Investigation of Aromatic Polyester Synthesis by the Chain-Growth Polycondensation Method

Synthesis of aromatic polyesters by chain-growth polycondensation of activated 4-hydroxybenzoic acid derivatives (1) was investigated. Model reactions of ester formation between active compounds (6 and 7) and phenolic nucleophile (8) gave the best result when 3-arylcarbonyl-2-benzothiazolone derivatives were used with tertiary amines in CH2Cl2. Therefore, we designed and synthesized 3-(4-hydroxy-3-octylbenzoyl)-2-benzothiazolone (1h) as a monomer. The M n− and M w/M n−conversion curves of the polymerization of 1h with initiator suggested that transesterification between polymer and monomer occurred in the late stage of polymerization. Investigation of the effects of bases and initiators revealed that the transesterification at the initiator end was minimized by using 3-(4-benzoyloxybenzoyl)-2-benzothiazolone (7h) and diisopropylethylamine as an initiator and a base, respectively. When the ratio of initiator 7h to monomer 1h was 20 mol %, the polycondensation proceeded in a chain-growth manner to give aromatic polyester having a controlled molecular weight and a low polydispersity. However, the polymerization of 1h with 10 mol % or less of 7h could not be controlled and gave step-growth polymer as well as chain-growth polymer. The model reaction demonstrated that transesterification would occur at the polymer main chain even by use of weak tertiary amines as a base.