Polymer International Polym Int 52:92 97 (2003) DOI: /pi.1037

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1 Polymer International Polym Int 52:92 97 (2003) DOI: /pi.1037 Synthesis and characterization of novel rod coil diblock copolymers of poly(methyl methacrylate) and liquid crystalline segments of poly(2,5-bis[(4-methoxyphenyl)oxycarbonyl] styrene) Hai-Liang Zhang, Xiaofang Chen, Xinhua Wan, Qi-Feng Zhou* and EM Woo Department of Polymer Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing , China Abstract: Liquid crystalline diblock copolymers with different molecular weights and low polydispersities were synthesized by atom transfer radical polymerization of methyl methacrylate (MMA) and 2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene (MPCS) monomers. The block architecture (coilconformation of MMA segment and rigid-rod of MPCS segment) of the copolymer was experimentally confirmed by a combination of 1 H nuclear magnetic resonance and gel permeation chromatograph techniques. The liquid crystalline behaviour of the copolymer was studied using differential scanning calorimetry and polarized optical microscope. It was found that the liquid crystalline behaviour was dependent on the number average molecular weight of the rigid segment. Only those copolymers with M n(gpc) of the rigid block above 9200g mol 1 could form liquid crystalline phases higher than the glass transition temperature of the rigid block. The random copolymers MPCS-co-MMA were also synthesized by conventional free radical polymerization. The molar content of MPCS in MPCS-co- MMA had to be higher than 71% to maintain liquid crystalline behaviour. # 2003 Society of Chemical Industry Keywords: atom transfer radical polymerization; diblock copolymer; mesogen-jacketed liquid crystalline polymer; rod coil diblock copolymer INTRODUCTION Self-assembly into a certain order is a common phenomenon in nature. Liquid crystalline polymers (LCP)s 1 and block copolymers (BCP)s 2 are two classes of synthetic materials that can readily undergo self-assembly. By combining both of these components in a single molecular system, the competition between their self-organizing behaviours offers opportunities for simultaneously creating ordered structures over many scales of length in polymer systems. 3 Combination of micro-phase separation and liquid crystallinity may result in new materials with superior and unanticipated properties. Such polymers may also serve as models to provide insight into the ordering of more complicated biological systems in which multiple ordering processes are present. 4 Several reviews have been published so far on this topic. 5,6 By definition, LC block copolymers contain at least one LC segment and may have structures that include rod coil, 7 11 side-group LC (SGLC) coil 12,13 and other block combinations. For mesogen-jacketed liquid crystal polymers (MJLCPs), characterized by mesogenic units laterally attached directly to the main chain, the mesophase is formed by the polymer chain as a whole, not only by the mesogenic units as in the case for most conventional side-chain polymers. In the latter polymers the main chains usually take a randomcoil conformation. In MJLCPs, main chains are expected to take an extended conformation and behave similarly to typical main-chain liquid crystal polymers with significant chain rigidity. Because MJLCPs can be conveniently synthesized by radical polymerization to obtain high molecular weights, and because homopolymers and copolymers with welldefined structure can be obtained by controlled/living radical polymerization, the controlled/living radical polymerization of MJLC monomers offers a convenient way to synthesize liquid crystalline rod-coil block copolymers with the desired length of the blocks. Wan et al 14,15 have first synthesized a series of * Correspondence to: Qi-Feng Zhou, Department of Polymer Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing, People s Republic of China qfzhou@pku.edu.cn Visiting Professor from Department of Chemical Engineering, Natioinal Cheng Kung University, Tainan, Taiwan Contract/grant sponsor: National Natural Science Foundation of China; contract/grant number: (Received 6 December 2001; revised version received 20 March 2002; accepted 8 May 2002) # 2003 Society of Chemical Industry. Polym Int /2003/$

