J. MATER. CHEM., 1991, 1(4), 611-619 61 1 Molecular Engineering of Liquid-crystalline Polymers by Living Polymerization. Part 13t.-Synthesis and Living Cationic Polymerization of (S)-( -)-2-Methylbutyl 4’4a-Vinyloxy)-alkoxybiphenyl-4-carboxylate with Undecanyl and Hexyl Alkyl Groups Virgil Percec,* Qiang Zheng and Myongsoo Lee Department of Macromolecular Science, Case Western Reserve University, Cleveland, OH 44 106, USA The synthesis and living cationic polymerization of (S)-(-)-2-methylbutyl 4-(I I-vinyloxy)undecyloxybiphenyl-4-carboxylate (13-11) and (S)-(-)-2-methylbutyl 4’-(6-vinyloxy)hexyloxybiphenyl-4-carboxylate (13-6) are described. Polymers with degrees of polymerization from 4 to 26 and polydispersities G1.10 were synthesized and characterized by differential scanning calorimetry (DSC) and thermal optical polarized microscopy.When determined from first heating and cooling DSC scans, poly(l3-11) exhibits monotropic s,, si and s, (unidentified) mesophases over the entire range of molecular weights. When determined from second and subsequent heating and first and subsequent cooling scans, poly(l3-11) with degrees of polymerization <10 exhibit enantiotropic s,, st and s, mesophases, and crystallization on the heating scan, while those with higher degrees of polymerization exhibit enantiotropic s,, si and s, mesophases only. Regardless of the thermal history of the sample, poly(l3-6) exhibits an enantiotropic s, phase, and the polymers with degrees of polymerization > 12 exhibit an enantiotropic s, mesophase as well.Keywords: Living cationic polymerization; Chirality; Smectic C phase; Vinyl ether Liquid-crystalline polymers exhibiting chiral mesophases, i.e. Experimental cholesteric’ and chiral smectic C (s,*),~are of both theoretical Materialsand technological interest. Liquid crystals exhibiting chiral smectic A (s:) mesophases were only recently discovered3 and 4-Hydroxybiphenyl (97%), dimethylsulphate (99Oh +), HBr to our knowledge, polymers exhibiting s: mesophases have (48% in H20), 9-borabicyclo[3.3.1]nonane (9-BBN dimer, not yet been reported. Side-chain liquid-crystalline polymers crystalline, 98%), 1 1-bromoundecan- 1-01 (98%), butyl vinyl exhibiting s: mesophases were reported from several different ether (98%), tetrabutylammonium hydrogen sulphate (TBAH) However, there is very little understanding (all from Aldrich), 1,lo-phenanthroline (anhydrous, 99”/0), laboratorie~.~,~-’~ of the molecular design of side-chain liquid-crystalline poly- palladium(I1) diacetate (both from Lancaster Synthesis), acetyl mers displaying s,*mesophases, and of the influence of various chloride (990/) and S-(-)-2-methylbutan-l-o1(95%) (both architectural parameters of these polymers on their dynam- from Fluka) were used as received.Methylene chloride (Fisher) The ability to synthesize side-chain liquid-crystalline was purified by being washed with concentrated sulphuric ic~.~*~-” polymers by a living polymerization technique would provide acid several times until the acid layer remained colourless, a very useful preparative tool which gives access to polymers then with water, dried (MgS04), refluxed over calcium hydride with narrow molecular weight distribution and controllable and freshly distilled under nitrogen before each use.Dimethyl molecular weight. sulphide (anhydrous, 99%, Aldich) was refluxed over 9-BBN Group-transfer polymerization of mesogenic methacrylates and then distilled under nitrogen. Trifluoromethanesulphonic can be used to prepare side-chain liquid-crystalline polymers acid (triflic acid, 98%, Aldrich) was distilled under vacuum. with narrow molecular weight distribution and controllable molecular weights. “3’’ However, this polymerization does Techniquesnot tolerate functional groups sensitive to the nucleophilic growing species and can be used mostly for the polymerization ‘H NMR (200 MHz) spectra were recorded on a Varian XL- of methacrylates.Cationic polymerization can be used to 200 spectrometer. Infrared (IR) spectra were recorded on a polymerize, under living conditions, mesogenic vinyl ethers Perkin-Elmer 1320 infrared spectrophotometer. Relative mol- containing a large variety of