- Email: [email protected]
Poly(methylphenylsilylene)-block-polystyrene copolymer prepared by the use of a chloromethylphenyl end-capped poly(methylphenylsilylene) as a macromolecular initiator in an atom transfer radical polymerisation of styrene
PII:
Eur. Polym. J. Vol. 34, No. 12, pp. 1829±1837, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0014-3057/98 $ - see front matter S0014-3057(98)00041-X
POLY(METHYLPHENYLSILYLENE)-BLOCK-POLYSTYRENE COPOLYMER PREPARED BY THE USE OF A CHLOROMETHYLPHENYL END-CAPPED POLY(METHYLPHENYLSILYLENE) AS A MACROMOLECULAR INITIATOR IN AN ATOM TRANSFER RADICAL POLYMERISATION OF STYRENE LAURENCE LUTSEN,1 GERARD P.-G. CORDINA,1 RICHARD G. JONES1* and FRANCOIS SCHUEÂ2 Centre for Materials Research, School of Physical Sciences, University of Kent, Canterbury CT2 7NR, U.K. and 2Laboratoire de Macromoleculaire Chemie, Universite des Sciences et Techniques du Languedoc, 34095 Montpellier Cedex 5, France
1
(Received 17 June 1997; accepted in ®nal form 21 October 1997) AbstractÐBlock copolymers of poly(methylphenylsilylene) and poly(styrene) have been conveniently synthesised by using a,o-di((chloromethyl)phenylethyl-dimethylsilyl)poly(methylphenylsilylene) as a macromolecular initiator for the bulk polymerisation of styrene in an atom transfer radical polymerisation reaction. Where appropriate, the structures of the precursor and the block copolymers have been con®rmed using size exclusion chromatography and NMR spectroscopy. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
Substituted polysilylenes, also called polysilanes, consisting of a linear chain backbone of silicon atoms bearing two substituents, usually unsubstituted aryl or alkyl groups, represent a relatively new class of polymeric materials with useful electronic properties [1]. They have been shown to have potential for application as photoresist materials [2± 4], photoconductors [5, 6] and as non-linear optical (NLO) materials [7±9], as well as ®nding application as photoinitiators in radical polymerisation [10, 11] and as ceramic precursors [12, 13]. With the exception of the preceramic application, these can all be associated with the delocalisation of the s-electrons of the catenated silicon atoms which is manifest in a strong UV absorption, commonly in the wavelength range 300±350 nm [14, 15]. However, in contrast with these attractive and useful properties, polysilylenes usually have poor mechanical properties which severely constrain the exploitation of their potential applications. The incorporation of the silylene chain within block copolymers that would be compatible with commodity polymers, could well present a way round this detractive feature. Notwithstanding this possibility, there is always interest in the synthesis and characterisation of new copolymers, with their potential to combine the physical and mechanical properties of the component homopolymers. However, here have been only a few reports describing the synthesis of block *To whom all correspondence should be addressed.
copolymers in which one of the blocks is a polysilylene. These, typically are of poly(styrene) and poly(methylphenylsilylyene), PMPS [16, 17], poly(isoprene) and PMPS [16], and of poly(methylmethacrylate), PMMA, and poly(1,1-dihexyl-2,2dimethyldisilylene) [18]. In addition, we have recently reported the synthesis and the characterisation of PMPS-block-PMMA by condensation of a,o-dichloropoly(methylphenylsilylene) with a living PMMA [19]. In common with the earlier syntheses, this method of preparation requires the pre-synthesis and careful handling under high vacuum of two reactive polymers and is followed by dicult puri®cation procedures. In contrast, in this paper we describe preliminary results of a convenient route to block copolymers of a polysilylene and polystyrene using only straightforward synthetic methods and standard polymer isolation procedures. Recently, an atom transfer radical polymerisation, ATRP, in which a 2,2'-bipyridine complexed Cu(I) halide is used as a mediating agent has been reported [20], see Scheme 1. In this polymerisation, CH3CHBrPh acts as an initiator which is converted into the requisite radical species by the co-ordination complex. The equilibrium depicted in the initiation reaction is maintained throughout the ensuing polymerisation and the consequential proximity of the complex and the propagating radical prevents mutual chain termination. Thus, ATRP is a living polymerisation providing for well-controlled polymerisations of monomers such as styrene and methyl methacrylate, and giving polymers with predetermined molecular weights and narrow distri-
1829
1830
L. Lutsen et al.
Scheme 1
butions. The initiating centres in these polymerisations are labile organic halides. We have recently demonstrated that poly(methylphenylsilylene)s in which the phenyl substituents have been partially bromomethylated can be used as precursors of a polystyrene-poly(methylphenylsilylene) graft copolymer [21]. Thus, polysilylenes bearing halomethylphenyl end-groups should make satisfactory macromolecular ATRP initiators for block copolymer synthesis. Such a structure has been prepared in accordance with Scheme 2 as the one active component of a mixture of products derived from PMPS. Its application as a macromolecular initiator is depicted in Scheme 3. EXPERIMENTAL
Materials Sodium was supplied by Fisons and dichloromethylphenylsilane by Lancaster Synthesis, all other reagents and
solvents were obtained from Aldrich. Their puri®cation is detailed within the following synthetic procedures.
