Synthesis and characterization of novel side-chain liquid crystalline polycarbonates, 4 - Synthesis of side-chain liquid crystalline polycarbonates with mesogenic groups having tails of different lengths

Side-chain liquid crystalline polycarbonates with alkoxyphenylbenzoate side groups, having a short spacer and tails ranging from 1 to 8 C-atoms, were synthesized. The polymers were prepared by an organo-zinc catalysed copolymerization of carbon dioxide and mesogenic 4-alkoxyphenyl 4-(2,3-epoxypropoxy)benzoates. Model polycarbonates were prepared by copolymerization of glycidyl phenyl ether (GPE) and carbon dioxide and by terpolymerization of GPE, propylene oxide and CO2. The copolymerizations of LC monomers were carried out at 70 degrees C in an autoclave at 60 atm CO2 pressure in dioxane as the solvent. Catalysts based on diethylzinc and water or diethylzinc and resorcinol were used. High catalyst concentrations (up to 20 mol-% on the basis of diethylzinc with respect to epoxide) and long reaction times (72 h) were found to be necessary for acceptable yields and reasonable molar masses. The resulting polymers had a high polydispersity and the maximum yield was about 30%. A major side product, formed directly from CO2 and epoxide, as well as indirectly by depolymerization, was the corresponding five-membered cyclic carbonate. The polymers on average contained about 30 mol-% of ether units and 70 mol-% of carbonate units due to a weak tendency of the epoxides to undergo homopolymerization. H-1 and C-13 NMR analysis revealed detailed information about the overall ether content and the distribution of the ether and carbonate groups in the chain, as well as the tacticity of the polymers. Surprisingly, the monomer distribution and the tacticity were molecular weight dependent. Some possible explanations for these phenomena are suggested.