of 2.68 at 500 kHz, and a dissipation factor of 0.00034.23 Compound (47), a mixture of BCB-bicapped fluoroarylether oligomers, can be cured in a vacuum to give a product whose Tg ranges from 176 to 349”C, depending on the value of TZ in the formula shown in Fig. 19. The dielectric constant of the polymers made from (47) is around 2.60 at 100 kHz, and the decomposition temperatures, both in air and under nitro- gen, are above 500°C. These properties make the polymers of (47) quite suitable for electronics applications. 28 Monomer l(26) is interesting in that it is a liquid at room temperature, and can be processed without the use of solvents. Whereas no properties were revealed by the authors 534 MICHAEL F. FARONA BA (33) NaOH/Et3N w H20/CH2C12 n - BCB-PC (56) - Fig. 27. Preparation of a BCB-teminated polycarbonate. for the final polymer, it is anticipated that it could find application in the electronics coating area. 22 5.4. High performance polymers Desirable properties of polymers used as high performance materials in composites and other applications are processibility, high thermal stability, chemical resistance, oxidative stability, toughness against fracture, and electroinsulative qualities. The polymers may be thermosetting or thermoplastic, depending on the type of material required for a specific application. 5.4.1. Polycarbonates The synthesis of bicapped and partially capped polycarbonates has been reported. 77-79 A general synthesis is exemplified in Fig. 27, wherein bisphenol A and phosgene react in equal amounts, and the polymerization is terminated on both ends by BCB-OH (33), to give BCB-PC (56). Of course, there is a great variety of linear polycarbonates available through this reaction; one may use many different hinds of diols in place of bisphenol A. The use of an aromatic trio1 will lead to branched polycarbonates. Partially terminated BCB-PC can be achieved by using one part of BCB-OH (33) and one part, e.g., p-tert-butylphenol as the capping groups. The molecular weights are controlled exclusively by, and are inversely proportional to, the amount of capping group added. The general method of synthesis can BENZOCYCLOBUTENES IN POLYMER CHEMISTRY 535 Table 6. Berwxyclobutene-polycarbonate properties Property mole/mole BCB-OH/Bisphenol A 1.00 0.60 0.30 0.10 0.03 Ba-PCs Mn (GPC) Disp. (GPC) Theo. ave. dp Ts (“C), precure Ts (“C), postcure T,(- 5%) (“C) log fj* (min) Tensile mod. (MPa) Film ductility Notch Izod (Jm--‘) 1327 500 2.65 2 130 207 415 2088 4128 11,633 718 1408 4518 2.91 2.93 2.58 2.7 4.3 11 125 130 139 194 181 170 - - - 2262 2262 low low - 59 - - 2.20 5.21 2041 2000 hkh high 198 235 31,894 11,803 2.70 32 150 160 469 6.37 1952 high 262 31,113 11,802 2.64 32 - 150 475 - 1862 high 801 ’ Bisphenol A-polycarbonate control (no BCB) also be used to prepare BCB-capped polyesters as well as BCB-capped polyestercarbo- nates. The preparation of the latter polymer involves the use of bisphenol A, phosgene, and a hydroxy-terminated polyester, along with the BCB capping agent. Crosslinking of BCB-PC to a completely insoluble matrix is achieved thermally. The crosslinking reaction most likely occurs through the process that produces poly(o- xylylene). Table 6 shows some physical and mechanical properties of the crosslinked polycarbonates. The DSC analysis of the linear BCB-PC polymers show a Tg or T,, depending on molecular weight, followed by a BCB homopolymerization exotherm. The rheology of the polymers depends on the initial molecular weights; higher molecular weight polymers show a viscosity-temperature behavior similar to the BA-PC until onset of the BCB reaction, whereupon the viscosity increases as the material crosslinks. Crosslinked BCB- PCs show remarkable strength and toughness over a wide range of crosslink densities. All showed yielding before failure under tension. Notched Izod impact strengths of 200-260 Jm- ’ were observed in samples having 0.3 m/m BCB/BA or less. The fracture toughness is relatively high and depends on the crosslink density of the material. Above Tg, crosslinked BCB-PCs have stable elastic moduli up to their decomposition temperatures. 