2 Novel rod coil diblock copolymers mesogen-jacketed liquid crystalline polymers (poly{2,5-bis[(4-methoxyphenyl) oxycarbonyl]styrene} (PMPCS)) with narrow polydispersity and a series of novel rod-coil diblock copolymers of polystyrene and PMPCS using the 2,2,6,6-tetramethy-1-piperidinyl-oxy (TEMPO) mediated free radical polymerization. Wu et al 16 have studied the self-assembly of these copolymers, and the results showed that the copolymer chains could self-assemble into a coil shell nano-structure as the dilute solution of PS-b-PMPCS was cooled down. However, only a rather limited number of monomers can be used and high polymerization temperatures are usually required. It is difficult to synthesize new rod-coil copolymers using the TEMPO method. In a previous paper, we reported the synthesis of PMPCS by atom transfer radical polymerization (ATRP). 17 In the present study, we aim at synthesizing a series of rod coil liquid crystal diblock copolymers containing both poly(methyl methacrylate) and PMPCS (PMMA-b- PMPCS) by ATRP processes. The liquid crystalline behaviour of the copolymers was studied using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). EXPERIMENTAL Materials Chlorobenzene (Acros, 99%) was purified by washing with concentrated sulphuric acid to remove thiophenes, followed by washing twice with water, once with 5% sodium carbonate solution, and again with water before being dried with anhydrous calcium chloride and being distilled. Methyl methacrylate (MMA) was purified by passing through a column of activated Al 2 O 3 ( mesh) to remove the inhibitor, stored over CaH 2, and then vacuum distilled before polymerization. Cuprous chloride was prepared from CuCl. 2H 2 O, purified by stirring in glacial acetic acid and washed with methanol, and then dried in vacuum. Sparteine (Aldrich) was distilled under reduced pressure over calcium hydride and stored under nitrogen atmosphere at 4 C in the dark. p-toluenesulfonyl chloride (p-tscl) was purchased from Aldrich and used without further purification. Tetrahydrofuran (THF) was used as received. MPCS was synthesized according to the literature. 18 Preparation of PMMA-Cl macroinitiators PMMA-Cl macroinitiators were prepared by the method described in Reference 19. Typically, MMA (10.00 g, mol), p-tscl (0.190 g, 0.01 mol), CuCl (0.100 g, 0.01 mol) and sparteine (0.469 g, 0.02 mol) were charged into a polymerization tube. After degassing with three freeze-pump-thaw cycles, the tube was sealed under vacuum. The tube was immersed in a thermostated water-bath at 50 C. At a certain time, the tube was withdrawn and cooled to room temperature. The reaction mixture was diluted with THF. After passing the solution through a column with activated Al 2 O 3 to remove the catalyst, and precipitation into n-hexane, PMMA was obtained and dried in a vacuum oven overnight at room temperature. Synthesis of PMMA-b-PMPCS PMMA-b-PMPCS diblock copolymers were synthesized by solution polymerization in chlorobenzene. In a typical run, a polymerization tube charged with g of PMMA-Cl macroinitiator ( mol), 0.268g of MPCS ( mol), 2.0mg of CuCl ( mol), 9.4mg of sparteine ( mol) and 2.760g of chlorobenzene was degassed three times and sealed under high vacuum. After the reaction was carried out at 90 C for 15 h, the tube was cooled to room temperature. The sample was diluted with THF and purified by passing through an activated Al 2 O 3 column. The mixture was poured into a large volume