Chain-Growth Condensation Polymerization Approach to Synthesis of Well-Defined Polybenzoxazole: Importance of Higher Reactivity of 3-Amino-4-Hydroxybenzoic Acid Ester Compared to 4-Amino-3-Hydroxybenzoic Acid Ester

Application of chain-growth condensation polymerization (CGCP) to obtain well-defined polybenzoxazole (PBO) was examined. CGCP of both phenyl 3-{(2-methoxyethoxy)methoxy (MEM-oxy)}-4-(octylamino)benzoate (1b) (para-substituted monomer) and phenyl 4-MEM-oxy-3-(octylamino)benzoate (3b) (meta-substituted monomer) was examined in the presence of metal disilazide base and phenyl 4-nitro- or methylbenzoate 2 as an initiator. Polymerization of the latter monomer, but not the former, afforded polymer with controlled molecular weight based on the feed ratio of monomer to initiator and with a narrow molecular weight distribution. Accordingly, monomer 3c, in which the octyl group on the amino nitrogen of 3b was replaced with a 4-(octyloxy)benzyl (OOB) group, was polymerized in the presence of lithium 1,1,1,3,3,3-hexamethyldisilazide (LiHMDS), phenyl 4-methylbenzoate (2b), and LiCl in THF at 0 degrees C to yield poly3c with well-defined molecular weight (M-n = 4520-9080) and low polydispersity (M-w/M-n 1.11). Treatment of poly3c with trifluoroacetic acid simultaneously removed the MEM and OOB groups, affording poly(o-hydroxyamide) (poly4) without scission of the amide linkages. Cyclodehydration of poly4 proceeded at 350 degrees C to yield PBO (poly5), which was insoluble in organic solvents and acids. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1730-1736