5.4.2. Poly(diketones) and poly(ether ketones) Commercial poly(ether ketones) are high-performance engineering thermoplastics and are used for coatings, adhesives and structural composites. In the amorphous state, the polymers are easier to process because of their lower Tg values and solubilities, but lose some of the high-performance properties associated with the semicrystalline materials. Crosslinking the amorphous polymers can restore the desirable properties of their semi- crystalline counterparts. One method of crosslinking was reported by Moore et aZ.,77 who incorporated BCB groups into the backbone of a known amorphous poly(arylene ether ketone) by the synth- esis shown in Fig. 28. Four polymers were prepared: a control whose x value was 1; another control whose x 536 MICHAEL F. FARONA 8 0 g &a 0 P + + 0 . 0 .z O=b 10 0 o=V I ,Q, 0 BJ5NZOCYCLOBUTENES IN POLYMER CHEMISTRY 537 Table 7. Comparison of properties of poly@K-BCB) and poly@ismaIeimide) Property Poly@K-BCB) Density (gems) Moisture uptake 24 h boil (%) Static flexural strength RT, MPa (ksi) Dynamic flex. modulus RT, GPa (ksi) 177°C GPa (ksi) 25O”C, GPa (ksi) Retention of modulus at 177°C (%) at 250°C (%) 2% Weight loss temp. Nz 0 air (“C) 1.20 1.30 1.90 - 182 (26.4) 162 (23.5) 4.07 (591) - 3.17 (460) 2.65 (384) 2.85 (413) - 78 59 70 - 389 - 362 - aCOMPIMIDEA. 65 PWR value was 0; and two copolymers whose x values were 0.75 and 0.5. The uncured polymers were soluble in common organic solvents, and thin films could be cast from the solvents or the melt. Molecular weights were around 100000 with polydispersities of ca. 2. The Ta values of the polymers were between 150 and 160 “C, those polymers containing BCB in their backbones crosslinked at temperatures above 3OO”C, and were stable up to their decomposition temperature of around 450°C. 1,3-Bis(be:nzocyclobutenoyl)benzene (DK-BCB), when crosslinked gives a high perfor- mance thermoset resin that can be processed for aerospace applications such as carbon fiber composites. It is processed as a partially polymerized species for resin transfer molding. The cured poly@K-BCB) exhibits higher temperature performance than bismaleimide resin systems, which are commonly used as composite matrices. Its Tg (330°C) is con- siderably higher than bismaleimide systems, and moisture uptake is less than epoxy or bismaleimide materials. Table 7 compares some properties of cured poly(DK-BCB) with those of a poly@ismaleimide). 78,79 Aging studies were conducted on cured poly(DK-BCB) at 275°C in air. After 2000 h, 91% of the unaged flexural modulus and 34% of the unaged flexural strength were retained. The surface structure of the polymer changed upon aging; hydrogen atoms were lost, chain scission occurred, and an anhydride compound formed as a result of a reaction with oxygen. ao The cured polymer also showed superior thermal fatigue and hot/wet properties when tested as a graphite fabric reinforced composite.81P82 Thermal oxidative stability evaluation of poly@K-BCB) composite showed it to be stable at 204°C for 4000 h in air, but mechanical properties decreased in the cured polymer composite above 316”C.83 Tan et ~1.‘~ reported a polyether ketone, polyether sulfone, and polyether benzoxazole bicapped with BCB. The cured materials exhibited the following Tg values: the ether ketone, 201; the ether sulfone, 264; and the ether benzoxazole, 282°C. The relative thermooxidative stabilities (10% degradation) according to TGA measurements were: ether sulfone (458) < ether ketone (491) < ether benzoxazole (513°C). The DSC results 538 MICHAEL F. FARONA Fig. 29. Polyether ketone, sulfone, and benzoxazole monomers. indicated that the materials all have excellent processing windows of at least 145°C. Figure 29 shows the structures of these monomers. 5.4.3. PoZy(maleimides) Some variations in polymaleimide resins with BCB end groups have been reported recently: among them, polymaleimides with ether linkages. Figure 30 shows the synthesis of a typical BCB-terminated ether-imide polymer. 