of methanol to precipitate the polymer. The resulting polymer was washed thoroughly with methanol and dried in a vacuum oven overnight at room temperature. Synthesis of random copolymer MPCS-co-MMA The copolymerization of MPCS and MMA was carried out in chlorobenzene solution at 60 C using 0.5%molequiv. AIBN as an initiator for 10h. The reaction mixture was treated as stated above. Measurements The yield of the polymerization was determined gravimetrically. The number average molecular weights M n and polydispersity M w were measured with a Waters 515 gel permeation chromatograph equipped with three Styragel columns (10 3, 10 4 and 10 5 Å) using THF as an eluent (1.0ml min 1 ) at 35 C. The column system was calibrated by a set of low polydispersity polystyrene standards. 1 H NMR spectra were obtained at 25 C on a Bruker ARX400 (400MHz) NMR spectrometer using CDCl 3 as solvent and tetramethylsilane (TMS) as the internal reference. Thermograms were obtained using a Perkin Elmer DSC-7 instrument. Heating and cooling rates were 10 C min 1. Polarized optical microscope observation was performed on a Leitz Laborlux 12 pol microscope with a Leitz 350 hot stage. RESULTS AND DISCUSSION PMMA-b-PMPCS was synthesized by sequential polymerization of MMA and MPCS, as shown in Scheme 1. A series of diblock copolymers PMMA-b-PMPCS were synthesized in this work and the results are summarized in Table 1. The controlled / living free radical polymerization of MMA was carried out in bulk. The conditions used were similar to those described in Reference 19. Two PMMA-Cl macroinitiators with different molecular weights and low polydispersity were synthesized (GPC results: Polym Int 52:92 97 (2003) 93

3 H-L Zhang et al Scheme 1. Synthetic route of PMMA-b-PMPCS. PMMA-Cl (a): M n =8000g mol 1, M w =1.05, PMMA-Cl (b): M n =16800g mol 1, M w =1.04). The resulting macroinitiator PMMA-Cl a was characterized by 1 H NMR spectroscopy (Fig 2(B)). The signals a, b and c at d , , ppm were ascribed to the repeating unit. Signal e is the resonance of the methyl ester protons in the terminal unit adjacent to the chlorine atom. Methylene protons adjacent to the initiating segment are responsible for signal f. Signalh can be assigned to the protons in the benzene ring at the end of the polymer chain. Signalg at around d 2.5 ppm is ascribed to the p-methyl protons, which is partially overlapped by signal d of the terminal methylene protons in the repeating unit. Thus the PMMA obtained was functionalized with an a-p-toluenesulfonyl group and a o-chlorine atom. By comparison of the integral ratio of signals c and h, the molecular weight of PMMA-Cl a was estimated to be 7600g mol 1, similarly to the value 8000g mol 1 measured by GPC. The chain extension was carried out in solution considering the high melting point of MPCS ( C) and the expected low solubility of PMMA-Cl or CuCl/Sp complex in MPCS. In chlorobenzene, MPCS, PMMA-Cl and the CuCl/Sp complex can be dissolved easily, to form a bright yellow transparent Figure 1. GPC traces of macroinitiator PMMA-Cl (a) and its related diblock copolymers with various number average molecular weights. solution. As the polymerization proceeded, the reaction mixture remained homogeneous. GPC results of diblock copolymers (D1, D2, D3, D4, D5 as labelled in Table 1) and prepolymer PMMA-Cl (a) are presented in Fig 1. After the chain extension reaction, an increase in molecular weight from M n =8000g mol 1, M w =1.05 to M n = 41400g mol 1, M w =1.19 was achieved. The polydispersity of the copolymer is a little higher than that of the macroinitiator, but is still lower than the theoretical value of 1.5 for controlled/living free radical polymerization. This is also the case for five other diblock copolymers (D6, D7, D8, D9, D10). Shoulder peaks were observable in the former four curves, especially in D2. The inevitable coupling termination among radicals, more