Synthesis of an (AB)4-type Star Block Copolymer of L-lactide and Cyclohexene Oxide from a Tetra-Arm Telechelic Macrophotoinitiator

Abstract

A multi-step reaction process was applied for the synthesis of a novel and well-defined starshaped telechelic macrophotoinitiator with four poly(L-lactide) (PLLA) arms connected to photoinitiating benzoin groups at the chain ends (PLLA-PI)4. To achieve this, 2,2-bis(hydroxymethyl)-1,3-propanediol was used as the initiator which constitutes the core of the star-shaped polymeric scaffold. Benzoin photoreactive end groups of the telechelic (PLLA-PI)4, capable of entering into further polymerization, allowed its use as a polymeric photoinitiator in photoinduced free radical promoted cationic polymerization of cyclohexene oxide (CHO) monomer at =350 nm to produce an (AB)4-type star-shaped block copolymer composed of both esteric L-lactide and etheric cyclohexene oxide chains on each arm, (PLLA-PCHO)4. Structural analysis and characterization of all intermediate and final compounds were done by a series of analytical and spectral methods. Molecular weights of the prepared polymers up to telechelic macrophotoinitiator (PLLA-PI)4 were determined based on 1 H-NMR (Mn H-NMR), GPC (Mn exp) analyses and theoretical calculations (Mn theo) and were found to be in good agreement with each other. Thermal properties and degradations of the prepared polymers were examined by thermogravimetric and differential thermal analyses (TG/DTA). The melting temperature for (PLLA-PCHO)4 was found higher compared to the other homo-type polymers in the literature. The thermogravimetric (TG) analyses showed that incorporation of the thermally stable PCHO block into the structure via photopolymerization improved its stability by increasing its decomposition temperature compared to the prepolymers.