85Y86 When polymer (57) is heated slowly from 200 to 250°C it first softens, and then hardens to a solid polymer as crosslinking occurs. The Ts of the crosslinked polymer is about 4O-50°C higher than that of the uncrosslinked polymer. An AB-BCB-maleimide (AB-BCB-MI) monomer containing an ether linkage was synthesized and converted to a resin.87 Figure 31 shows the structure of monomer (58) 4-maleimidophenyl-4’-benzocyclobutenyl ether. When fabricated into a carbon fiber com- posite, poly(AB-BCB-MI) shows excellent toughness in compression after impact while maintaining a very good composite strength in open hole compression. Monomer (58) is a solid at room temperature with a melting point of 117°C. Its DSC polymerization exotherm begins at about 18O”C, passes through a maximum at 261°C and ends around 290°C. Therefore, the window exists between 125 and 180°C for melt processing. Because of its sustained low viscosity, poly(AB-BCB-MI) is particularly suitable for resin transfer mold- ing. Some properties of poly(AB-BCB-MI) are listed in Table 8. This monomer (58) has all the advantages of using BCB as the reactive polymerization center. The polymerization is primarily through chain extension (BCB with the maleimide dienophile). The polymerization is activated by heat, and since it is a single component system, precise stoichiometric control is not required. There is no solvent present, nor are gaseous products evolved during polymerization, thereby simplifying the fabrication of void-free composite parts. The carbon-fiber composites of this material have good open BENZOCYCLOBUTENES IN POLYMER CHEMISTRY 539 0 A0 0 4 0 NH2 -Y + F 0 m--.&_o+xF 0 (57) Fig. 30. Synthesis of a BCB-capped maleimide-ether polymer. hole composite strength (in the 15&177”C range), excellent toughness (compression after impact of 331 MPa), and resistance to organic solvents. A variety of AB-BCB-MI monomers, analogous in structure with compound (58), have been reported. All of the materials prepared from the cured monomers possess high Tg values and moduli, and excellent toughness which makes them suitable candidates for matrix resins in fiber reinforced composites.88 0 \ af19 / / N \ \ 0 Fig. 31. Structure of an AB-BCB-maleimide monomer. 540 MICHAEL F. FARONA Table 8. Neat resin properties of poly(AB-BCB-MI) Property Density Tl? TGA temp. at 1% loss in air and Nz CLTE RT flexural strength RT flexural modulus Dielectric constant at 1 kHz at 1OOkHz Dissipation factor at 1 kHz at 100 kHz Fracture toughness (G1c) Value 1.29 gem” 270°C 390°C 60 /.&m/“C 181 Pa (26.3 ksi) 350 MPa (50.8 ksi) 3.61 3.56 0.03 0.011 1500 J/M2 Some maleimide copolymers were reported with comonomers (26)” and (37),% both of which contained a small amount of a free-radical inhibitor, namely 2,6-di-tert-butyl-4- methylphenol. The free-radical inhibitor increased the pot life of the molten monomer mixture and increased the toughness of the cured resin by 30-40 Jm- 2. The bis(maleimide) used as the comonomer in each case was l,l’-(methylenedi-4,1-phenylene)bis(maleimide). The copolymers have improved physical and mechanical properties compared to the bis(maleimide) homopolymers. For example, improvements in onset temperature, adhesion, thermal stability, oxidative stability, solvent resistance, Tg, water absorption, dielectric constant, elongation at break and toughness were all claimed. A particularly advantageous property of the copolymers is their excellent long-term thermal stability at elevated temperatures. They typically exhibit less than a 2.5% weight loss when heated in air at 300°C for 66 h. An interesting feature of the copolymer of the bis(maleimide) with monomer (26) is a second point of curing. In addition to the poly(o-xylylene) that can be formed from BCB groups, Die&Alder reactions can also occur at the ethylene groups. It has also been reported that a dicyanate ester/bis(maleimide) copolymer gains im- proved toughness in the cured state when a bis-BCB, which serves as the curing site, is added as one of the monomers to make a terpolymer.