Synthesis of copolyesters based on substituted and non-substituted lactones towards the control of their crystallinity and their potential effect on hydrolytic degradation in the design of soft medical devices

Abstract

A series of copolymers based on ε-caprolactone (ε-CL) in combination with lactone monomers substituted with alkyl groups (4 and 6 carbon atoms), specifically δ-decalactone (δ-DL), ε-decalactone (ε-DL) and δ-dodecalactone (δ-DD), as well as a copolymer using two substituted lactone monomers with alkyl groups (ε-DL and δ-DD) were synthesized in different molar ratios. The objective of the synthesis of these copolymers was to evaluate the effects of branching in the polymer backbone on the crystallinity and the thermal properties of the synthesized materials. All copolymers were obtained via ring-opening polymerization with high conversion values for both comonomers using neodymium isopropoxide (Nd(i-Pr)₃) as the initiator, and their compositions were determined by ¹H NMR and ¹³C NMR. The molar masses (M_n and M_w) and distributions were obtained by GPC measurements. Such measurements showed that a majority of the copolymers exhibited dispersities (Ɖ) in the range of 1.2–1.6 and M_n in the range of 15–40 kDa. First- and second-order transitions such as melting, crystallization and glass transition, as well as the crystallization degree (melting enthalpy), were determined by DSC analysis. Copolymers based on ε-CL developed interesting behaviors, wherein the copolymers with higher percentages of this monomer exhibited semicrystalline behavior, while the copolymers with a higher percentage of the comonomers ε-DL, δ-DL or δ-DD showed amorphous behavior. In contrast, the copolymers synthesized using both monomers from the alkyl group-substituted lactone developed fully amorphous features, regardless of their composition. These changes in the crystalline features of the synthesized copolymers suggest that the content of short branchings on the copolymer backbone will significantly modify their rates of hydrolytic degradation and their potential use in the development of different soft medical devices.