functional especially ecular weights of polymers were measured by gel permeation with the initiating system CF3S03H-S(CH3)2.20 chromatography (GPC) with a Perkin-Elmer Series 10 LC The goal of this paper is to describe the synthesis and living instrument equipped with LC-100 column oven and a Nelson cationic polymerization of (S)-(-)-2-methylbutyl 4-(1 1 -vinyl- Analytical 900 series integrator data station.A set of two oxy)undecyloxylbiphenyl-4-carboxylate(13-11)and (S)-(-)-2- Perkin-Elmer PL gel columns of 5x102 and 104A with methylbutyl 4’-(6-vinyloxy)hexyloxybiphenyl-4-carboxylate CHC1, as solvent (1 cm3 min- I) were used. The measurements (13-6). The mesomorphic behaviour of poly(l3-11) and were made at 40 “C using the UV detector. Polystyrene poly(13-6) will be discussed as a function of molecular weight. standards were used for the calibration plot. High-perform- ance liquid chromatography (HPLC) experiments were per- formed with the same instrument. Absolute number average molecular weights were determined by ‘H NMR spectroscopy t Part 12. V. Percec, C. S. Wang and M. Lee, Polym.Bull., in the by analysing the chain ends of the resulting polymers. A press. Perkin-Elmer DSC-4 differential scanning calorimeter 612 equipped with a TADS data station was used to determine the thermal transitions, which were reported as the maxima and minima of their endothermic and exothermic peaks. In all cases, heating and cooling rates were 20 "C min-' unless specified. Glass-transition temperatures (Tg) were read at the middle of the change in the heat capacity. First heating scans differed from second and subsequent heating scans. However, second and subsequent heating scans were identical. A Carl- Zeiss optical polarized microscope (magnification 100x) equipped with a Mettler FP 82 hot stage and a Mettler FP 800 central processor was used to observe the thermal tran- sition and to analyse the anisotropic textures.Synthesis of Monomers Monomers were synthesized as outlined in Scheme 1. 4-Methoxybiphenyl (2) 4-Hydroxybiphenyl (127.8 g, 0.75 mol) was dissolved in aque- ous sodium hydroxide (2.250 dm3, 1.5 mol dm-3) at 55 "C. Dimethyl sulphate (189 g, 1.5 mol) was added slowly so that the temperature did not exceed 60 "C. The temperature was then raised to 70 "C for 30 min and the reaction mixture was allowed to cool to room temperature. The resulting white 1 2 2 - Br(CH2),-OH + n-C,HgCH=CHz Cl?ICI CHp=CH -O(CH,)@r 7 -n 8 9 -n (n=6.11) 4 CHJCOZH 13-11 Scheme 1 Synthesis of (S)-(-)-2-methylbutyl 4'-(1 1 -vinyloxy)undecyl-oxybiphenyl-4-carboxylate (13-11) and (S)-(-)-2-methylbutyl 4-(6-vinyloxy)hexyloxybiphenyl-4-carboxylate(13-6) J.MATER. CHEM., 1991, VOL. 1 precipitate was filtered and recrystallized from 95% ethanol to yield white crystals (68.5 g, 50%); purity, 99.8% (HPLC); m.p., 87.0-87.5 "C (lit.,21" 89.0 "C; lit.216 80.5 "C); dH (CDCl,, TMS) 3.81 (s, 3 H, OCH3), 6.92 (d, 2 H, 3-H, 5-H), 7.22 (d, 2 H, 2-H, 6-H), 7.35 (m, 2 H, 2'-H, 6'-H), 7.47 (m, 3 H, 3-H, 4-H, 5-H). 4-Acetyl-4-Methoxybiphenyl (3) 4-Methoxybiphenyl (2) (56 g, 0.303 mol) was dissolved in 500 cm3 dry methylene chloride in a 2000 cm3 three-necked round-bottom flask, equipped with dropping funnel and con- denser. The solution was cooled to 0-2 "C. Anhydrous alu- minium chloride (48.4 g, 0.36 mol) was added quickly to give a green solution.Acetyl chloride (28.5g, 0.363 mol) was then added over 20-30 min, and the reaction mixture was refluxed for 2 h. Ice-cooled, concentrated HC1 (300 cm3) was added to the cooled mixture to decompose the yellow-green complex. Then water (200 cm3) was added and the mixture was heated to remove most of the methylene chloride. A light yellow- brown solid separated from the reaction mixture. The resulting solid was washed twice with diethyl ether (200 cm3 each time) to remove the isomeric 3-acetyl-4'-methoxybiphenyl, which is soluble in diethyl ether. The solid was then recrystallized from isopropyl alcohol (24 cm3 g-') to yield colourless flakes (64 g, 93%); purity, 99.5% (HPLC); m.p., 153-154 "C (lit.,21b 153-154 "C); 8H (CDCl,, TMS) 2.61 (s, 3 H, COCH,), 3.88 (s, 3 H, OCH3), 7.00 (m, 2 H, 3'-H, 5'-H), 7.58 (d, 2 H, 