Synthesis and characterization of biodegradable homopolymers and block copolymers based on 1,5-dioxepan-2-one

Homopolymers of 1,5-dioxepan-2-one (DXO) and block copolymers of DXO and epsilon-caprolactone (epsilon-CL) have been synthesized with aluminum isopropoxide as an initiator in toluene and tetrahydrofuran (THF). The homopolymerization is first order with respect to both monomer and initiator, and the end-group analysis agrees with a coordination insertion mechanism based on the acyl-oxygen cleavage of the DXO ring. Living poly(epsilon-caprolactone) (PCL) and poly(1,5-dioxepan-2-one) (PDXO) chains are very efficient macroinitiators for the polymerization of DXO and epsilon-CL, respectively, with formation of block copolymers of a narrow molecular weight distribution. Size-exclusion chromatography (SEC) and C-13 NMR confirm the blocky structure of the copolymers, in agreement with DSC that show a melting endotherm for the PCL block and two glass transitions characteristic of the amorphous phases of PDXO and PCL. Because of the crystallinity of the PCL block (T(m) = 60-degrees-C) and the low glass transition temperature of the amorphous PDXO block (T(g) = -39-degrees-C), poly(epsilon-CL-b-DXO-b-epsilon-CL) triblocks have the potential of thermoplastic elastomers. Block copolymers of epsilon-CL and DXO are also sensitive to hydrolysis which makes them possible candidates for biomedical applications. Initiation of the DXO polymerization with functional diethylaluminum alkoxides is also discussed.