| Free-radical solution polymerization is an important commercial method to produce acrylic resins for paints, adhesives and coatings. In response to increasingly tighter environmental regulations, the polymerization method has been moving toward lower solvent levels and higher temperatures. At high temperatures (140--200°C), many side reactions---such as thermal initiation, chain transfer, beta-scission, and branching---play an important role in controlling polymerization rate and the molecular structure of the polymer, and thus they affect strongly the properties of the final product. Detailed information on the polymer structure, such as branching frequency and type of end groups, is required to deduce the reaction mechanism and obtain kinetic parameter values.; In this work, free-radical polymerization of n-butyl acrylate (nBA) was carried out in a Mettler Toledo RC1e Calorimeter, at temperatures ranging from 120 to 180°C, with or without initiators [2,2'-Azobis (2,4,4-trimethylpentane), and 2,2'-Azobis-isobutane). The initial monomer concentrations were 10, 20 and 40 wt%, with xylene being solvent. Samples were collected during the polymerization to obtain conversion and molecular weight measurements by gravimetry and gel permeation chromatography (GPC), respectively. Selected samples were used to characterize the polymer structural characteristics. Fourier transfer mass spectrometry (FTMS) using an electrospray ionization (ESI) interface identified two predominant initiating species and two terminating species. 13C and 1H NMR corroborated the FTMS/ESI results. Quantification of the structural properties was based upon 13C and 1H NMR spectra.; A comprehensive reaction mechanism that includes the predominant side reactions at high temperature, was proposed. A mathematical model was developed for a batch nBA polymerization reactor. Rate constants were estimated for back-biting, tertiary radical propagation, beta-scission, and terminal double bond reactions. The model can predict satisfactorily conversion, polymer average molecular weights, branching frequency, and number of the terminal double bonds. |