2'-H, 6'-H), 7.64 (d, 2 H, 2-H, 6-H), 7.98 (d, 2 H, 3-H, 5-H).4'-Methoxybiphenyl-4-carboxyIicAcid (4) A sodium hypobromite solution, prepared at 0-5 "Cby adding bromine (34cm3, 0.659mol) very slowly into a solution of sodium hydroxide (95.2 g, 2.38 mol) in water (450 cm3), was added slowly to a solution of 4-acetyl-4'-methoxybiphenyl (3) (34 g, 0.15 mol) in 1100 cm3 of 1,4-dioxane over 1 h. The temperature of the reaction mixture was allowed to rise to 35-40 "C. After being stirred for an additional 15 min, the sparingly soluble sodium salt solution was treated with enough sodium hydrogen sulphite (52.5 g, 0.505 mol) to destroy the excess of hypobromite.The hot solution was then acidified to yield 4'-methoxybiphenyl-4-carboxylicacid (29 g, 84.5%) of sufficient purity to be used in the reduction step; m.p., 253- 254 "C (lit.,22 258 "C); aH (C2H6]acetone, TMS) 3.86 (s, 3 H, OCH,), 7.05 (m, 2 H, 3'-H, 5'-H), 7.68 (d, 2 H, 2'-H, 6'-H), 7.75 (d, 2 H, 2-H, 6-H), 8.08 (d, 2 H, 3-H, 5-H). 4'-Hydroxybiphenyl-4-carboxylicAcid (10) Compound 4 (29 g, 0.127 mol) was dissolved in 1160 cm3 of boiling acetic acid. A solution of 48% hydrobromic acid (230 cm3, 2.03 mol) was added and the reaction mixture was heated at reflux temperature for 12-13 h. The reaction mixture was then poured into water (3 dm3), and allowed to cool to room temperature. The resulting precipitate was separated, washed with water and dried to yield the crude solid product (27 g, 99%0).The product was purified by dissolving in meth- anol, and treating the solution with activated carbon to give a colourless solution, which was poured into water to yield a white powder (25 g, 91%); m.p., 293-294 "C (lit.,23 293- 294 "C); dH (['H,]acetone, TMS) 3.03 (s, 1 H, OH), 6.98 (d, 2 H, 3'-H, 5'-H), 7.58 (d, 2 H, 2'-H, 6'-H), 7.71 (d, 2 H, 2-H, 6-H), 8.07 (d, 2 H, 3-H, 5-H); v,,, (Nujol, KBr plate)/cm-' 3300-2400 (OH of COOH), 1670 (C=O). S-(-)-2-Methylbutyl Toluene-p-sulphonate (6) S-(-)-2-Methylbutan-l-o1 (44g, 0.5 mol) was added slowly to a solution of toluene-p-sulphonyl chloride (190.6 g, 1 mol) in dry pyridine (300 cm3) at 0 0C.24The reaction mixture was stirred overnight at room temperature.The resulting solution J. MATER. CHEM., 1991, VOL. 1 was poured into water (300 cm3) and extracted with diethyl ether. The ether layer was dried (MgS04) and the diethyl ether was removed on a rotary evaporator to yield a colourless oil (120 g, 99"/), purity, 99% (HPLC); dH (CDC13, TMS) 0.85 [m, 6 H, CH(CH3)CH2CH3], 1.02- 1.39 (m, 2 H, CHCH2CH3), 1.61 [m, 1 H, CH2CH(CH3)CH2], 2.40 (s, 3 H, ArCH3), 3.83 (m, 2 H OCH,), 7.22 (d, 2 H, 3-H, 5-H), 7.71 (d, 2 H, 2-H, 6-H). 1,lO-Phenanthrolinepalladium(II) Diacetate To a stirred solution of palladium(I1) diacetate (1.783 g, 7.94 mmol) in dry benzene (60 cm3) was added a solution of 1,lO-phenanthroline (1.5 g, 8.32 mmol) in benzene (70 cm3) over 30min.The mixture was stirred for 4h and the yellow precipitate was filtered and washed with benzene and light petroleum to yield a yellow solid (3.2g, 99"/), m.p., 233- 234 "C (lit.,25, 234 "C); 8H (CDC13, TMS) 2.16 [s, 6 H, OCO(CH,)J, 7.77 (m, 2 H, 4-H, 7-H), 7.99 (s, 2 H, 5-H, 6-H), 8.56-8.67 (m, 4 H, 2-H, 9-H). 11-Brornoundecyl Vinyl Ether (9-11)19' A solution of 11-bromoundecan-1-01 (8 g, 0.23 mol), 1,lO- phenanthrolinepalladium(1I) diacetate (0.41 8 g, 1.03 mmol), dry chloroform (20cm3) and butyl vinyl ether (85cm3) was heated at reflux overnight (12-14 h). The resulting light-yellow solution, obtained after gravity filtration, was distilled on a rotary evaporator to remove the excess butyl vinyl ether and chloroform.The remaining yellow oil was purified by column chromatography (silica gel, CH2C12 as eluent) to yield a light- yellow oil (8.1 g, 92%); 6, (CDC13, TMS) 1.27-1.81 (m, 18 H, OCH2[CH2I9), 3.38 (t, 2 H, CH2Br), 3.64 (t, 2 H, =CHOCH2-), 3.96 (d, 1 H, =CH2, trans), 4.14 (d, 1 H, =CH2, cis), 6.39-9.51 (m, 1 H,=CH-0-). 6-Brornohexyl Vinyl Ether (9-6) Compound 9-6 was synthesized by the same procedure as the one used for in the synthesis of 9-11.6-Bromohexan-l-o1(12g, 66.3 mmol) and 1,lo-phenanthrolinepalladium(rr) diacetate (0.872 g, 2.16 mmol), dry chloroform (20 cm3) and butyl vinyl ether (177 cm3) were heated at reflux overnight. The resulting product was purified to produce a light-yellow oil (12.2g, 89Yo). 