Synthesis and Biodegradability of Monoalkylated Caprolactone Copolymers

The authors first synthesized 4-alkylated caprolactones (4ACLs) from 4-alkyl-cyclohexanones via Baeyer-Villiger reaction. The 4ACLs obtained from 4-methyl-, 4-ethyl, and 4-t -butylcyclohexanone were abbreviated as 4MCL, 4ECL, and 4BCL, respectively. Copolymers of L-lactide (L-LA) with 4ACL or e-caprolactone (CL) and homopolymers of the monomers were prepared using tin (II) octylate as catalyst. Thermal and NMR analyses of the polymers indicated poly(L-LA) and poly)CL) to be crystalline, while poly(4ACLs) to be noncrystalline polymers, and L-LA/4ACL copolymers to be crystalline or noncrystalline polymers depending on compositon and to have random sequences. Flexibility (elongation at break) of the copolymers was much improved compared to poly(L-LA), with melting temperature and mechanical properties of poly(L-LA) maintained as much as possible. The biodegradability of the polymers by enzymes (Proteinase K, lipoprotein lipase) and in seawater was examined. Enzymatic degradation of copolymers followed the order, copoly(L-LA/CL)>copoly(L-LA/4MCL)>copoly(L-LA/4ECL)>copoly(L-LA/4BCL), suggesting that the greater steric hindrance of alkyl side chains of 4ACL, the less was degradability. 1H NMR spectra of water-soluble degraded products of L-LA/4MCL copolymer indicated that Proteinase K ultimately degraded the copolymer into hydroxy acids (viz., L-lactic acid and 6-hydroxy-4-methylcaproic acid) of its constitutive units.