High-Temperature Homopolymerization of Ethyl Acrylate and n-Butyl Acrylate: Polymer Characterization

Abstract

This paper presents a thorough molecular characterization of ethyl acrylate (EA) and n-butyl acrylate (nBA) homopolymers made at high temperature (140−180 °C) to high conversions (50−90%) in xylene isomers without the use of added thermal initiator. Electrospray ionization/Fourier transform mass spectrometry (ESI/FTMS) analysis shows four dominant chain types formed during high-temperature polymerization. Chains initiated by β-scission radicals and by xylol radicals that grow and eventually terminate to form terminally saturated and unsaturated chains. These chain structures suggest the underlying secondary mechanisms in high-temperature acrylate polymerization include β-scission (disproportionation) of the carbon-centered tertiary radical that is most likely formed via intramolecular chain transfer. Additionally, chain transfer to solvent, xylene in this case, also plays an important mechanistic role. Results from 1D NMR using 13C, 1H, and distortionless enhancement polarization transfer (DEPT) corroborate the ESI/FTMS results and additionally predict (i) 2 branch points per chain on average for EA homopolymer with a number-average molecular weight of 4000 and (ii) 1.25 branch points per chain on average for nBA homopolymer with a number-average molecular weight of 3300. The presence of branch points indicates propagation of the midchain tertiary radical does occur to significant extent under the conditions of the experiments. Neither the ESI/FTMS nor NMR results suggest a mechanistic route by which the acrylates initiate polymerization without added thermal initiator.