6H (CDCl3, TMS) 1.42-1.84 (m, 8 H, OCH2-[CH2]4), 3.38 (t, 2 H, CH,Br), 3.65 (t, 2 H, =CHOCH2), 3.93 (d, 1 H, =CH2, trans), 4.14 (d, 1 H, =CH2, cis), 6.38-9.49 (m, 1 H,=CHO-).Potassium 4'-Hydroxybiphenyl-4-carboxylate (1 1) Compound 10 (20 g, 0.293 mol) was dissolved in methanol (500 cm3). The solution was titrated with a solution of KOH (1 mol dm-3) in CH30H using phenolphthalein as indicator. The solution was then poured into diethyl ether (1.5 dm3) to give a white precipitate. The precipitate was filtered and dried to yield the product (20.2g, 96%). The formation of the potassium carboxylate was confirmed by IR. After complete reaction, the carbonyl peak of the carboxylic acid at 1670 cm- was shifted down to 1585 cm-' owing to the more single-bond character of the carbonyl group of the potassium carboxylate.(S)-(-)-2-Methylbutyl4'-Hydroxybiphenyl-4-carboxylate(12) To a solution of potassium salt 11 (20.2 g, 0.08 mol) and TBAH (4 g) in dry DMSO (300 cm3) was added (S)-2-methyl- butyl toluene-p-sulphonate (20.1 g, 0.0826 mol) (6).After being stirred at 60 "C for 20 h, the clear light-yellow solution was poured into water (1.2 dm3). The resulting precipitate was filtered off. The crude product was dissolved in methanol (400cm3) to give a light-brown solution, which was treated with activated carbon to produce a colourless solution. The white solid, obtained after the solvent was distilled, was 613 recrystallized from a mixture of methanol and water (1.25 :1.0, v/v) to yield crystals (22.1 g, 98.2%), purity, 99% (HPLC); m.p., 115.8 "C (DSC); 6, (CDCl,, TMS): 1.02 [m, 6 H, CH(CH3)-CH2CH3], 1.33-1.49 (m, 2 H, CHCH,CH,), 1.90 (m, 1 H, -CH-), 4.24 (m, 2 H,-OCH2-), 5.18 (s, 1 H, -OH), 6.98 (d, 2 H, 3'-H, 5'-H), 7.56 (d, 2 H, 2'-H, 6-H), 7.63 (d, 2 H, 2-H, 6-H), 8.1 1 (d, 2 H, 3-H, 5-H).(S)-(-)-2-MethyEbutyl 4'-( 1 1 -Vinyloxy)undecyloxybiphenyl-4-carboxylate (13-11) To a mixture of potassium carbonate (5.9 g, 0.0378 mol) and acetone (90 cm3) was added ester 12 (4.3 g, 0.015 mol). After being stirred for 2 h at 60 "C, the mixture turned yellow. Then, 11-bromoundecyl vinyl ether (4.0 g, 0.014 mol) and dry DMSO (5cm3) were added and the reaction mixture was stirred for 20 h at 60 "C. The reaction mixture was poured into water (250cm3) to give a white precipitate, which was extracted with chloroform.The chloroform solution was dried (MgS04) and the solvent was removed in a rotary evaporator. The resulting solid was recrystallized from methanol to yield the monomer 13-11 (5.2g, 75%), purity, 97% (HPLC). The monomer was further purified by column chromatography (silica gel, CH2C12 as eluent) to give 4.3 g (62%); purity, 99.9% (HPLC); m.p., 48.0 "C (DSC); 6H (CDCl,, TMS) 0.99 (m, 6 H, CH(CH3)CH2CH3), 1.29 (m, 16 H, OCH2CH2[CH,],, and CHCH2CH3), 1.63 (m, 2H, CH2CH,0Ar), 1.78 (m, 3 H,=CHOCH2CH2- and -CH2CH(CH3)CH2-), 3.64 (t, 2 H,=CHOCH2-), 3.98 (m, 3 H, -CH20Ar and =CH2, trans), 4.13-4.17 (m, 3 H,-C02CH2-and =CH2 cis), 6.40-6.51 (m, 1 H,=CHO-), 6.96 (d, 2 H, 3'-H, 5'-H), 7.54 (d, 2 H, 2'-H, 6'-H), 7.60 (d, 2 H, 2-H, 6-H), 8.07 (d, 2 H, 3-H, 5-H).(S)-(-)-2-Methylbutyl 4-(6-Vinyloxy)hexyloxybipheny~-4-carboxylate (13-6) Compound 13-6was synthesized by the same procedure as the one used for the preparation of 13-11. Starting with alcohol 12 (5 g, 0.0176 mol), bromoether 9-6 (3.646 g, 0.0176 mol) and potassium carbonate (6.9 g), ester 13-6 (5.5 g, 76%) was obtained, purity, 99% (HPLC); m.p., 38.5 "C (DSC); 6H (CDC13, TMS) 0.97 [m, 6 H,-CH(CH3)CH2CH3], 1.29 (m, 2H, CHCH,CH,), 1.46 (m, 4H,= CHOCH2CH2[CH2I2CH2), 1.65 (m, 2 H,-CH2CH20Ar-), 1.78 [m, 3 H,-OCH2CH2- and -Ch2CH(CH3)CH2-1, 3.65 (t, 2 H,=CHOCH,-), 3.96 (m, 3 H, =CH2, trans, and -CH20Ar), 4.14-4.18 (m, 3 H, =CH2, cis, and-C02CH2-), 6.40-6.51 (m, 1 H, =CHO-), 6.96 (d, 2 H, 3'-H, 5'-H), 7.50 (d, 2 H, 2'-H, 6'-H), 7.58 (d, 2 H, 2-H, 6-H), 8.06 (d, 2 H, 3-H, 5-H). Cationic Polymerizations Polymerizations were carried out in glass flasks equipped with Teflon stopcocks and rubber septa under argon atmos- phere at 0 "C for 1 h.All glassware was dried overnight at 180 "C. The monomer was further dried under vacuum over- night in the polymerization flask. Then the flask was filled with argon, cooled to 0°C and the methylene chloride, dimethyl sulphide and triflic acid were added viu a syringe. The monomer concentration was ca. 10 wt.