Synthesis and characterization of hydrolytically degradable poly(N-vinylcaprolactam) copolymers with in-chain ester groups

Poly(N-vinylcaprolactam) (PNVCL) is attracting increasing interest as a polymer for biomedical application. However, it is not biodegradable, which is an important drawback for such applications. The present paper describes the synthesis and characterization of novel thermosensitive PNVCL copolymers able to hydrolytically degrade, which were prepared by the RAFT/MADIX copolymerization of NVCL and 5,6-benzo-2-methylene-1,3-dioxepane (BMDO). The RAFT/MADIX polymerization process displayed a moderate degree of control under the experimental conditions employed as proven by GPC measurements. The formation of the NVCL-BMDO copolymers was demonstrated by H-1 NMR analyses which showed the presence of the in-chain ester groups resulted from the ring-opening polymerization of BMDO. The BMDO content of the copolymers was much lower than in the feed, indicating a higher reactivity of NVCL, which was confirmed through the estimation of the monomer reactivity ratios by applying the non-linear least squares method to fit the experimental results to the Lowry-Meyer integrated form of the Mayo-Lewis copolymer composition equation. The glass transition temperature of the copolymers diminished with the BMDO unit concentration within the chain. The phase transition temperature of the copolymers in 0.5-wt% aqueous solution decreased with the BMDO content as proven by transmittance measurements, in agreement with the increasing hydrophobic character. The degradability of the copolymers was demonstrated by the hydrolysis of the in-chain ester groups in 1-N KOH solution. The degraded polymer displayed a higher phase transition temperature than the original polymer, as expected. The results described within this paper may find applications for the synthesis of new biomaterials.