‘l 5.4.4. Bentocyclobutenes in rigid-rod molecular composites An alternative to a chopped fiber-reinforced composite is a molecular composite, where, in lieu of the fiber, one approach is to disperse an intrinsically rigid rod-like heterocyclic polymer in a flexible, coil-like heterocyclic polymer. A disadvantage to these types of molecular composites is phase separation between the two polymers. To overcome the propensity to phase separate, Tan and Arnold attached compound (42), a potentially thermosetting polymer, to a rigid-rod polymer, poly(amic amide).g2 The structure of the polymer (59) is shown in Fig. 32. When (59) is heated, it is converted into poly(4,4’- biphenyl pyromellitimide). Around 230°C the BCB reacts to form the thermosetting plastic, presumably through the formation of poly(o-xylylene). BENZOCYCLOBUTENE!SINPOLYMBRCHEMISTRY 541 - t i N-C I II CH2 0 I CH3 n Fig. 32. Structure of a trans-isomer of poly(amic amide)-BCB polymer. Tan et al., have reported the synthesis of some polyamides containing BCB as the crosslinking agent. 93y94 These thermosetting polymers can be applied to molecular compo- sites with suitable rigid-rod polymers. Figure 33 shows the structures of the polymers investigated. The cured polymers (62) and (64) are tough while (63) is brittle. Cured (62) shows an onset of weight loss around 35O”C, and the tensile strengths of (62) and (64) are 11.7 and 14.4 ksi, respectively. The morphologies of thin films of (62) and rigid rod polymer poly@-phenylene benzo- bisthiazole) (PBZT) were studied by wide-angle X-ray diffraction (WAXD) and high resolution scanning electron microscopy (SEM). The SEM micrographs showed a mor- phology of spherical domains (polyamide) resting on a network structure (PBZT). The morphology of the composite did not change after curing, indicating that phase separation is minimal. That phase separation is small or nonexistent was shown by WAXD patterns where the scattering intensifies of the pristine and cured samples were essentially identical. 6. REACTIONS OF BCB AND POTENTIAL APPLICATION TO POLYMERS 6.1. Photoresponsive polymers In general, photoresponsive polymers or additives blended with them undergo a struc- tural change in the presence of light. Reactions involving a photochemical ring opening or closure are particularly interesting since they create a change in optical density or refractive 542 MICHAEL F. FARONA (62) (63) w Fig. 33. Polyamide-BCB rigid-rod polymers. index. In order for these reactions to be of utility in imaging, they must be capable of occurring in the solid state. Cameron and Frechet have prepared polymers that undergo ring closure in the solid state by photochemical means, and the reaction, at least for one of them, is thermally reversible. 95 An alternating copolymer of maleic anhydride and 2,4,6-triisopropylbenzophenone-4- vinyl ether (60) was synthesized by a free radical process. Similarly, the copolymer of styrene and the vinyl ether-benzophenone monomer (61) was also synthesized; this copoly- mer was particularly desirable because the relative amounts of the monomers could be varied in the copolymer. Figure 34 shows the structures for the copolymers (60) and (61). The photochemical reaction of these copolymers involves the formation of a hydroxy- benzocyclobutenyl group. The photochemical reaction is shown in Fig. 35 for (61); the photocyclization is reversible thermally for this copolymer. Both (60) and (61) undergo photocyclization in the solid state, but only (61) is thermally reversible. The cycle was performed for a total of three runs, and during each run the efficiency decreased, and the polymer slowly deteriorated. It should be noted that while this photoreaction is thermally reversible, it is not reversible photochemically. 