Apparatus and procedures 1 H, 13C, 29Si NMR spectra were recorded at probe temperature using a JEOL GX-270 spectrometer from solutions in CDCl3. Chromium III acetylacetonate (0.02 M) was used as a spin-relaxation agent for 29Si NMR. Molecular weights of the polymers and copolymers were estimated by size exclusion chromatography, SEC, using equipment supplied by Polymer Laboratories Ltd. All determinations were carried out at room temperature on a dual column bank of 500 and 104 AÊ using THF as eluent at a ¯ow rate of 1.0 ml minÿ1. The system was equipped with a dierential refractive index (RI) detector, and a variable wavelength UV-visible spectrophotometric detector set either at l = 254 nm or l = 334 nm. For the polymer systems under investigation, the absorption at l = 334 nm is unique to the silicon backbone of PMPS. Polystyrene standards were used as calibrants for both the UV and RI determinations. No signi®cant dierences in the estimated molecular weights were observed between
Scheme 2
Block copolymers synthesised by using poly(methylphenylsilylene)
1831
Scheme 3 UV and RI determinations of the either the parent PMPS or the product copolymer. Refractive indices were determined using an Abbe refractometer and SEC peaks were resolved using Peaksolve2, a peak ®tting package for Windows2 supplied by Galactic Industries Corporation.
Synthesis of a,o-di((4-chloromethylphenylethyl)dimethylsilyl)-poly(methylphenylsilylene) In accordance with the reaction depicted in Fig. 2, a,odi((4-chloromethylphenylethyl)dimethylsilyl)poly(methylphenylsilylene) was synthesised by a modi®cation of the standard Wurtz-type reductive-coupling of dichloromethyl-
phenylsilane with sodium metal in boiling toluene. The procedures were as follows: Sucient freshly cut sodium metal to ensure a 20% stoichiometric excess was introduced to a dry, 500 ml threenecked round bottom ¯ask ®tted with an overhead stirrer, a condenser and a dropping funnel. The sodium was carefully washed with successive aliquots of petroleum ether, after which it was transformed to a ®ne dispersion by rapid stirring whilst heating. Dry toluene (300 ml) was added and the was mixture brought to re¯ux prior to the addition of dichloromethylphenylsilane (15 g). When the addition of the monomer was complete, the reaction mixture was rapidly stirred under re¯ux for 2 h, during which
Fig. 1. 1H NMR spectrum of the PMPS macroinitiator, a,o-di((chloromethyl)phenylethyl-dimethylsilyl)po ly(methylphenylsilylene).
1832
L. Lutsen et al.
Scheme 4 time it turned from grey to purple, the colour associated with the formation of colloidal sodium [22] and indicative of high conversion [23]. At this stage reductive-coupling is still possible as chain ends associated with the residual sodium surface are incipient silyl anions, thus a dilute solution of (4-chloromethylphenylethyl)dimethylchlorosilylene, CDCS, in toluene (2 g in 10 ml) was then added cautiously. The reaction conditions were maintained for 30 min to allow for the coupling of at least some of the chain ends that remained associated with the sodium surface with Si±Cl groups of CDCS, after which the system was allowed to cool to room temperature. The polymer, hereafter called the PMPS macroinitiator, was then precipitated dropwise in methanol, and reprecipitated several times from THF solution before being vacuum dried at 508C for 24 h. The 1H NMR spectrum of the macroinitiator is shown in Fig. 1. The polymer does not have a well-de®ned structure for at least ®ve signi®cant resonances are observed between 0.8 and 3.5 ppm in addition to the broad peaks associated with the aromatic and methyl protons of the main chain substituents. If the end groups were exclusively those depicted in Fig. 3 then the only resonances that would be expected in this range would be those associated with the chloromethyl group (a singlet) and the non-equivalent methylene groups (a pair of triplets).* However, a number of alternative end group structures can arise from reaction of the capping agent at either of its chloro-functions, or from the reaction of uncapped chain ends with methanol in the ®nal quenching reaction. These are depicted in Scheme 4. Accordingly, the discernible resonances from 0.8 to about 3 ppm are assigned either to methyl protons in ±SiMe2OMe, ±SiMe2Cl and ± SiMe(Ph)OMe end groups, or to methylene protons in a *The resonances associated with the non-equivalent methyls of terminal SiMe2 groups in the desired structure are expected to be contained within the broad band extending from ÿ1 to 0.7 ppm.
corresponding variety of environments, i.e. ± C6H4CH2SiMe2OMe, ±C6H4CH2SiMe2Cl and ±C6H4CH2 SiMe(Ph)OMe. Signi®cantly, however, the resonance at 3.5 ppm corresponds to the protons of chloromethyl groups. The 29Si NMR spectrum is shown in Fig. 2. The three peaks observed at ÿ39.2, ÿ39.8 and ÿ41.2 ppm correspond to heterotactic, syndiotactic and isotactic triad con®gurations within the silicon backbone. The sharp peaks at 2.21 and ÿ21.96 ppm are assigned to the silicon atoms of terminal ±SiOMe and ±SiMe2 groups respectively. The signi®cant numbers of uncapped end groups are again evident.
ATRP of styrene initiated by poly(methylphenylsilylene) macro-initiator The polymerisation, represented in Fig. 3, was carried out in a dried Schlenck tube equipped with a magnetic stirring bar. The PMPS macroinitiator (0.5 g), CuBr (0.043 g), 2,2'-bipyridyl (0.14 g) and freshly distilled, inhibitor-free styrene (5 ml) were placed in the Schlenck tube, degassed by three freeze-pump-thaw cycles and then stirred at 1208C under vacuum. After 20 h, toluene was added and the mixture was ®ltered to remove all catalyst residues. The ®ltrate was then added dropwise to stirred methanol in order to isolate polymer which was then puri®ed by repeated reprecipitation from THF solution into methanol before ®nally drying in a vacuum oven at 508C over 24 h.
