Investigation into branching in acrylate polymerization: advanced characterization and kinetics

Abstract

Branching has been investigated in poly(acrylic acid) (PAA) and poly(2-ethylhexyl acrylate) (P2EHA) synthesized by conventional radical polymerization with and without chain transfer agent (CTA) at different temperatures and initial monomer concentrations. The average number of branches per monomer unit (i.e., degree of branching) was quantified by solutionstate 13C NMR spectroscopy (solution-state and melt-state for PAA and P2EHA respectively). The heterogeneity of branching in PAA (dispersity of the electrophoretic mobility distributions) was measured by capillary electrophoresis in the critical conditions (CE-CC). For PAA, the degree of branching (DB) increases with the reaction temperature due to a rise in the frequency of reactions leading to branches, while the heterogeneity of branching remains steady. DB is lower in polymer synthesized with CTA. This decrease is due to either the CTA quenching the mid-chain radicals or a reduction of the rate of chain transfer to polymer relative to (chain-end) propagation. No influence of initial monomer concentration on DB and on the heterogeneity of branching was observed. Results were slightly different for the P2EHA. Electrospray ionization-mass spectrometry analyses have revealed the occurrence of β-scission and of termination products, which was not the case for PAA. The DB of P2EHA is also reduced by the presence of a CTA but its increase with temperature is not observed anymore when the β-scission becomes important. Backbiting rate coefficients of PAA and P2EHA were evaluated from the DB values. Further characterization of PAA by CE-CC was carried out. The mechanism of separation has been studied in depth. The effects of ionic strength and the addition of silver on the selectivity of separation were investigated. The optimal separation of branched poly(sodium acrylate) (PNaA) was obtained at high sodium borate buffer concentration, as the different peaks (corresponding to different branching densities) in a sample are completely resolved in electropherograms and no adsorption of polymer on the capillary was observed. No influence of the addition of silver on the selectivity of separation was observed. The potential complexation between PNaA and buffer and its effect on the analysis were investigated. The improvement of separation of PNaA by branching has allowed new findings on the dispersity of electrophoretic mobility distribution that were not previously observable. Finally, alternative methods to size exclusion chromatography (SEC) were tested on PAA to obtain size-based characterization: diffusion coefficients and hydrodynamic radii of PAAs can be obtained by Taylor dispersion analysis and the Mn can be obtained by quantification of end groups with 1H solution-state NMR spectroscopy.