6.2. Reactions of o-quinodimethanes with multiple bonds containing heteroatoms Whereas BCB itself is not known to undergo [4 + 21 cycloadditions with multiple bonds containing heteroatoms, 1-hydroxy-BCB (l-BCB-OH) (65) does so readily. For example, l-BCB-OH is converted to the corresponding o-quinodimethane derivative in boiling toluene. If (65) is converted to the oxy anion by use of an appropriate base, then o- quinodimethane reactions occur below room temperature.% The l-BCB-OH parent or its BENZOCYCLQBUTENES IN POLYMER CHEMISTRY w (61) Fig. 34. Structure of two photoresponsive copolymers. derivatives are made conveniently by dehydrohalogenating a haloarene with NaNH, to the corresponding benzyne, treating with a ketene dimethyl- or diethylacetal, and hydrolysing the crude dimethoxybenzocyclobutene quantitatively to the benzocyclobutenone. This species is then reduced by NaBl& to l-BCB-OH in high yields.97 It should be pointed out that whereas the reactions with the BCB oxy anion appear to be those of o-quinodi- methane (i.e., [4 + 21 cycloadditions with an appropriate dienophile), the actual reactive species is its tautomer: o-tolualdehyde anion.96 Figure 36 shows the transformation of (65). Olofson et al., took advantage of the recent developments in l-BCB-OH chemistry i0 hr (A* 29oMl) - d’ - Fig. 35. Photocyclization and thermal reversibility of a photoresponsive polymer. 544 MICHAEL F. FARONA (65) 1BCB-OH Fig. 36. Formation of tolualdehyde anion from l-BCB-OH. carry out regioselective syntheses of a variety of anthracenes. These authors allowed reaction of l-BCB-OH with various benzynes to give, after dehydration, the regioselective anthracenes.98 Table 9 shows a few of the reactions to produce anthracenes. Using l-BCB-OH (65) derivatives as surrogates for o-tolualdehyde anions, Olofson et al., showed that these intermediates can be trapped by aldehydes.W A thermal reaction of (65) with aldehydes in refluxing toluene gave only a 10% yield of the product; however, when the tolualdehyde anion was generated with potassium t-butoxide or lithium 2,2,6,6- tetramethylpiperidide (LTMP), the yield of product was 95-98%. The reaction ingredients Table 9. Synthesis of anthracenes lBCB-OH OH BenzvneFVecunor OH Produet % Yield Me P CF3 45 35 40 BENZOCYCLOBUTENES IN POLYMER CHEhtISTRY 545 Table 10. Reactions of l-BCB-OH derivatives with aldehydes lBCB-OH m 0 OH P 0 OH Me Q’- 0 OH Me m 0 OH H,$?= fl 0 OMe OH R (CH&ZH - C-H T cm* 0 H 96 87 were mixed at - 78”C, allowed to warm to room temperature, and the product was isolated after a reaction time of one hour. Table 10 shows some reactions of tolualdehyde anion derivatives with various aldehydes. It should be pointed out that the benzopyranols formed from cyclization of the anion of l-BCB-OH with aldehydes can be easily oxidized to 3,4- dihydroisocoumarins enroute to natural products. Continuing with the cyclization reactions of the anion of l-BCB-OH, Olofson and coworkers carried out reactions with nitriles to give 3-substituted isoquinolines, some of which are pharmaceutically active. Table 11 gives some products of the reaction of the hydroxybenzocyclobutene anions with nitriles. The intermediate compound of cyclization is analogous to that of the aldehydes, with an OH substitutent on the carbon atom adjacent to the heteroatom. However, in the case of reaction with nitriles, dehydration occurs during acidification of the reaction mixture, and only the isoq,uinoline is isolated. The l-hydroxy-o-quinodimethane intermediate can be generated photochemically from 546 MICHAEL F. FARONA Table 11. Reactions of l-BCB-OH anions with nitriles 1 BCB-OH OH OH OMC OH OMC OH OH Nitrile Product % Yield PhCEN QQr 47 QQP 55 49 66 80 2-methylbenzaldehyde and trapped with trifluoroacetone to give a bicyclic hemiacetal product. loa The reaction is shown in Fig. 37. o-Quinodimethanes also undergo [4 + 21 cycloaddition reactions with silylketenes. Thus, 1-methoxy-BCB undergoes ring opening in boiling toluene, and can be trapped by a silylketene to give an isochromene.