RESULTS AND DISCUSSION
Amongst the various end groups of the modi®ed PMPS only the benzyl chloride moieties are sensitive in ATRP. Thus, it is to be expected that the product mixture from the ATRP reaction will consist of unmodi®ed PMPS as well as AB and ABA block copolymers. Figure 3 depicts the RI size
Block copolymers synthesised by using poly(methylphenylsilylene)
Fig. 2.
1833
29
Si NMR spectrum of the PMPS macroinitiator.
exclusion chromatograms of the PMPS macroinitiator (Mn=4550, Mw=26 850, Mp=3100) and the copolymer product (Mn=46 547, Mw=493 400, Mp=371 400). It is clear from the low molecular weight region of the chromatogram for the copolymer that some precursor polymer does indeed remain. Assuming that polystyrene standards are acceptable calibrants for SEC determinations of PMPS, then from the Mn values a ratio of styrene to sily-
lene units of 9.2 is calculated. On the assumption that the ATRP reaction had resulted in an even distribution of all of the available styrene between the macroinitiator molecules, then it is readily calculated that this ratio should be about 12. Full conversion of the styrene could not have been achieved and this ®gure would accordingly be lower. However, the Mn value for the product mixture is depressed by the presence of polymer that is not fully endcapped, so the styrene units are actually
Fig. 3. SEC chromatograms of the PMPS macroinitiator (- - -) and the copolymer product (ÐÐÐ) obtained using RI detection.
1834
L. Lutsen et al.
Fig. 4. Resolved peaks ( ) and curve ®t (- - -) of the SEC chromatogram of the copolymer product (ÐÐÐ) depicted in Fig. 3.
only distributed over a portion of the product such that within the copolymer fraction a signi®cantly higher ratio of 53 styrene to silylene units is found. Figure 4 shows the resolved peaks and curve ®t for the copolymer product, and Table 1 lists the molecular weight parameters and proportions of the three peaks. The molecular weights of the second of the two high molecular weight peaks are approximately twice those of the ®rst, from which it is concluded that they represent ABA and AB block structures respectively. The Mw for the low molecular weight region compares very favourably with that of the macroinitiator. This region constitutes only about 11% of the total area of the chromatogram. The refractive index increments, dn/dc, for PMPS and PS in dilute solution in THF were determined to be 0.21 and 0.22 respectively, so assuming that similar values apply to the copolymer fractions, the ®gure of 11% corresponds to the amount of PMPS remaining in the copolymer. However, these considerations are not sucient to explain the high styrene content of the copolymer product and it must be assumed that during the course of product isolation there were signi®cant losses from the low molecular weight fraction of the product mixture. The Mn value of fraction 1 of Fig. 4 being signi®cantly greater than Mn of the macroinitiator is consistent with this notion.
Figure 5 shows the SEC determinations of the product mixture monitored using the variable wavelength detector set at 254 nm and 334 nm, respectively. Dierences in the relative intensities of the peaks corresponding to the dierent molecular weight fractions are to be expected. At 254 nm the detector is mainly sensitive to the pp* transitions associated with the aromatic moieties. In contrast, at 334 nm it is only sensitive to the ss* transitions associated with the backbones of the polysilylene chains. The greater ratio of the high to low molecular weight peaks in the 254 nm chromatogram accords with there being, on average, one aromatic group per repeat unit of polymer molecule in both the high and low molecular weight regions, i.e. in both the block copolymer and the remaining precursor polysilanes. In contrast, the corresponding ratio in the 334 nm chromatogram is much lower since the average number of catenated silicon atoms per repeat unit in the copolymer, which gives rise to this absorption, is much lower than in either of the precursor polymers. Nonetheless, a high molecular weight peak is evident and its presence con®rms that the product contains PMPS units linked to the predominant polystyrene, PS, units. It is also possible to resolve the peaks of the chromatograms of Fig. 5. However, the relative values of the absorption coecients of the macroinitiator and the copolymers, which vary with com-
Table 1. Relative proportions and molecular weight parameters for the resolved peaks of Fig. 4 Fraction
wt%
1 2 3
11.3 49.8 38.9
Mn
Mw 6540 206 600 527 100
PD 27 550 393 600 754 700
4.21 1.91 1.43
Block copolymers synthesised by using poly(methylphenylsilylene)
Fig. 5. SEC chromatograms of the copolymer obtained using UV detection at 254 nm (ÐÐÐ) and 334 nm (- - -).
Fig. 6. 1H NMR spectrum of the block copolymer.
1835
1836
L. Lutsen et al.
Fig. 7.