likely when the conversion reaches 90%, most probably accounts for the presented bimodal peaks. It must be mentioned that M n values measured by Table 1. GPC, NMR, DSC data and liquid crystallinity of copolymers Diblock copolymer Macro-initiator Yield (%) M n a 10 4 (g mol 1 ) GPC results M n 10 4 b M n 10 4 c (g mol 1 ) M w (g mol 1 ) d F MPCS Mol fractions of MPCS (%) in the copolymers f MPCS e T g ( C) Liquid crystallinity D1 a No D2 a No D3 a Yes D4 a Yes D5 a Yes D6 b No D7 b No D8 b Yes D9 b Yes D10 b Yes a The theoretical molecular weight of copolymers calculated on the basis of M n of PMMA-Cl by GPC. b The measured M n of the PMMA-block-PMPCS diblock copolymer by GPC. c The M n of the PMPCS block in its copolymer was estimated by GPC. d Calculated by molar fraction of MPCS in the feed. e Molar fraction of MPCS in the copolymer determined by 1 H NMR spectroscopy. 94 Polym Int 52:92 97 (2003)

4 Novel rod coil diblock copolymers Figure 2. 1 H NMR spectra of (A) PMPCS; (B) PMMA-Cl (a); and (C) D4. GPC of the copolymers are much lower than the actual ones, which is ascribed to the difference in hydrodynamic properties of PMPCS and PSt, the latter being used for standards in GPC measurements. A qualitative explanation for this is to be seen in the much larger mass per unit length of PMPCS than that of the calibration standard PSt. The true molar mass should therefore be higher than the measured one. There is, however, an opposing effect that has to do with the stiffening of the polymer chain imposed by the spatially demanding substituents. This leads to an increase in hydrodynamic volume, causing the GPC molar mass to become larger than the actual one. For reported cases in homopolymers, 17 the latter effect obviously does not overcompensate the former one. The resulting copolymers were characterized by 1 H NMR spectroscopy. Figure 2 gives examples of the 1 H NMR spectra of homopolymer PMPCS, macroinitiator PMMA-Cl a and its related diblock copolymer, recorded in CDCl 3, with TMS as an internal reference. It is worth noting that the expected resonance of the main chain protons of PMPCS, signal a at d ppm, is weak in Fig 2A. The shielding effect of the mesogen group, 2,5-bis[(4- methoxyphenyl)oxycarbonyl]benzyl, on the main chain group lowers the resonance intensity of CH 2 CH. In a recent article, Armes et al 20 discussed the similar phenomenon occurring in poly(2-(dimethylamino)ethyl methacrylate-blockmethacrylic acid) (DMAEMA-b-PMAA) zwitterionic copolymer. At elevated temperatures, the hydrophobic DMAEMA block formed the micelle core. The shield effect of the hydrophilic PMAA block decreased the resonance intensities of DMAEMA. The presence of the shield effect from the large side-chain mesogen group on the main chain gives evidence of the mesogen-jacket effect, which we previously postulated for MJLCPs. 21 In Fig 2(C), the signals a, b, and c at d , , 3.60ppm are ascribed to the repeating MMA unit. Characteristic resonance of benzene ring (signals c, d ) and OCH 3 protons (signal b ) in MPCS units are both present in the 1 H NMR spectrum of the copolymer. These data illustrate the presence of both MMA and MPCS blocks in the polymer chain. The chain extension reactions with various molar fractions of MPCS in the feed (the total number of moles is the sum of the moles of MPCS and the moles of MMA in the PMMA-Cl macroinitiator), F MPCS, range from 10% to 60%, and the corresponding mole fractions of MPCS in the diblock copolymers, f MPCS, are listed in Table 1. The composition of the diblock copolymers was determined by means of 1 H NMR spectroscopy from the integral ratio of the signals originated from the PMMA block at 0.85ppm, 1.02ppm, 1.21ppm (signal a, 3H, CH 3 ) and from the PMPCS block at 6.40ppm, 6.70ppm (signal c, 8H, aromatic protons) according to the equation: f MPCS ¼ I A =8 I A =8 þ I M =3 in which I A and I M are the integrals of