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 starshapedtelechelic macrophotoinitiator with four poly(L-lactide) (PLLA) arms connected to photoinitiatingbenzoin groups at the chain ends $(PLLA-PI)4$. To achieve this, 2,2-bis(hydroxymethyl)-1,3-propanediol wasused as the initiator which constitutes the core of the star-shaped polymeric scaffold. Benzoin photoreactiveend groups of the telechelic (PLLA-PI)4, capable of entering into further polymerization, allowed its use asa polymeric photoinitiator in photoinduced free radical promoted cationic polymerization of cyclohexeneoxide (CHO) monomer at =350 nm to produce an (AB)4-type star-shaped block copolymer composed ofboth esteric L-lactide and etheric cyclohexene oxide chains on each arm, $(PLLA-PCHO)4$. Structural analysisand characterization of all intermediate and final compounds were done by a series of analytical and spectralmethods. Molecular weights of the prepared polymers up to telechelic macrophotoinitiator (PLLA-PI)4 weredetermined based on $1H-NMR$ ($Mn$ H-NMR), GPC ($Mn$ exp) analyses and theoretical calculations (Mn theo) andwere found to be in good agreement with each other. Thermal properties and degradations of the preparedpolymers were examined by thermogravimetric and differential thermal analyses (TG/DTA). The meltingtemperature for $(PLLA-PCHO)4$ was found higher compared to the other homo-type polymers in theliterature. The thermogravimetric (TG) analyses showed that incorporation of the thermally stable PCHOblock into the structure via photopolymerization improved its stability by increasing its decompositiontemperature compared to the prepolymers.