13
C NMR of the block copolymer.
position, are unknown so this does not lead to additional quantitative information that is meaningful. However, it appears from Fig. 5 that the low molecular weight region does constitute a signi®cantly greater proportion of the isolated polymer than was calculated from the RI chromatogram so, taking a qualitative view, the high molecular weight of the product copolymer is more readily rationalised. The 1H NMR spectrum of the block copolymer is shown in Fig. 6. Although the spectrum reveals the dominance of the polystyrene in the copolymer through the high intensities of the aromatic reson*The remainder of these resonances associated with the structure of the macroinitiator are swamped by the intense signal of the methine protons of the styrene blocks.
ances and those associated with the methylene and methine protons of its backbone, resonances associated with methyl substituents of the polysilane backbone are discernible between 0 and ÿ1 ppm. Weak signals at 0.85, 2.0 and 2.5 ppm are identically placed to three of the strongest resonances assigned to protons in the macroinitiator* but the weak signal which corresponded to the chloromethylphenyl groups of the macroinitiator is shifted to 3.7 ppm. Within the block copolymer this resonance is associated with the structure of the terminal ±CH2±CH(Ph)Cl group. From the resonance integrals a ratio of styrene units to silylene units of 45 can be calculated which, given the inaccuracies and uncertainties in both determinations, is not greatly dierent from the value obtained from SEC. Inevitably, the 29Si NMR spectrum of the block copolymer (not shown) is extremely weak.
Block copolymers synthesised by using poly(methylphenylsilylene)
Nevertheless, it contains the same features as those shown in Fig. 2. Slightly more revealing is the 13C NMR spectrum of the copolymer shown in Fig. 7. In addition to the resonances at 40±50 ppm, and 120±130 ppm and 145 ppm associated with the aliphatic and aromatic carbon atoms of polystyrene there is a very low intensity peak at around 135 ppm which is characteristic of the phenyl carbon atoms of PMPS. Unfortunately the resonance associated with the methyl carbon atoms of PMPS expected at ÿ7 ppm is not discernible above the noise level. CONCLUSION
A mixture of AB and ABA block copolymers of polystyrene and poly(methylphenylsilylene) has been prepared by an ATRP reaction using a chloromethylphenyl end-capped poly(methylphenylsilylene) as a macroinitiator. The most compelling evidence for the formation of the block copolymer as opposed to a mixture of homopolymers comes from SEC analysis which reveals that the high molecular weight fraction in the product mixture possesses a chromophore that absorbs at 334 nm which is characteristic of a polysilane. This new synthetic procedure is a much more convenient route to silylene/vinyl block copolymers than those involving polymer±polymer reaction methodologies. The results reported here are only viewed as preliminary. Our investigations are presently directed towards control of block lengths and alternative methods whereby initiating centres for an ATRP reaction can be attached to the chain ends of the precursor PMPS and other polysilanes. AcknowledgementsÐWe gratefully acknowledge the award of postdoctoral fellowships to LL within the framework of the E.U. Human Capital and Mobility Programme, and to GP-GC by the U.K. Engineering and Physical Sciences Research Council. The research was conducted as part of the EU Network Programme ARPEGE which is concerned with studies of the synthesis and characterisation of polymers for application in electronics and electrical engineering.
1837
REFERENCES
1. Miller, R. D., Angew. Chem. Int. Ed. Engl. Adv. Mater., 1989, 28, 1733. 2. Zeigler, J. M., Harrah, L. A. and Johnson, A. W., SPIE Adv. Resist Technol. Proc. II, 1985, 539, 166. 3. Miller, R. D., Polym. News, 1987, 12, 326. 4. Yagci, Y., Onen, A. and Schnabel, W., Macromolecules, 1991, 24, 462. 5. Abkowitz, M. and Stolka, M., Philos. Mag. Lett., 1988, 58, 239. 6. Dohmaru, T., Oka, K., Yajima, T., Miyamoto, M., Nakayama, Y., Kawamura, T. and West, R., Philos. Mag. B, 1995, 71, 1069. 7. Kajzar, F., Messier, J. and Rosilio, C., J. Appl. Phys., 1986, 60, 3040. 8. Yang, L., Wang, Q. Z., Ho, P. P., Dorsinville, R., Alfano, R. R., Zou, W. K. and Yang, N. L., Appl. Phys. Lett., 1988, 53, 1245. 9. Hasegawa, T., Iwasa, Y., Koda, T., Kishida, H., Tokura, Y., Wada, S., Tahiro, H., Tachibana, H., Matsumoto, M. and Miller, R. D., Synth. Met., 1995, 71, 1679. 10. Wol, A. R. and West, R., Appl. Organomet., 1987, 1, 7. 11. Yucessan, D., Hostoygar, H., Denizligil, S. and Yagci, Y., Die Angew. Makromol. Chem., 1994, 221, 207. 12. Yajima, S., Hasegawa, Y., Hayashi, J. and Ijmura, M., J. Mater. Sci., 1978, 13, 2569. 13. Mazdyasni, K., West, R. and David, L. D., J. Am. Chem. Soc., 1978, 61, 504. 14. West, R., J. Organomet. Chem., 1986, 300, 327. 15. Michl, J. and Miller, R. D., Chem. Rev., 1989, 89, 1359. 16. Fossum, E., Love, J. A. and Matyjaszewski, K., J. Organomet. Chem., 1995, 499, 253. 17. Demoustier-Champagne, S., de Mahieu, A. F., Devaux, J., Fayt, R. and Teyssie, Ph., J. Polym. Chem., 1993, 31, 2009. 18. Sakurai, H., Polym. Prepr., 1990, 31(2), 230. 19. Lutsen, L. and Jones, R. G., Polym. Int., in press. 20. Wang, J. S. and Matyjaszeski, K., Macromolecules, 1995, 28, 7901. 21. Jones, R. G. and Holder, S. J. Makromol. Chem., 1997, 198, 3571. 22. Jones, R. G., Ben®eld, R. E., Cragg, R. H. and Swain, A. C., Nature, 1991, 353, 340. 23. Jones, R. G., Ben®eld, R. E., Cragg, R. H., Swain, A. C. and Webb, S. J., Macromolecules, 1993, 26, 4878.