% of the solvent volume and the dimethyl sulphide concentration was 20 times larger than that of the initiator. The polymer molecular weight was controlled by the monomer :initiator ([MIo :[II0) ratio. After the polymerization was quenched with ammoniacal methanol, the reaction mixture was precipitated into meth- anol.The filtered polymers were dried, and precipitated from methylene chloride solution into methanol several times until GPC traces showed no trace amounts of unreacted monomer. The polymerization results are summarized in Tables 1 and 2. Table 1 Cationic polymerization of (S)-(-)-2-methylbutyl 4-(11-vinyloxy)undecyloxybiphenyl-4-carboxylate(13-11)"and characterization of the resulting polymersb ~ ~~~~~~~ ~~~~~~~~ ~ DP phase-transition temperatures/ "C and corresponding enthalpy changes/kJ mol -polymer [MI,: [I], yield (%) M, x lop3 M,/M, GPC NMR heating cooling ~~~ ~ 4 88 1.8 1.07 4 5 g -1.4 k 52.8 (19.8) SA 92.1 (6.23) i i 84.4 (6.10) SA 42.9 (0.121) sC* g -3.1 SX 5.8 (1.05) SX 13.4 (-0.71) sC* 26.4 (1.30) k -0.11 (3.76) SX -8.1 g 32.7 (-1.17y k 47.5 (10.2) sA91.5 (6.14) i 7 93 3.0 1.10 6 6 g 4.2 k 56.8 (18.1) SA 104.8 (6.14) i i 97.5 (5.85) sA44.9 (0.142) sc* g 1.2 SX 15.5 (2.72) sC* 31.7 (-) k 37.4 (-0.627)" k 48.4 (2.59) SX -1.7 g (4.01) SA 104.5 (5.89) i 10 90 4.7 1.08 10 10 g 6.9 k 56.2 (16.8) SA 108.7 (6.02) i i 102.5 (5.81) SA 46.3 (0.159) sC* g 5.2 SX 19.4 (2.63) sC* 35.2 (-) k 38.2 (-0.418)" k 48.3 9.5 (2.72) sx 1.8 g (1.42) SA 108.5 (5.77) i 14 90 6.7 1.09 14 12 g 10.1 k 56.8 (15.4) sA 114.4 (5.56) i i 108.4 (5.43) SA 47.1 (0.180) sC* g 6.8 SX 20.8 (2.13) sC* 51.1 (0.142) SA 114.3 (5.56) i 12.7 (2.01) sx 5.1 g 18 93 8.2 1.08 17 18 g 11.1 k 57.2 (12.9) sA 118.6 (5.68) i i 112.3 (5.35) SA 48.4 (0.159) sC* g 8.2 SX 24.9 (2.09) sC* 52.2 (0.159) SA 118.1 (5.56) i 15.6 (2.05) sx 7.9 g 23 91 11.5 1.07 24 24 g 10.5 k 60.4 (15.0) sA 119.8 (5.56) i i 113.8 (5.31) SA 48.2 (0.159) sC* g 9.5 SX 24.0 (2.09) sC* 52.3 (0.121) sA 119.7 (5.27) i 15.7 (2.09) sx 7.5 g 30 92 12.1 1.10 26 26 g 11.5 k 62.2 (15.5) sA 122.8 (5.60) i i 115.4 (5.14) sA 48.8 (0.142) sC* g 10.1 SX 26.1 (2.34) sC* 53.3 (0.142) SA 121.8 (5.23) 16.7 (2.59) sx 7.9 g ~~ ~~~~~ ~~~~~ ~ Polymerization temperature, 0 "C; polymerization solvent, methylene chloride; [MI, =0.208; [(CH,),S], :[I], =20; polymerization time, 1 h.Data on first line are from first heating and on second line are from second heating scan.Crystallization during heating. Table 2 Cationic polymerization of (S)-(-)-2-methylbutyl 4-(6-vinyloxy)hexyloxylbiphenyl-4-carboxylate"(13-6) and characterization of the resulting polymersb ~~ ~ ~ DP phase-transition temperatures/ "C and corresponding enthalpy changes/kJ mol polymer [MI,: [I], yield (YO) M,XIO-3 M,/M, GPC NMR heating cooling 5 70 2.9 1.04 6 6 g 2.4 SA 90.8 (4.39) i i 83.5 (4.51) SA -4.1 g g 1.3 SA 90.4 (4.47) i 8 78 3.9 1.09 10 9 g 8.2 SA 95.9 (4.60) i i 87.7 (4.43) SA -0.5 g g 7.5 SA 95.5 (4.56) i 12 87 5.0 1.06 12 15 g 16.9 sA 103.2 (4.51) i i 95.1 (4.60) SA 8.8 g g 14.2 sA 102.6 (4.56) i 18 86 6.9 1.07 17 18 g 20.1 sX 40.1 (0.920) sA 106.9 (4.47) i i 99.8 (4.51) SA 29.9 (0.794) SX 13.5 g 19.3 SX 38.6 (0.418) sA 106.6 (4.68) i 23 91 7.5 1.10 19 23 g 25.9 SX 45.6 (1.630) sA 108.5 (4.47) i i 101.2 (4.60) sA 38.3 (0.501) sx 18.2 g 24.1 SX 46.7 (0.209) sA 108.3 (4.56) i 30 85 10.0 1.09 25 31 g 29.3 sX 49.7 (1.05) sA 111.7 (4.72) i i 102.4 (4.72) SA 48.1 (0.418) SX 21.4 g 27.9 sX 58.0 (0.418) sA 110.3 (4.47) i Table 1, except [M],=0.244.As for Table 1. J. MATER. CHEM., 1991, VOL. 1 Although polymer yields are lower than expected owing to losses during the purification process, conversions are almost quantitative in all cases. Results and Discussion In the area of low molar mass liquid crystals, there are some empirical rules that can be used to design compounds dis- playing chiral smectic C (s;) mesophases.26 Such rules are not available for the design of side-chain liquid-crystalline poly- mers exhibiting s; phase~.