Real-Time 1H and 31P NMR spectroscopy of the copolymerization of cyclic phosphoesters and trimethylene carbonate reveals transesterification from gradient to random copolymers

Abstract

Copolymerization is the general approach to combine properties of different polymers in a single material. We prepared the first trimethylene carbonate (TMC) phosphoester (PE) copolymers by statistical copolymerization using either stannous octanoate or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the respective catalyst. Variation of comonomer feed, reaction temperature and solvent, resulted in a library of copolymers with molar masses of up to 83,000 g/mol and moderate molar mass dispersities (<1.8). The monomer consumptions during the copolymerizations were followed by real-time 1H and 31P NMR spectroscopy in solution and in the bulk. As ring-opening copolymerization of cyclic esters often leads to gradient copolymers, also TMC and PE copolymerizations resulted in gradient copolymers, i.e. P(TMC-grad-PE)s, with different gradient strength depending on the copolymerization conditions. If a suitable transesterification catalyst like stannous octanoate was used, randomization of the gradient copolymer occurred. The 31P NMR shift of incorporated phosphoester units was used as a probe to monitor the randomization of the gradient copolymers in real-time by 31P NMR spectroscopy. The P-comonomer acts as an ideal NMR probe for in situ monitoring the changes of the copolymers’ microstructure during the transesterification by following the evolution of diads and triads. This paper focusses on the randomization procedure and gives detailed information about the copolymer structure and the kinetics of the copolymerization. The data give the possibility to understand randomization processes on a molecular scale and to fine-tune properties of polycarbonate-polyphosphoester copolymers in future applications.