“i Figure 38 shows this reaction. The implications of the chemistry presented above toward applications in polymer synthesis are easy to envisage. Oxygen atoms attached to the l-position of BCB lower the temperature of the thermal ring opening by some 9O”C, compared to the unsubstituted parent. This can be desirable in the cure of certain polymers which may be somewhat heat sensitive, but otherwise exhibit favorable material properties. Even lower cure tempera- tures (below room temperature) can be achieved by converting l-BCB-OH to the anion. It can be envisaged that bifunctional l-BCB-OH monomers can be synthesized or that mono- functional l-BCB-OH monomers can be incorporated into polymers, and then be allowed to react or cure at relatively low temperatures. BENZOCYCLOBUTENES IN POLYMER CHEMISTRY 547 Fig. 37. Photochemical generation and trapping of 1-hydroxy-o-quinodimethane. The reports that show that l-BCB-OH anion can undergo [4 + 21 cycloadditions with multiple bonds containing heteroatoms allows researchers the opportunity to investigate new types of BCB-containing polymers and new types of cures, possibly leading to new materials. 6.3. Regio- and stereoselective processes Fukumoto and coworkers have devised the synthesis of steroid compounds by intramo- lecular BCB Die&Alder reactions, and examined the stereochemistry of the products obtained. 102--105 For example, compound (66), containing a BCB group and an internal alkene function, was synthesized and subjected to thermolysis. An intramolecular Diels- Alder reaction between the o-quinodimethane and double bond produced the transfused des-AB-trienic steroid molecule (67), as a single enantiomer. The structures are shown in Fig. 39. A similar intramolecular Die&Alder reaction of o-quinodimethane with a trifluoro- methyl ally1 group led to the four possible stereochemical products; the stereochemistry depended 1a:rgely on the protecting substituent. Thus, compound (68) was synthesized and + Fig. 38. Reaction of 1-methoxy-BCB with a silylketene. 548 MICHAEL F. FARONA (67) Fig. 39. Stereospeciiic intramolecular Diels-Alder reaction with BCB. thermolyzed to general structure (69). Figure 40 shows the conversion of (68) to (69), while Table 12 lists the distribution of enantiomers. The esters acetate and p-nitrobenzoate, as well as the terahydropyran protecting groups give primarily the cis-fused cycloadducts C and D favoring the the cis-anti isomer, while the silyl ethers afforded the transfused adducts A and B favoring the trans-anti isomer.lM Using the above information, these researchers were able to design a BCB-containing molecule to carry out, through an intramolecular Diels-Alder reaction, a total synthesis of an 18,18,18-trifluorosteroid.105 Jones and coworkers lo6 carried out a controlled intramolecular Diels-Alder reaction with BCB and a bulky substituent to improve the synthesis of the precursor to pisiferol. They synthesized a BCB with a Z-cyan0 group on the four-membered ring and a phenyl- sulfonate substituent on the alkene group. Steric repulsion between between the Z-cyan0 BENZOCYCLOBUTENES IN POLYMER CHEMISTRY 549 Me.0 Me0 (69) Fig. 40. Intramolecular DiebAlder reaction of a trifluoromethylallyl group with BCB. group and an exo-directed SOzPh substituent would force the sulfonate to an endo position and the connecting chain exo during an intramolecular Diels-Alder reaction, so that the desired transfusion product would be favored. Thermolysis of (70) gave the stereo isomers (71) and (72:), with a trans:cis ratio of 1.5:1. This was a considerable improvement over previous attempts to synthesize (71). Figure 41 shows the structures of the compounds in this reaction. Charlton e,t al. lo7 allowed a reaction between 1-phenyl-1-hydroxybenzocyclobutene with the fumarate of methyl (R)-mandelate and isolated a single diastereomer in high yield. The cycloaddition is endo, with addition occurring between the re face of the (S,S)-dienophile and the si face of the o-quinodimethane. Figure 42 shows this reaction. The analogous reaction using 1-phenyl-BCB with the fumarate of methyl (R)-mandelate was unselective. Chromium tricarbonyl complexes of syn-1-hydroxy-BCB have been