~-'~A classic example comes from our laboratory where repeated attempts to synthesize side- chain liquid-crystalline polymers exhibiting s; mesophases led to polymers exhibiting an sA me~ophase.~~.~~Therefore, we decided to perform a series of systematic investigations aimed to derive some empirical rules useful for the molecular engin- eering of side-chain liquid-crystalline polymers exhibiting s; mesophases.In some previous publications, we have reported the synthesis and characterization of some polymers contain- ing various polymer backbones and spacer length, and side groups derived from 4-[S-(-)-2-methyl- l-butyoxyl-4'- (hydroxy)-a-methylstilbene.'5~27' The influence of polymer backbones, spacer length and mesogenic group length on the ability to generate a sz mesophase was discussed.Here we will first discuss the synthesis and living cationic polymerization of (S)-(-)-2-methylbutyl 4'-( 1 1 -vinyloxy)un- decyloxybiphenyl-4-carboxylate13-11 and (S)-(-)-2-methyl- 9hmh me 615 butyl 4'-(6-vinyloxy)hexyloxybiphenyl-4-carboxylate13-6. In the second part, we will discuss their mesomorphic behaviour. Scheme 1 outlines the synthesis of vinyl ethers 13-11 and 13-6. The cationic polymerization of both monomers was initiated with the system CF3S03H-SMe2 and was performed at 0 "C in CH2C12.'9*20 The polymerization mechanism is described in Scheme 2.It is essential that the monomers used in these polymerization experiments are completely free of pro- tonic impurities. In order to achieve this degree of purity, after the purification by conventional techniques, the monomer is purified by passing through a chromatographic column con- taining silica gel and using methylene chloride as eluent. Poly- merization results are summarized in Tables 1 and 2. In both tables, conversions are less than quantitative owing to polymer losses during the purification process. However, at the end of the polymerization, HPLC and GPC traces showed that the monomer conversion was ca. 100%. Although the molecular weights determined by GPC and reported in Tables 1 and 2 are relative to polystyrene standards, they demonstrate that the ratio of [MIo: [II0 provides a very good control of the polymer molecular weight.In addition, all polydispersities of poly(l3-11) and poly(13-6) are <1.10. Absolute number aver- age molecular weights and degrees of polymerization were determined by 200 MHz 'H NMR spectroscopy. A representa-tive 'H NMR spectrum together with its protonic assignments is presented in Fig. 1. Degrees of polymerization were deter- mined by measuring the ratio of the doublet at 6 6.97 us. the j CH3CH (CH2CH), -2 CH2CHOCH3 I I I 0 0 0 CH2 CH2 CH2 i CH2 CH2 CH2 n (CH2)n -4 (CH2)n -4 (CH2)n -4 o CH2 CH2 CH2 I CH2 CH2 CH2 gI I I 0 0 08\ \ \ c=o c=o c=o I I I ? ? ? CH2 CH2 CH2 f CH -CH~*CH-CH~ *CH-CH~ CH2 CH2 CH2\ k CH3 CH3 CH3kp r H-8 7 6 5 4 6 (PPm) O*P (1.29) m,n (156) I flk9179\N V (1.18) 3 2 1 Fig.1 200 MHz 'H NMR spectrum of poly(l3-11) with theoretical DP=4 J. MATER. CHEM., 1991, VOL. 1 12 - I0 s=10- Q COpR' (R'=-CHp HCHpCH3) F CH3 8\ LOpW COpR' QQQ QQQ COpR' CO2W COpR' COpR' cop COpR' Scheme 2 Cationic polymerization of 13-11 and 13-6 broad triplet at 6 4.64.The degrees of polymerization deter- mined by NMR are summarized in Tables 1 and 2 and, unex- pectedly, especially for the case of poly(l3-11), they agree quite well with the results obtained by GPC. Fig. 2 presents the plots of M, determined by GPC and NMR and MJM, us. t U0 Q, 26 --.s3 u 8-Q m' 6-X 2= 4-O! I I I I I I 0 5 10 15 20 25 30 : 5 [Mlo/[Ilo I -A-A-A-A-A-A-0 5 10 15 20 25 30 35 [MI0 /[I10 Fig. 2 The dependence of the number average molecular weight (M,) determined by GPC (0)and by NMR (m)and of the polydispersity (M,/M,) of (a)poly(l3-11) and (b) poly(13-6) on the [MIo: [IlO ratio (A) II'i'2vSC*sA -20 10 40 70 100 130 -20 10 40 70 100 130 -20 10 40 70 100 130 T/"C T/"C TIT Fig. 3 DSC traces displayed during (a) the first heating scan (b), the first cooling scan and (c) the second heating scan by poly(l3-11) with different degrees of polymerization (DP) determined by