J. MATER. CHEM., 1994, 4(9), 1359-1363 Synthesis of High Refractive Index Acrylic Copolymers Thomas P. Davis,*a Michael J. Gallagher,b Millagahamada G. Ranasingheb and Michael D. Zammit? a School of Chemical Engineering and Industrial Chemistry and bSchool of Chemistry, The University of New South Wales, P.O. Box 7, Kensington, New South Wales 2033, Australia A series of brominated carbazole-containing monomers with a polymerizable acrylic or methacrylic functionality were synthesized and characterized. These monomers were polymerized thermally and characterized by differential scanning calorimetry (DSC) and refractive index via Brewster's angle. The monomer 4-(1,3,6,8-tetrabrom0-9-carbazolyl)-l -butyl acrylate (4b) was copolymerized with methyl methacrylate (MJ yielding the reactivity ratios r, =1.08 and r, =0.93; these values were used in the Skeist procedure to predict compositional drift with conversion. The Alfrey-Price Q and e values for (4b) were 0.86 and 0.46,respectively. The copolymers were all colourless and transparent, despite the absence of an azeotrope for this comonomer pair, and their refractive indices were found to lie between those of the constituent homopolymers.Copolymers rich in methyl methacrylate (MMA) were found to be soluble, yet those rich in 4b were found to be intractable. The attainment of high refractive index (nD)in polymeric materials has been an important goal for several years. One commercial application is that of spectacle lens manufacture where a high refractive index allows the design of lighter weight products.The preferred monomer for many years has been ally1 diglycol carbonate (ADC) with a homopolymer refractive index of 1.51, although alternative materials are currently being developed. Other applications include wave- guides, optical fibres and adhesives for optical components where refractive-index matching is of critical importance. Poly(N-vinyl carbazole) (PVK) has a very high refractive index of 1.68 and also possesses excellent thermal stability and useful mechanical properties. However, it is extremely brittle and is relatively intractable.' The aim of the work reported here was to enhance the tractability of PVK whilst retaining the large refractive index value that the carbazole moiety induces.A study by Minns and Gaudiana2 suggested that this might be achieved by maintaining the carbazole chromophore in the side-chain but at a greater distance from the polymer backbone. The insertion of a straight-chain alkylene group between the nitrogen atom and the polymerizable group was proposed to achieve this end. The inclusion of a spacer chain lowers the refractive index, so this is compensated by halogenating the carbazole rings, with bromination or iodination expected to yield the greatest effect. Homopolymers of these monomers were prepared by Minns and Gaudiana' and found to be fairly intractable and insoluble except in 1-chloro-, 1-bromo- and 1-methyl-naphthalene. One of the goals of this work was to improve the tractability of the materials by copolymerization and to provide a range of different copolymers with different refractive indices which would enable the refractive index matching for adhesive applications.The large refractive index between the co-monomers indicated that the ideal situation would be to locate the azeotrope composition for copolymerization, thereby enabling the production of homogeneous and there- fore transparent copolymers. Experimental and Results 300 MHz H1 NMR spectra were recorded on a Bruker AC-300F spectrometer at 300 K, and are reported in ppm from internal tetramethylsilane. Data are reported as follows: chemical shift [multiplicity (s, singlet; d, doublet; q, quartet; qu, quintet; m, multiplet), coupling constant in Hz, integra- tion, interpretation].All solid-state C13 NMR spectra were recorded on a Bruker MSL300 spectrometer at 300 K and a rotational speed of 4-9 kHz in 4 mm zirconia rotors. Thin-layer chromatography (TLC) was carried out