prepared, treated with base to create the oxy anion, and then allowed to react with various dienophiles. The Cr(CO)3 group would be expected to block one face of the o-quinodomethane diene, and thus direct the dienophile to the opposite face. Removal of the Cr(CO), group would leave a product enriched in one enantiomer. Reactions with the chromium carbonyl complex of the o-quinodimethane are regio- specific as with any Die&Alder reaction: the hydroxy group on the o-quinodimethane 550 MICHAELF.FARONA Table 12. Product ratios of compound (69) CH3C = 0 I 1.1 1.2 1 2.8 p-N02Bz 1.1 0.6 1 2.7 0.6 1.8 1 1.6 TBSb 0.6 3.1 1 0.8 TIPSC 0.4 3.1 1 1.4 8 THP = tetrabydropyran; b TBS = t-butyldimetbylsilyMluoromethane ’ TIPS = triisopropylsilyltrifluoromethanesulfonate (70) t (71) - - - - (72) X = SOzPh Fig. 41. Synthesis of a precursor to pisiferol. BENZOCYCLOBUIENFS IN POLYMER CHEMIsTRY 551 Fig. 4;‘. Reaction of 1-phenyl-1-hydroxy-BCB with the fumarate of methyl (R)-mandelate. Table 13. PrIoducts of the reactions of various dienophiles with Cr(CO), complexed o-quinodimethane Complex 05 CN 0 74 552 MICHAEL F. FARONA + c60 - " rl= 1,70% n = 2,27% n=3, trace + c60 - H d : ’ No’ F ” mixtureofn=l andn=2 trace of n = 3 Fig. 43. Some BCB reactions with &. and the substituent on the dienophile predominantly end up on the resulting six-membered ring in adjacent positions. However, because the Cr(C0)3 group blocks one face of the o-quinodimethane, results of Diels-Alder reactions are selectively (anti-l-tetrahydro- naphthol)Cr(C0)3 complexes. With ester- and nitrile-substituted alkenes, the major diastereomer is the cis-product (endo-addition), whereas vinyl sulfones largely prefer exo-addition to give the trans product.108P109 Table 13 shows the products, after removal of Cr(C0)3 group, of the reaction of various dienophiles with the Cr(C0)3 complex of the anion of l-hydroxy-o-quinodimethane. Knowledge of the regio- and stereoselective reactions of intramolecular and Cr(CO)3- complexed o-quinodimethane reactions can allow the synthesis of specially polymers tailored with specific properties. 6.4. BuckminsterjXerene-BCB chemktry Buckminsterfullerene (Cm) is multifunctional and undergoes Diels-Alder reactions across 6,6 ring junctions; Cm acts as the dienophile. Two recent reviews have appeared, one on cycloaddition reactions,11o and another on macromolecular fullerene chemistry. ‘I1 Fullerene is known to undergo [4 + 21 cycloadditions with o-quinodimethane intermediates generated from various BCB molecules; up to three BCB molecules have been accommo- dated on one Cm nucleus. The BCB group may contain additional functional groups that are susceptible to polymerization. Thus, C& can very easily be incorporated into polymers. Figure 43 shows some typical BCB reactions with Cso.l12 BENZOCYCLOBUTE~ IN POLYMER CHEMISTRY 553 Co' OH :I toluene + c60 reflw Fig. 44. Reaction of l-BCB-OH with G. The reactions with BCB molecules unsubstituted on the four membered ring are carried out in a high boiling solvent, typically 1,2,4-trichlorobenzene. However, as noted pre- viously, 1-hydroxy-BCB undergoes ring opening at substantially lower temperatures; the reaction of l,-BCB-OH and Cm was carried out in boiling toluene, a reasonably good solvent for Ctio, affording a 59% yield of product.‘13 The reaction is shown in Fig. 44. Since Cm is multifunctional and can undergo [4 + 21 cycloadditions with o-quinodi- methanes at any of the 6,6 junctions, it is clear that a bis-BCB molecule can theoretically form polymers where the monomers are perfectly alternating. The possibility of branching is likely, but the primary reaction on Cm seems to be kinetically preferred to the secondary reaction (note the distribution of products in Fig. 43). With an excess of bis-BCB with respect to C6a, crosslinking should occur. It is curious that reactions of bis-BCB molecules with fullerenes have not yet been reported considering the great number of reports on fullerene reactions that have appeared. 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