GPC.Values of DP are printed on the left-hand side of each DSC scan J. MATER. CHEM., 1991, VOL. 1 [M]o/[I]o obtained for the polymerizations of 13-11 [Fig.2(a)] and 13-6 [Fig. 2(b)]. These plots demonstrate that within this range of molecular weights both monomers poly- merize through a living polymerization mechanism. As expected, the plots of absolute and relative M,us. [M]o/[I]o provide different slopes [Fig. 2(b)]. Fig. 3 presents the DSC traces of poly(l3-11) with various degrees of polymerization. As we can observe from this figure, the DSC curves of the first heating scan [Fig. 3(a)] differ from those of the second heating scan [Fig. 3(c)]. However, second and subsequent heating scans exhibit identical DSC traces. First and subsequent cooling scans also exhibit identical DSC traces [Fig. 3(b)]. On the first heating scan, all polymers exhibit a glass-transition temperature followed by a crystalline phase which melts into an S, mesophase.The s,-isotropic transition temperature has a stronger dependence on molecu- lar weight than that of the melting transition temperature [Fig. 3(a),Table 13. On the cooling DSC scans, all poly (13-11)exhibit an isotropic-s, followed by sA-sz and sE-sx phase transitions [Fig. 3(b)]. The nature of the sx phase was not identified. On the second heating scan, the phase behaviour of poly(l3-11) is strongly dependent on the molecular weight of the polymer [Fig. 3(c)]. Poly(l3-11) with degrees of polymeriz- ation <10 undergo the transition from S, to sz phase followed by crystallization through endothermic and exothermic peaks. The crystalline phase melts into an S, phase [Fig.3(c), Table 11. Poly(l3-11) with DP=4 has an additional exother- mic peak on the second heating scan [Fig. 3(c)]. This peak is due to the completion of the sx phase formation. The endo- therm due to sx-sz and the exotherm of the sz-k phase tran- sitions overlap [Fig. 3(c)]. Therefore, poly(l3-11) with degrees of polymerization <10 exhibit very narrow enantiotropic sx, sz and S, mesophases and a crystallization on the heating scan when their data are collected from the second DSC scans. Poly(l3-11) with degrees of polymerization >10 do not crys- tallize on the heating scan [Fig. 3(c)]. Subsequently, their DSC traces show very distinct transitions from sx to s: and from sz to S, mesophases. Therefore, when the thermal transitions of these polymers are collected from second and subsequent heating and first and subsequent cooling DSC traces, they exhibit a quite broad enantiotropic s,?j phase [Fig.3(b), (c), Table I]. However, if these polymers are annealed within their sz phase below the melting transition temperatures determined from the first heating scan [Fig. 3(a), (c)], they crystallize. This means that under equilibrium condition, poly(l3-11) exhibit an enantiotropic S, and a monotropic sc* mesophase. The difference between the behaviour of low and high molecular weight poly(l3-11) is determined by the difference between the crystallization ability of these two series of polymers. The low molecular weight polymers exhibit a high rate of crystalliz- ation and therefore, can crystallize on the second heating scan, while the higher molecular weight polymers have a much lower rate of crystallization and, subsequently, they do not crystallize on the second heating scan.This represents a classic example of the 'polymer effect' which shows how the kinetically con- trolled crystallization process affects the stability of a thermo- dynamically controlled mesophase. Similar examples of this behaviour are available both from 0ur'~7" and from other laboratories. l8 The thermal transition temperatures collected from Fig. 3 are summarized in Table 1 and plotted in Fig. 4(a) (data from the first heating scan), Fig. 4(b) (data from the first cooling scan) and Fig. 4(c) (data from the second heating scan). Plate 1 presents representative optical polarized micrographs of the sAand sc* phases exhibited by poly(l3-11).The sA meso-phase displays a focal conic-texture (Plate la). The S,-S; phase transition is accompanied by the formation of equidistant lines on the focal conic te~ture.