Eur. Polym. J. Vol. 34, No. 12, pp. 1829±1837, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0014-3057/98 $ - see front matter S0014-3057(98)00041-X
POLY(METHYLPHENYLSILYLENE)-BLOCK-POLYSTYRENE COPOLYMER PREPARED BY THE USE OF A CHLOROMETHYLPHENYL END-CAPPED POLY(METHYLPHENYLSILYLENE) AS A MACROMOLECULAR INITIATOR IN AN ATOM TRANSFER RADICAL POLYMERISATION OF STYRENE LAURENCE LUTSEN,1 GERARD P.-G. CORDINA,1 RICHARD G. JONES1* and FRANCOIS SCHUEÂ2 Centre for Materials Research, School of Physical Sciences, University of Kent, Canterbury CT2 7NR, U.K. and 2Laboratoire de Macromoleculaire Chemie, Universite des Sciences et Techniques du Languedoc, 34095 Montpellier Cedex 5, France
1
(Received 17 June 1997; accepted in ®nal form 21 October 1997) AbstractÐBlock copolymers of poly(methylphenylsilylene) and poly(styrene) have been conveniently synthesised by using a,o-di((chloromethyl)phenylethyl-dimethylsilyl)poly(methylphenylsilylene) as a macromolecular initiator for the bulk polymerisation of styrene in an atom transfer radical polymerisation reaction. Where appropriate, the structures of the precursor and the block copolymers have been con®rmed using size exclusion chromatography and NMR spectroscopy. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
Substituted polysilylenes, also called polysilanes, consisting of a linear chain backbone of silicon atoms bearing two substituents, usually unsubstituted aryl or alkyl groups, represent a relatively new class of polymeric materials with useful electronic properties [1]. They have been shown to have potential for application as photoresist materials [2± 4], photoconductors [5, 6] and as non-linear optical (NLO) materials [7±9], as well as ®nding application as photoinitiators in radical polymerisation [10, 11] and as ceramic precursors [12, 13]. With the exception of the preceramic application, these can all be associated with the delocalisation of the s-electrons of the catenated silicon atoms which is manifest in a strong UV absorption, commonly in the wavelength range 300±350 nm [14, 15]. However, in contrast with these attractive and useful properties, polysilylenes usually have poor mechanical properties which severely constrain the exploitation of their potential applications. The incorporation of the silylene chain within block copolymers that would be compatible with commodity polymers, could well present a way round this detractive feature. Notwithstanding this possibility, there is always interest in the synthesis and characterisation of new copolymers, with their potential to combine the physical and mechanical properties of the component homopolymers. However, here have been only a few reports describing the synthesis of block *To whom all correspondence should be addressed.
copolymers in which one of the blocks is a polysilylene. These, typically are of poly(styrene) and poly(methylphenylsilylyene), PMPS [16, 17], poly(isoprene) and PMPS [16], and of poly(methylmethacrylate), PMMA, and poly(1,1-dihexyl-2,2dimethyldisilylene) [18]. In addition, we have recently reported the synthesis and the characterisation of PMPS-block-PMMA by condensation of a,o-dichloropoly(methylphenylsilylene) with a living PMMA [19]. In common with the earlier syntheses, this method of preparation requires the pre-synthesis and careful handling under high vacuum of two reactive polymers and is followed by dicult puri®cation procedures. In contrast, in this paper we describe preliminary results of a convenient route to block copolymers of a polysilylene and polystyrene using only straightforward synthetic methods and standard polymer isolation procedures. Recently, an atom transfer radical polymerisation, ATRP, in which a 2,2'-bipyridine complexed Cu(I) halide is used as a mediating agent has been reported [20], see Scheme 1. In this polymerisation, CH3CHBrPh acts as an initiator which is converted into the requisite radical species by the co-ordination complex. The equilibrium depicted in the initiation reaction is maintained throughout the ensuing polymerisation and the consequential proximity of the complex and the propagating radical prevents mutual chain termination. Thus, ATRP is a living polymerisation providing for well-controlled polymerisations of monomers such as styrene and methyl methacrylate, and giving polymers with predetermined molecular weights and narrow distri-
1829
1830
L. Lutsen et al.
Scheme 1
butions. The initiating centres in these polymerisations are labile organic halides. We have recently demonstrated that poly(methylphenylsilylene)s in which the phenyl substituents have been partially bromomethylated can be used as precursors of a polystyrene-poly(methylphenylsilylene) graft copolymer [21]. Thus, polysilylenes bearing halomethylphenyl end-groups should make satisfactory macromolecular ATRP initiators for block copolymer synthesis. Such a structure has been prepared in accordance with Scheme 2 as the one active component of a mixture of products derived from PMPS. Its application as a macromolecular initiator is depicted in Scheme 3. EXPERIMENTAL
Materials Sodium was supplied by Fisons and dichloromethylphenylsilane by Lancaster Synthesis, all other reagents and
solvents were obtained from Aldrich. Their puri®cation is detailed within the following synthetic procedures.