on Kieselgel 60 F-254 precoated silica-gel TLC plates obtained from Merck. The plates were run on the eluting solvents stated. All plates were detected by observation under UV light. All DSC thermograms were performed on a DuPont 910 DSC with nitrogen flow rate of 40ml min-I. All samples were performed at a heating rate of 20 "C rnin-l from 20 to 400 "C with a sample size of 10mg. The only distinctive feature of the DSC thermograms was the glass-transition temperature, Tg,which was taken as the temperature at which the midpoint of the heat capacity change at transition was achieved.All reaction temperatures refer to those of the reaction mixture. Reactions requiring an inert atmosphere were carried out under a blanket of argon with a positive pressure. Monomer Synthesis The synthetic methods used were based on those reported by Minns and Gaudiana.2 As several modifications were clevel- oped, the full experimental details are reported. Preparation of 1,3,6,8-Tetrabromocarbazole(2) 1,3,6,8-Tetrabromocarbazolewas prepared from carbazole 1 by the method of Pielich~wski.~ Into a 11 round-bottamed flask were placed carbazole (12 g, 72 mmol) and glacial acetic acid (150ml). The flask was fitted with a condenser and topped with a calcium chloride drying tube leading to a water trap to remove evolved HBr.The mixture was magnetically stirred at room temperature and a solution of bromine (16 ml, 0.31 mol) in acetic acid (200 ml) was added dropwise. The mixture was then heated in an oil bath at 95 "C for 21 h. The reaction was monitored by TLC using a mixture of hexane and ethyl acetate in a ratio of 5 : 1 as the developing solvent. The Rf values for 1 and 2 were 0.51 and 0.71, respectively. After the suspension had been cooled to ambient temperature it was filtered, recrystallized from toluene (ca. 400 ml) and dried to yield 2 (26.6 g, 76% yield); mp 233-235 "C, (lit.2 mp 233-235 "C). 'H NMR (CDC13) 6 7.75 (s, 2H, H2, H7), 8.05 (s, 2H, H4, H'), 11.51 (s, lH, NH). Preparation of 9-(4-Bromobutyl)-1,3,6,8-tetrabromocarbazole (3)The following reagents were placed in a reaction flask fitted with a condenser; product 2 (10 g, 41 mmol), powdered anhy- drous potassium carbonate (16 g, 0.12 mol), anhydrous aceto- nitrile (200 ml) and 1,4-dibromobutane (50 ml, 0.83 mol).The reaction vessel was purged with argon and this atmosphere was maintained throughout the reaction. The reaction mixture was stirred and heated at reflux in an oil bath at 95°C for 19 h. The reaction was monitored as before by a 7 :1 mixture of hexane and ethyl acetate. Rf values for 2 and 3 were 0.49 and 0.66, respectively. Acetonitrile was removed by evaporation and the remaining yellow suspension was subjected to liquid-liquid extraction using dichloromethane and distilled water. The organic com- ponent was dried over sodium sulfate, filtered and concen- trated.The product was precipitated by the addition of methanol, filtered and dried in a desiccator to yield 3 (10.6 g, yield 82%), mp 142-144 "C, (lit.2 mp 142-144 "C). 'H NMR (CDC1,) 6: 1.93 (m, 4H), 3.41 (t, J 7, 2H, CH,Br), 5.12 (t, J 7, 2H, CH,N), 7.76 (d, J 2, 2H), 8.00 (d, J 2, 2H). Preparation of4-( 1,3,6,8-Tetrabromo-9-carbazolyl)-1-butyl Methacrylate (4a) A solution of tetrabutylammonium methacrylate was prepared in a 100ml round-bottomed flask by the addition of meth- acrylic acid (0.6 ml, 7.2 mmol), methanol (60 ml) and 7.2 ml of a 1 moll-' solution of tetrabutylammonium hydroxide in methanol.The pH of the solution was then adjusted by the addition of the ammonium salt until basic conditions pre- vailed. The translucent solution was then acidified by the addition of a few drops of the acid. The solvents were then removed and acetonitrile (30ml) was then added and removed by rotary evaporation. The resulting oil was then dissolved in acetonitrile (60ml), and a solution of 3 (4.1 g, 6.6 mmol) in warm toluene (60ml) was added. The solution was stirred under an argon atmosphere at 45°C for 12 h. TLC was used to monitor