the signals originated from the PMPCS block at d ppm and the PMMA block at d ppm, respectively. As shown in Table 1, f MPCS is nearly equal to F MPCS. Calculated from f MPCS and M n estimated by NMR analysis of PMMA-Cl (a), one can obtain the number average molecular weight of the corresponding diblock copolymers (D1: , D2: , D3: , D4: , D5: g mol 1 ), which are similar to the theoretical values. Figure 3 shows that the glass transition temperatures (T g ) of neat PMPCS (M n =19400g mol 1, M w =1.19) and PMMA-Cl (a) are 120 C and 112 C, respectively. Only one glass transition is observed in the diblock copolymer. The value T g of all the PMMA-b-PMPCS is much higher than that of the prepolymer PMMA-Cl and a little higher than that of PMPCS with high molecular weight. This phenomenon has not yet been reported elsewhere to our knowledge, and detailed investigations are still underway. The liquid-crystalline behaviour of the copolymers was indicated by the optical textures (Fig 4) formed above T g (given by DSC) as observed by POM. The results show that only those copolymers bearing the rigid block with M n greater than 9200g mol 1 Polym Int 52:92 97 (2003) 95

5 H-L Zhang et al Figure 3. DSC second heating curves of PMPCS, PMMA-Cl (a) and the corresponding diblock copolymers. Figure 4. Representative polarized optical micrograph (200magnification) of the texture of D4 at 200 C. demonstrated liquid crystalline behaviour above the glass transition temperature of the rigid PMPCS. The birefringence did not disappear until the decomposition temperature. Figure 4 shows a representative photomicrograph of the texture of D4 taken at 200 C which is well above T g. It is interesting to note the fact that with a sufficiently high degree of polymerization of the rigid block (M n of the PMPCS block greater than 9200g mol 1 ), the mole fraction of MPCS in the block copolymer can be as low as 19% for the polymer to form a liquid crystalline phase. Wan et al. 22 have studied the relationship between the composition and liquid crystallinity of MPCS-co-St random copolymer. It was found that the molar content of MPCS in this copolymer had to be higher than 79% to maintain liquid crystallinity. In this work, we also synthesized a series of MPCS-co-MMA random copolymers. It also showed that the molar content of MPCS in MPCS-co- MMA has to be higher than 71% to maintain the liquid crystallinity (Table 2). The significant difference in the MPCS content necessary for the two kinds of random copolymers and our resulting block copolymers to exhibit liquid crystallinity would again suggest the block architecture of the targeted copolymers. Wan et al 15 have synthesized diblock copolymers PS-b-PMPCS using TEMPO mediated free radical polymerization. They discovered that two copolymers with M n of the rigid block equal to 5400 and 10800g mol 1, respectively, are thermotropic liquid crystals. In a previous paper, 17 we reported the synthesis of PMPCS with different molecular weight via ATRP. The liquid-crystalline behaviour of those polymers with M n ranging from 3800 to 17400g mol 1 was studied using DSC and POM. It was found that only the polymers whose M n is above 10200g mol 1 could exhibit a liquid crystalline phase. The significant PMPCS length necessary for copolymers to exhibit liquid crystallinity might be related to the difference in the end groups ( Cl/Br for ATRP, TEMPO for NMP) or side reactions. Certainly, further work is needed before any concrete conclusion can be drawn. In contrast with conventional side-chain liquidcrystal polymers with regard to molecular architecture, the mesogenic units of MJLCPs are linked to the main chain without or through only short spacers by lateral attachment. For PMPCS, only one covalent bond is used to connect the main chain and the mesogenic unit. The high population of rigid, bulky side-groups would form a jacket along the polymer chain and thereby force the main chain to take a rod-like conformation. Early studies on lyotropic behaviour, banded textures and solution properties have proved the rod-like nature of MJLCPs. 