Apparatus and procedures 1 H, 13C, 29Si NMR spectra were recorded at probe temperature using a JEOL GX-270 spectrometer from solutions in CDCl3. Chromium III acetylacetonate (0.02 M) was used as a spin-relaxation agent for 29Si NMR. Molecular weights of the polymers and copolymers were estimated by size exclusion chromatography, SEC, using equipment supplied by Polymer Laboratories Ltd. All determinations were carried out at room temperature on a dual column bank of 500 and 104 AÊ using THF as eluent at a ¯ow rate of 1.0 ml minÿ1. The system was equipped with a dierential refractive index (RI) detector, and a variable wavelength UV-visible spectrophotometric detector set either at l = 254 nm or l = 334 nm. For the polymer systems under investigation, the absorption at l = 334 nm is unique to the silicon backbone of PMPS. Polystyrene standards were used as calibrants for both the UV and RI determinations. No signi®cant dierences in the estimated molecular weights were observed between
Scheme 2
Block copolymers synthesised by using poly(methylphenylsilylene)
1831
Scheme 3 UV and RI determinations of the either the parent PMPS or the product copolymer. Refractive indices were determined using an Abbe refractometer and SEC peaks were resolved using Peaksolve2, a peak ®tting package for Windows2 supplied by Galactic Industries Corporation.
Synthesis of a,o-di((4-chloromethylphenylethyl)dimethylsilyl)-poly(methylphenylsilylene) In accordance with the reaction depicted in Fig. 2, a,odi((4-chloromethylphenylethyl)dimethylsilyl)poly(methylphenylsilylene) was synthesised by a modi®cation of the standard Wurtz-type reductive-coupling of dichloromethyl-
phenylsilane with sodium metal in boiling toluene. The procedures were as follows: Sucient freshly cut sodium metal to ensure a 20% stoichiometric excess was introduced to a dry, 500 ml threenecked round bottom ¯ask ®tted with an overhead stirrer, a condenser and a dropping funnel. The sodium was carefully washed with successive aliquots of petroleum ether, after which it was transformed to a ®ne dispersion by rapid stirring whilst heating. Dry toluene (300 ml) was added and the was mixture brought to re¯ux prior to the addition of dichloromethylphenylsilane (15 g). When the addition of the monomer was complete, the reaction mixture was rapidly stirred under re¯ux for 2 h, during which
Fig. 1. 1H NMR spectrum of the PMPS macroinitiator, a,o-di((chloromethyl)phenylethyl-dimethylsilyl)po ly(methylphenylsilylene).
1832
L. Lutsen et al.
Scheme 4 time it turned from grey to purple, the colour associated with the formation of colloidal sodium [22] and indicative of high conversion [23]. At this stage reductive-coupling is still possible as chain ends associated with the residual sodium surface are incipient silyl anions, thus a dilute solution of (4-chloromethylphenylethyl)dimethylchlorosilylene, CDCS, in toluene (2 g in 10 ml) was then added cautiously. The reaction conditions were maintained for 30 min to allow for the coupling of at least some of the chain ends that remained associated with the sodium surface with Si±Cl groups of CDCS, after which the system was allowed to cool to room temperature. The polymer, hereafter called the PMPS macroinitiator, was then precipitated dropwise in methanol, and reprecipitated several times from THF solution before being vacuum dried at 508C for 24 h. The 1H NMR spectrum of the macroinitiator is shown in Fig. 1. The polymer does not have a well-de®ned structure for at least ®ve signi®cant resonances are observed between 0.8 and 3.5 ppm in addition to the broad peaks associated with the aromatic and methyl protons of the main chain substituents. If the end groups were exclusively those depicted in Fig. 3 then the only resonances that would be expected in this range would be those associated with the chloromethyl group (a singlet) and the non-equivalent methylene groups (a pair of triplets).* However, a number of alternative end group structures can arise from reaction of the capping agent at either of its chloro-functions, or from the reaction of uncapped chain ends with methanol in the ®nal quenching reaction. These are depicted in Scheme 4. Accordingly, the discernible resonances from 0.8 to about 3 ppm are assigned either to methyl protons in ±SiMe2OMe, ±SiMe2Cl and ± SiMe(Ph)OMe end groups, or to methylene protons in a *The resonances associated with the non-equivalent methyls of terminal SiMe2 groups in the desired structure are expected to be contained within the broad band extending from ÿ1 to 0.7 ppm.
corresponding variety of environments, i.e. ± C6H4CH2SiMe2OMe, ±C6H4CH2SiMe2Cl and ±C6H4CH2 SiMe(Ph)OMe. Signi®cantly, however, the resonance at 3.5 ppm corresponds to the protons of chloromethyl groups. The 29Si NMR spectrum is shown in Fig. 2. The three peaks observed at ÿ39.2, ÿ39.8 and ÿ41.2 ppm correspond to heterotactic, syndiotactic and isotactic triad con®gurations within the silicon backbone. The sharp peaks at 2.21 and ÿ21.96 ppm are assigned to the silicon atoms of terminal ±SiOMe and ±SiMe2 groups respectively. The signi®cant numbers of uncapped end groups are again evident.
ATRP of styrene initiated by poly(methylphenylsilylene) macro-initiator The polymerisation, represented in Fig. 3, was carried out in a dried Schlenck tube equipped with a magnetic stirring bar. The PMPS macroinitiator (0.5 g), CuBr (0.043 g), 2,2'-bipyridyl (0.14 g) and freshly distilled, inhibitor-free styrene (5 ml) were placed in the Schlenck tube, degassed by three freeze-pump-thaw cycles and then stirred at 1208C under vacuum. After 20 h, toluene was added and the mixture was ®ltered to remove all catalyst residues. The ®ltrate was then added dropwise to stirred methanol in order to isolate polymer which was then puri®ed by repeated reprecipitation from THF solution into methanol before ®nally drying in a vacuum oven at 508C over 24 h.
