Chromatographic examination of the chemical composition and sequence distribution of copolymers from ethyl and benzyl diazoacetate

Abstract

Polymers, especially copolymers, are highly complex samples and, therefore, require various setups for their thorough characterization. In this work, one- and two-dimensional chromatographic approaches were applied to characterize two homopolymers and two dissimilar copolymers prepared by rhodium-mediated carbene polymerization using ethyl and benzyl diazoacetate as the monomers: poly-(EA-ran-BnA) and poly((EA)(b)-(BnA-ran-EA)(b)). Different strategies of synthesis suggested that poly-(EA-ran-BnA) was an approximately 1:1 random copolymer and that poly((EA)(b)-(BnA-ran-EA)(b)) was a block-type copolymer of which the (BnA-ran-EA)(b) block is dominated by BnA. The hydrodynamic volume of the investigated polymers turned out to be comparable, i.e. size exclusion chromatography (SEC) did not separate them. Temperature-gradient interaction chromatography was not feasible and one-dimensional (solvent-)gradient-elution liquid chromatography (GELC) suffered from coelution problems. Pyrolysis-gas-chromatography (Py-GC-MS) experiments allowed determining the monomer ratio. SEC with UV and refractive-index detection indicated the presence of molecular-weight-dependent chemical heterogeneity for the poly((EA)(b)-(BnA-ran-EA)(b)) copolymer. This finding was confirmed with off-line SEC//Py-GC-MS. Only a comprehensive two-dimensional GELC×SEC setup enabled the separation of high-molecular-weight material of the less-retained homopolymer from low-molecular-weight copolymeric material. In this way it was possible to detect some high-molecular-weight homopolymeric material in one of the copolymers.