23,24 Wu et al 16 have proven that these novel rod coil diblock copolymers comprising a polystyrene block and a MJLCPs block have unique supermolecular structure and self-assembly behaviour which, we believe, can be affected by the Molar fraction of MPCS (%) in the copolymers GPC results Table 2. Characteristics of random copolymers MPCS-co-MMA Copolymer Yield (%) Theor Measured by 1 H NMR M n 10 4 (g mol 1 ) M w T g ( C) Liquid crystallinity No No Yes Yes Yes 96 Polym Int 52:92 97 (2003)

6 Novel rod coil diblock copolymers PMPCS length and its liquid crystallinity. Detailed investigations are proceeding. CONCLUSION Liquid crystalline diblock copolymers with low polydispersities were successfully synthesized by atom transfer radical polymerization of methyl methacrylate and 2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene. 1 H NMR and GPC characterizations confirmed the block architecture of the copolymers, and the liquidcrystalline behaviour of the copolymers was investigated using DSC and POM. It was observed that all copolymers showed one T g, but only those copolymers with M n of the rigid block higher than 9200g mol 1 (with a molar content of MPCS in the copolymer of 19%) could form a liquid crystalline phase above the glass transition temperature of the rigid block. The clearing point of the liquid crystalline phase could not be experimentally detected because it was higher than the polymer decomposition temperature. The random copolymers MPCS-co-MMA were also synthesized by conventional free radical polymerization. The molar content of MPCS in MPCS-co-MMA had to be higher than 71% to maintain liquid crystallinity. REFERENCES 1 Weiss RA and Ober CK (Eds) Liquid-Crystalline Polymers, ACS Symp Ser, Vol 435, ACS, Washington, DC (1990). 2 Bates FS, Science Washington 251:898 (1991). 3 Adams J and Gronski W, Makromol Chem, Rapid Commun 10:553 (1989). 4 Gallot B, Progr Polym Sci 21:1035 (1996). 5 Mao G and Ober CK, Acta Polym 48:405 (1997). 6 Walther M and Finkelmann H, Progr Polym Sci 21:951 (1996). 7 Stupp SI, LeBonheur V, Walker K, Li LS, Huggins KE, Keser M and Amstutz A, Science 276:384 (1997). 8 Lee M, Cho BK, Kim H, Yoon JY and Zin WC, J Am Chem Soc 120:9168 (1998). 9 Chen JT, Thomas EL, Ober CK and Mao GP, Science Washington 273:343 (1996). 10 Jenekhe SA and Chen XL, Science Washington 283:372 (1999). 11 Widawski G, Rawiso M and Francois B, Nature London 369:387 (1994). 12 Anthamatten M, Zheng WY and Hammond PT, Macromolecules 32:4838 (1999). 13 Schneider A, Zanna JJ, Yamada M, Finkelmann H and Thomann R, Macromolecules 33:649 (2000). 14 Wan XH, Tu HL and Zhou QF, Chin J Polym Sci 17:189 (1999). 15 Wan XH, Tu YF, Zhang D and Zhou QF, Polym Int 49:243 (2000). 16 Tu YF, Wan XF, Zhang D, Zhou QF and Wu C, J Am Chem Soc 122:10201 (2000). 17 Zhang HL, Yu ZN, Wan XH and Zhou QF, Polymer 43:2357 (2002). 18 Zhang D, Liu YX, Wan XF and Zhou QF, Macromolecules 32:5183 (1999). 19 Yu B and Ruckenstein E, J Polym Sci Part A: Polym Chem 37:4191 (1999). 20 Lowe AB, Billingham NC and Armes SP, Macromolecules 31:5991 (1998). 21 Zhou QF, Wan XH, Zhu XL, Zhang D and Feng XD, in Liquid Crystalline Polymer Systems Technological Advances, ed by Isayev AI, Kyu T and Cheng SZD, ACS Symposium Series 632, American Chemical Society: Washington DC, pp (1996). 22 Wan XH, Tu HL, Tu YF, Zhang D and Zhou QF, Synthesis and characterization of random and rod-coil diblock copolymers consisting of styrene and 2,5-bis[(4-methoxypheny)]oxycarbonyl]-styrene, Chin J Polym Sci (in press). 23 Zhou QF, Wan XF, Zhu XL, Zhang F and Feng XD, Mol Cryst Liq Cryst 231:107 (1993). 24 Wan XF, Zhang F, Wu P, Zhang D, Feng XD and Zhou QF, Macromol Symp 96:207 (1995). Polym Int 52:92 97 (2003) 97