RESULTS AND DISCUSSION
Amongst the various end groups of the modi®ed PMPS only the benzyl chloride moieties are sensitive in ATRP. Thus, it is to be expected that the product mixture from the ATRP reaction will consist of unmodi®ed PMPS as well as AB and ABA block copolymers. Figure 3 depicts the RI size
Block copolymers synthesised by using poly(methylphenylsilylene)
Fig. 2.
1833
29
Si NMR spectrum of the PMPS macroinitiator.
exclusion chromatograms of the PMPS macroinitiator (Mn=4550, Mw=26 850, Mp=3100) and the copolymer product (Mn=46 547, Mw=493 400, Mp=371 400). It is clear from the low molecular weight region of the chromatogram for the copolymer that some precursor polymer does indeed remain. Assuming that polystyrene standards are acceptable calibrants for SEC determinations of PMPS, then from the Mn values a ratio of styrene to sily-
lene units of 9.2 is calculated. On the assumption that the ATRP reaction had resulted in an even distribution of all of the available styrene between the macroinitiator molecules, then it is readily calculated that this ratio should be about 12. Full conversion of the styrene could not have been achieved and this ®gure would accordingly be lower. However, the Mn value for the product mixture is depressed by the presence of polymer that is not fully endcapped, so the styrene units are actually
Fig. 3. SEC chromatograms of the PMPS macroinitiator (- - -) and the copolymer product (ÐÐÐ) obtained using RI detection.
1834
L. Lutsen et al.
Fig. 4. Resolved peaks ( ) and curve ®t (- - -) of the SEC chromatogram of the copolymer product (ÐÐÐ) depicted in Fig. 3.
only distributed over a portion of the product such that within the copolymer fraction a signi®cantly higher ratio of 53 styrene to silylene units is found. Figure 4 shows the resolved peaks and curve ®t for the copolymer product, and Table 1 lists the molecular weight parameters and proportions of the three peaks. The molecular weights of the second of the two high molecular weight peaks are approximately twice those of the ®rst, from which it is concluded that they represent ABA and AB block structures respectively. The Mw for the low molecular weight region compares very favourably with that of the macroinitiator. This region constitutes only about 11% of the total area of the chromatogram. The refractive index increments, dn/dc, for PMPS and PS in dilute solution in THF were determined to be 0.21 and 0.22 respectively, so assuming that similar values apply to the copolymer fractions, the ®gure of 11% corresponds to the amount of PMPS remaining in the copolymer. However, these considerations are not sucient to explain the high styrene content of the copolymer product and it must be assumed that during the course of product isolation there were signi®cant losses from the low molecular weight fraction of the product mixture. The Mn value of fraction 1 of Fig. 4 being signi®cantly greater than Mn of the macroinitiator is consistent with this notion.
Figure 5 shows the SEC determinations of the product mixture monitored using the variable wavelength detector set at 254 nm and 334 nm, respectively. Dierences in the relative intensities of the peaks corresponding to the dierent molecular weight fractions are to be expected. At 254 nm the detector is mainly sensitive to the pp* transitions associated with the aromatic moieties. In contrast, at 334 nm it is only sensitive to the ss* transitions associated with the backbones of the polysilylene chains. The greater ratio of the high to low molecular weight peaks in the 254 nm chromatogram accords with there being, on average, one aromatic group per repeat unit of polymer molecule in both the high and low molecular weight regions, i.e. in both the block copolymer and the remaining precursor polysilanes. In contrast, the corresponding ratio in the 334 nm chromatogram is much lower since the average number of catenated silicon atoms per repeat unit in the copolymer, which gives rise to this absorption, is much lower than in either of the precursor polymers. Nonetheless, a high molecular weight peak is evident and its presence con®rms that the product contains PMPS units linked to the predominant polystyrene, PS, units. It is also possible to resolve the peaks of the chromatograms of Fig. 5. However, the relative values of the absorption coecients of the macroinitiator and the copolymers, which vary with com-
Table 1. Relative proportions and molecular weight parameters for the resolved peaks of Fig. 4 Fraction
wt%
1 2 3
11.3 49.8 38.9
Mn
Mw 6540 206 600 527 100
PD 27 550 393 600 754 700
4.21 1.91 1.43
Block copolymers synthesised by using poly(methylphenylsilylene)
Fig. 5. SEC chromatograms of the copolymer obtained using UV detection at 254 nm (ÐÐÐ) and 334 nm (- - -).
Fig. 6. 1H NMR spectrum of the block copolymer.
1835
1836
L. Lutsen et al.
Fig. 7.
13
C NMR of the block copolymer.
position, are unknown so this does not lead to additional quantitative information that is meaningful. However, it appears from Fig. 5 that the low molecular weight region does constitute a signi®cantly greater proportion of the isolated polymer than was calculated from the RI chromatogram so, taking a qualitative view, the high molecular weight of the product copolymer is more readily rationalised. The 1H NMR spectrum of the block copolymer is shown in Fig. 6. Although the spectrum reveals the dominance of the polystyrene in the copolymer through the high intensities of the aromatic reson*The remainder of these resonances associated with the structure of the macroinitiator are swamped by the intense signal of the methine protons of the styrene blocks.
ances and those associated with the methylene and methine protons of its backbone, resonances associated with methyl substituents of the polysilane backbone are discernible between 0 and ÿ1 ppm. Weak signals at 0.85, 2.0 and 2.5 ppm are identically placed to three of the strongest resonances assigned to protons in the macroinitiator* but the weak signal which corresponded to the chloromethylphenyl groups of the macroinitiator is shifted to 3.7 ppm. Within the block copolymer this resonance is associated with the structure of the terminal ±CH2±CH(Ph)Cl group. From the resonance integrals a ratio of styrene units to silylene units of 45 can be calculated which, given the inaccuracies and uncertainties in both determinations, is not greatly dierent from the value obtained from SEC. Inevitably, the 29Si NMR spectrum of the block copolymer (not shown) is extremely weak.