this reaction, with the eluting solvent being a 1:1 dichloromethane-hexane mixture. Rfvalues for 3 and 4a were 0.82 and 0.43, respectively. The solvents were removed and the residue was extracted with ether (l00ml) and water (200ml).The organic phase 7 -w& 1 Br 2 8r I mC(CH3)CH2I Bf I * (cp)4Br Br@q*Br Bf (Cy4 Br Br@Q Br 4a 3 OCOCHCHz /I Br Br 4b Scheme 1 Preparation of monomers 4a and 4b J. MATER. C'HEM., 1994, VOL. 4 was then dried with anhydrous sodium sulfate and the solid product was isolated from the ether. Further purification was achieved by reprecipitating the solid from dichloromethane solution by the addition of an excess of methanol. The white crystals were dried to yield 4a (3.15 g, 76%) mp 152-153 "C. (lit.2 mp 152-153 "C). 'H NMR (CDCl,) 6: 1.75 (m, 4H), 1.95 (m, H, CH,), 4.15 (t, J 6 H, 2H, CH20), 5.15 (t, J 8, 2H, CH,N), 5.55 (d, J 2, lH, vinyl), 6.05 (d, J 2, lH, vinyl) 7.8 (d, J 2, 2H), 8.05 (d, J 2, 2H).Preparation of 4-( 1,3,6,8-Tetrabromo-9-carhazolyl)-l-but~l Acrylate (4b) This monomer was prepared by a procedure analogous to that described in the preparation of monomer 4a. The reaction was monitored by TLC with the eluting solvent being a 1:1 dichloromethane-hexane mixture. The R, values for 3 and 4b were 0.82 and 0.39, respectively. Yield 85%, mp 130-132 "C. 'H NMR (CDCl,) 6: 1.25 (t, J 4, lH, -HC=), 1.75 (m, 4H), 4.2 (t, J 4, 2H, CHZO), 5.15 (t, J 8, 2H, CH,N), 5.8 (d, J 8, lH, vinyl), 6.1 (d, J 8, lH, vinyl), 6.4 (d, J 16.5, lH, vinyl), 7.8 (s, 2H), 8.05 (s, 2H). Homo polymerization* The homopolymers were prepared from their monomers via thermal polymerization in the absence of any initiator. The reaction was effected under reduced pressure at a temperature sufficient to melt the monomers, above 160"C, for 1h.The resulting polymers were dissolved in 1-chloronapthalene (15 ml for 1 g polymer) at 100"C. After dissolution had occurred, the solution was further diluted by the addition of toluene (8 ml). The polymer was then isolated from the solution by precipitation in cold dichloromethane. The product was washed with dichloromethane and dried to yield conversions in excess of 75%. Copolymerization Copolymer systems were prepared that encompassed the entire feed ratio system between monomers MMA and 4b. Two situations arose, that of an intractable copolymer in the high 4b feed ratio set, and soluble copolymers in the high MMA feed ratio set.Precipitation Copolymerization A mixture of monomer 4b (l.Og, 0.16mmol), MMA (0.16 mmol), toluene and lauroyl peroxide (0.0025 g) was purged with argon and heated to 70°C. The reaction was allowed to run for 4 h. During this time the solid polymer precipitated from the solution. This was isolated, dried and weighed to yield a conversion of 10%. Solid-state NMR confirmed that the precipitate was a copolymer. Solution Copolymerization Homogenous copolymerization was successfully attempted by maintaining high feed ratios of MMA. The same procedure as detailed above was followed, and no precipitation occurred. The copolymer was precipitated from the reaction mixture using diethyl ether. The composition of the copolymers was determined by 'H NMR.The feed compositions, copolymer compositions and yields are given in Table 1. Also see Fig. 1 for the 'H NMR spectrum of the soluble copolymer (5 :95% feed ratio 4b :MMA) and Fig. 2 for the DSC thermogram. J. MATER. CHEM., 1994, VOL. 4 Table 1 Table of feed composition, copolymer composition, (determined by 'H NMR) and yield; note: (1 ~4b) fi (feed comp.) F, (copoly. comp.) yield (YO) 4.87 3.8 6.17 4.98 3.09 13.8 9.49 5.93 10.4 10 4.78 5.04 10.36 4.67 4.90 13.97 7.74 4.5 14.8 8.17 4.37 15.15 8.77 15 20 11.47 9.4 20.4 13.7 3.7 20.48 8.47 3.48 20.73 11.26 13.7 24.86 13.5 5.0 Refractive Index Measurement The refractive indices of the carbazole homopolymers were beyond the measurement range of the Abbk refractometer.Consequently a Brewster's angle technique was employed. This procedure utilises a vertically polarised He-Ne laser (1= 633 nm). Transparent polymer films were prepared by a hot-melt procedure and placed in the laser beam. By starting with the sample reflecting the laser light back onto the source, then rotating the sample, on a rotatable angle table, and observing when the reflected light passes through a minimum of intensity, the Brewster angle can be read. The refractive index (nD) is then the tangent of that angle. iI 1 --LA 1 0.8~ 227.63"C -110.11 "C 2 Om4'3 g 0.2 CI Qa,x 0.0. -0.2' 304.25 'C -0.4$0 iio iio zio 260 3io-TIOC Fig. 2 DSC thermogram of copolymer from feed ratio 5 :95%, 4b :MMA.Tg= 110"C and showing the two thermal degradation characteristics of the copolymer at 227.6 and 304.2 "C Discussion Homopolymerization The primary difficulty encountered in this work m'as the intractability of the polymers. The carbazole-con taining homopolymers are soluble in the reported solvents, with no additional solvents being found. Unfortunately the corre-sponding carbazole-containing monomers were fount1 to be -7I -3 8.08.0 7.07.0 6.06.0 5.05.0 4.04.0 3.03.0 2.02.0 11.o.o Fig. 1 'H NMR of copolymer from feed ratio 5 :95%, 4b :MMA only sparingly soluble in the naphthalenes; in fact no common solvent could be found for both the monomers and polymers. Copolymerization The reactivity ratios were determined for the copolymerization of 4b (M,) with MMA (M2) using the Fineman-Ross4 and Kelen-Tudos' procedures.An attempt was also made at using the non-linear error-in-all-variables (EVM) approach,6 but this would not converge. This may well be because the data were collected over a limited range because of the difficulty in maintaining a homogenous polymerisation. The average reactivity ratio values obtained were rl = 1.08& 0.005 and r2= 0.93& 0.004. Thus there is no azeotrope for this ideal copolym- erisation. The introduction of the alkyl spacer group removed any reactivity the halogenated carbazole moiety would have induced in the reacting acrylate unit. Thus we see that the reactivity ratios are virtually the same with a slight preference for the synthesized monomer.The acrylate functionality of the synthesized monomer has significantly altered the reactiv- ity of the carbazole compared with N-vinyl carbazole. Also the Alfrey-Price7 Q and e values for 4b are 0.86 and 0.46, respectively. These reactivity ratio values were used with the integrated form of the Skeist equation to predict compositional drift.' Here one calculates C (conversion) when fl changes during this conversion from a value fli to a value flj. The most convenient form of this equation is the integrated form given by Meyer and Lowry.' where a=r2/(1-r2); /3 =rl/( 1-rl); y =(1-rlr2)/[( 1-rl) x ( 1-4; 6 =( 1-r2)/(2 -r1 -r2), A plot of conversion us. comonomer feed composition is shown in Fig.3. Despite the absence of an azeotrope it is clear that compositional drift is minimal across the composi- tional range. The only significant drift occurs at conversions >95%. The compositions of the copolymers were deduced from NMR data. The MMA monomer '-0-CH,' peak (3.6 ppm) was used, along with the aromatic peaks (7.7-8.2ppm), for the synthesized monomer 4b. All the copolymers, from both the precipitation copoly- merization and the homogenous copolymerization yielded colourless transparent films. This is despite the large refractive index differences between the constituent comonomers. This can be attributed to the production of virtually homogenous -J 0.2 0.4 0.6 0.8 1.o f, (feed composition) Fig.3 Comonomer feed composition uersus conversion J. MATER. CHEM., 1994, VOL. 4 copolymers (with respect to composition), as indicated by the compositional drift equation. It is thus possible to synthesize a range of transparent copolymers for refractive index matching applications. The Tgsand refractive indices of the copolymers are given in Table 2. There is not a simple linear relationship between the feed composition and a weighted average of the two homopolymer refractive indices. One possible