~~~'~~'~*'~ The DSC traces of the first and second heating and of the 617 130, 704 k30i 10--04--0-0-/0-0 glassy/O-1 0 I I I 1 I 1301 (b) / SA 70-9i2 50-A-A-A-A-A-/ 30-SC' degree of polymerization 130 I / o~~-~-o-o-5-0-glassy 0 5 10 15 20 25 30 degree of polymerization Fig. 4 The dependence of phase-transition temperatures on the degree of polymerization determined by GPC of poly(l3-11): (a)data from the first heating scan, 0,q;.H, T(k-s,); 0,T(sA-i);(b) data from the first cooling scan, H, T(1-sA); A, T(s~-s$);+, T(s$-s,); 0, q;(c) data from the second heating scan, 0,q; 0, T(sx-sE); H, T(k-SA); A, T(S,*-SA);0,T(SA-i) J.MATER. CHEM., 1991, VOL. 1 t t 0 0 U a Q, 25.0DSA Jb -10 20 50 80 110 -10 20 50 80 110 -10 20 50 80 110 T/"C T/"C T/"C Fig. 5 DSC traces displayed during (a) the first heating scan, (b) the first cooling scan and (c) the second heating scan by poly(13-6) with different degrees of polymerization determined by GPC (DP). DP is printed on the top of each DSC scan first cooling scans of poly(13-6) with various molecular weights 120 I I are presented in Fig. 5(a)-(c). Over the entire range of molecu- lar weights, these polymers exhibit an enantiotropic sAphase.Polymers with degrees of polymerization 17 exhibit also an enantiotropic sx phase. The only difference between the first and second or subsequent heating scans consists of the fact that the enthalpy changes of the s,-sA phase-transition tem- peratures from the second heating and first cooling scans are 0 lower than those from the first heating scans (Table 2). This is i2 60-&---*--A due to the close proximity of the sx phase to the glass tran- 40-fA' sx sition of poly(13-6). Owing to this proximity, the sx phase is kinetically controlled. A similar behaviour was observed with 20-other polymer^.'^*'^ The phase-transition temperatures col- lected from the first and second heating scans are plotted in 0-Fig.6(a),while those from the first cooling scan in Fig. 6(b). As we can observe from both Fig. 5(a) and 5(b),the slopes of -20 ! I the dependences of sX-sA and sA-sX transition temperatures I I I I I 0 5 10 15 20 25 30 us. the degree of polymerization are steeper than the slope of degree of polymerization the dependence of < us. degrees of polymerization. Conse- quently, poly(13-6) with degrees of polymerization <17 exhibit120 a virtual sx mesophase. These results can be explained by using the thermodynamic schemes published previously.28 A general discussion on the polymer backbone effects on the phase behaviour of side-chain liquid-crystalline polymers will be published elsewhere.29 Some brief discussion on the same topic has been already p~blished.~'*~' Additional experiments on the synthesis and characterization of side-chain liquid-crystal- line polymers and copolymers with narrow molecular weight 40g60] distribution, well defined molecular weights, and exhibiting A /A-sz mesophases are in progress.The availability of these well defined polymers will allow the elucidation of the influence of various parameters such as molecular weight and polydispers- ity on the dynamics of the sz .mesophases exhibited by side- chain liquid-crystalline polymers. Financial support from the Office of Naval Research, DARPA -204 II I I 1 I 0 5 10 15 20 25 30 and an unrestricted Hercules Incorporated Aid-to-Education degree of polymerization grant is gratefully acknowledged.Fig. 6 The dependence of phase-transition temperatures on the degree Referencesof polymerization of poly(13-6) (determined by GPC): (a) data from the first heating (fh)and the second heating scan (sh), 0, (fh); A, 1 V. P. Shibaev and Ya. S. Freidzon, in Side Chain Liquid Crystal T(sX-SA) (fh);0,T(SA-i) (fh); 0, Tg(sh); A, T(s~-s,)(sh); .,T(SA-i) Polymers, ed. C. B. McArdle, Chapman and Hall, New York, (sh);(b)data from the first cooling scan, .,T(i-sA); A, T(s,-s,); 0,T, 1989, p. 260. J. MATER. CHEM., 1991, VOL. 1 Plate 1 Representative optical polarized micrographs (100 x) of (a) the sA mesophase displayed by poly(l3-11) (DP= 10) at 90 "C on the cooling scan; (b)the sc* mesophase displayed by poly(l3-11) (DP= 10) at 31 "C on the cooling scan V.Percec et al. (Facing p. 618) J. 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