Block copolymers synthesised by using poly(methylphenylsilylene)
Nevertheless, it contains the same features as those shown in Fig. 2. Slightly more revealing is the 13C NMR spectrum of the copolymer shown in Fig. 7. In addition to the resonances at 40±50 ppm, and 120±130 ppm and 145 ppm associated with the aliphatic and aromatic carbon atoms of polystyrene there is a very low intensity peak at around 135 ppm which is characteristic of the phenyl carbon atoms of PMPS. Unfortunately the resonance associated with the methyl carbon atoms of PMPS expected at ÿ7 ppm is not discernible above the noise level. CONCLUSION
A mixture of AB and ABA block copolymers of polystyrene and poly(methylphenylsilylene) has been prepared by an ATRP reaction using a chloromethylphenyl end-capped poly(methylphenylsilylene) as a macroinitiator. The most compelling evidence for the formation of the block copolymer as opposed to a mixture of homopolymers comes from SEC analysis which reveals that the high molecular weight fraction in the product mixture possesses a chromophore that absorbs at 334 nm which is characteristic of a polysilane. This new synthetic procedure is a much more convenient route to silylene/vinyl block copolymers than those involving polymer±polymer reaction methodologies. The results reported here are only viewed as preliminary. Our investigations are presently directed towards control of block lengths and alternative methods whereby initiating centres for an ATRP reaction can be attached to the chain ends of the precursor PMPS and other polysilanes. AcknowledgementsÐWe gratefully acknowledge the award of postdoctoral fellowships to LL within the framework of the E.U. Human Capital and Mobility Programme, and to GP-GC by the U.K. Engineering and Physical Sciences Research Council. The research was conducted as part of the EU Network Programme ARPEGE which is concerned with studies of the synthesis and characterisation of polymers for application in electronics and electrical engineering.
1837
REFERENCES
1. Miller, R. D., Angew. Chem. Int. Ed. Engl. Adv. Mater., 1989, 28, 1733. 2. Zeigler, J. M., Harrah, L. A. and Johnson, A. W., SPIE Adv. Resist Technol. Proc. II, 1985, 539, 166. 3. Miller, R. D., Polym. News, 1987, 12, 326. 4. Yagci, Y., Onen, A. and Schnabel, W., Macromolecules, 1991, 24, 462. 5. Abkowitz, M. and Stolka, M., Philos. Mag. Lett., 1988, 58, 239. 6. Dohmaru, T., Oka, K., Yajima, T., Miyamoto, M., Nakayama, Y., Kawamura, T. and West, R., Philos. Mag. B, 1995, 71, 1069. 7. Kajzar, F., Messier, J. and Rosilio, C., J. Appl. Phys., 1986, 60, 3040. 8. Yang, L., Wang, Q. Z., Ho, P. P., Dorsinville, R., Alfano, R. R., Zou, W. K. and Yang, N. L., Appl. Phys. Lett., 1988, 53, 1245. 9. Hasegawa, T., Iwasa, Y., Koda, T., Kishida, H., Tokura, Y., Wada, S., Tahiro, H., Tachibana, H., Matsumoto, M. and Miller, R. D., Synth. Met., 1995, 71, 1679. 10. Wol, A. R. and West, R., Appl. Organomet., 1987, 1, 7. 11. Yucessan, D., Hostoygar, H., Denizligil, S. and Yagci, Y., Die Angew. Makromol. Chem., 1994, 221, 207. 12. Yajima, S., Hasegawa, Y., Hayashi, J. and Ijmura, M., J. Mater. Sci., 1978, 13, 2569. 13. Mazdyasni, K., West, R. and David, L. D., J. Am. Chem. Soc., 1978, 61, 504. 14. West, R., J. Organomet. Chem., 1986, 300, 327. 15. Michl, J. and Miller, R. D., Chem. Rev., 1989, 89, 1359. 16. Fossum, E., Love, J. A. and Matyjaszewski, K., J. Organomet. Chem., 1995, 499, 253. 17. Demoustier-Champagne, S., de Mahieu, A. F., Devaux, J., Fayt, R. and Teyssie, Ph., J. Polym. Chem., 1993, 31, 2009. 18. Sakurai, H., Polym. Prepr., 1990, 31(2), 230. 19. Lutsen, L. and Jones, R. G., Polym. Int., in press. 20. Wang, J. S. and Matyjaszeski, K., Macromolecules, 1995, 28, 7901. 21. Jones, R. G. and Holder, S. J. Makromol. Chem., 1997, 198, 3571. 22. Jones, R. G., Ben®eld, R. E., Cragg, R. H. and Swain, A. C., Nature, 1991, 353, 340. 23. Jones, R. G., Ben®eld, R. E., Cragg, R. H., Swain, A. C. and Webb, S. J., Macromolecules, 1993, 26, 4878.