explanation for this is that there may be a strong solvent effect on the copolymerization, so that the two different copolymerization approaches may have yielded significantly different compositions. There is strong evidence for this from work by Ledwith et al.," who found a 'bootstrap'-type effect for the copolymerisation of NVK with MMA.This offers the possibility of manipulating the copolymer composition (and therefore the refractive index of the copolymers) not by just controlling the monomer feed ratio but also by manipulating medium effects. The goal of improving the tractability of the carbazole by copolymerization was not generally attained. In fact the copolymers with a high carbazole content were more intractable than the homocarbazole polymer. The solubility parameter of a 50 :50 mol% copolymer was calculated to be 10.9 (cal cm3)0.' mol-'. However, no solvents could be found which would dissolve this copolymer, including chloronaph- thalene. The paper from which the monomer synthesis was derived2 also provided a method for the heptahalogenation of the carbazole unit.The heptabrominated monomers were synthesized, but no copolymerization was performed. The increase in bromination had an increase in refractive index for the homopolymer, and it is expected that copolymers would display the same reactivity as the monomer reported here, with an according increase in Tgand refractive index. Finally, there are only three transitions that occur on the DSC traces. The first transition is the glass-transition tempera- ture, which is shown in Fig. 2 to be 110.1 "C.The other two transitions on this thermogram can be explained by attributing those peaks to the thermal degradation of acrylate materials." Those methyl methacrylate oligomers containing unsaturated end groups degrade at ca.255 "C (227.6 "C), whilst acrylate chains containing saturated terminal units degrade at tempera- tures in excess of 300 "C (304.2 "C). The DSC thermograms indicate that the copolymers are amorphous and exhibit the degradation features of PMMA. Conclusions (1) The bromination of the carbazole rings does- compensate for the potential refractive index reduction caused by intac- ing a spacer chain between the nitrogen atom and the polymerizable group. (2) Copolymers of monomer 4b with MMA formed from feed compositions 40-95% 4b were insoluble in all common solvents. Therefore copolymerization with MMA in high 4b feed did not improve the tractability of these carbazole polymers.However, copolymers rich in MMA were found to be soluble in many common solvents and hence commercially interesting. (3) All the copolymers of 4b with MMA were colourless Table 2 Glass-transition temperatures and refractive indices of soluble copolymers fi (copoly. comp. 4b) glass-transition temp. (TJC) n, 0.0309 110.71 1.502 0.0593 125.87 1.520 0.0877 135.78 1.540 0.1126 128.45 1.541 J. MATER. CHEM., 1994, VOL. 4 1363 and transparent with refractive indices between those of the 2 R. A. Minns and R. A. Gaudiana, J.M.S. Pure Appl. Chem, 1992, constituent homopolymers. A29, 19. J. Pielichowski and J. Kyziol, Monatsh. Chem., 1974, 105, 1306. M. Fineman and S. D. Ross, J. Polym. Sci., 1949,5259. T. Kelen and F. Tudos, J. Macromol. Sci. Chem., 1975, A9, 1.We thank Professor M. Gal and Mr. John Tann for their P. M. Rielly and H. Patino-Leal, Technometrics,1981,23 221. assistance with the refractive index determinations, and Mrs T. Alfrey Jr. and C. C. Price, J. Polym. Sci., 1946,2, 101. Hilda Stender, Mrs Than Vo Ngoc and Dr. Jim Hook for I. Skeist,J. Am. Chem. SOC.,1946,68, 1781. processing the NMR samples. V. E. Meyer and G. G. Lowry, J. Polym. Sci., Polym. Chern., 1965, 3,2843. 10 A. Ledwith, G. Galli, E. Chiellini and R. Solaro, Poljm. Bull., 1979, 1,491. 11 P. Cacioli, G. Moad, E. Rizzardo, A. K. Serclis andReferences D. H. Solomon, Polym. Bull., 1984,11,325. 1 D. Bailey, D. Tirrell and 0.Vogel, J. Macromol. Sci. Chem., 1978, A12, 661. Paper 4/011045; Received 23rd February, 1994