Effects of Reversible Addition Fragmentation Transfer (RAFT) on Branching in Vinyl Acetate Bulk Polymerization

Abstract

A detailed mathematical model has been formulated for branching due to chain transfer to polymer in reversible addition fragmentation chain transfer (RAFT) polymerization. The traditionally adopted mechanism for RAFT polymerization has been modified by the inclusion of a relaxation process. It is proposed that this step occurs during the period immediately after a chain radical is released from the RAFT transfer intermediate. While undergoing relaxation, the so-called unrelaxed radicals are assumed to be capable of propagation, intermolecular chain transfer to polymer, and bimolecular termination, but not intramolecular chain transfer to polymer which only becomes possible when the radical is relaxed. The use of these assumptions indicates that RAFT can reduce the overall rate of branching without significantly affecting other key measurements such as conversion and average molecular weights. If relaxation is neglected, RAFT would appear not to affect the rate of branching. However, by reducing the average molecular weight, RAFT polymerization would still result in more linear polymers as compared to conventional methods. † This paper is dedicated to Professor Charles Liotta who has always been a good colleague, friend, and mentor. In addition to being a walking encyclopedia of chemical knowledge, Charlie is always good for a laugh (F.J.S.

RAFT Polymerization of Vinyl Esters: Synthesis and Applications

“…A slight decrease in molecular weight was observed as the concentration of RAFT agent was increased, with dispersities ranging from 1.5 to 3.5. In all cases molecular weight in the presence of the RAFT agent was greater than that obtained in the absence of the RAFT agent, which was ascribed to the suppression of chain transfer to polymer or monomer in RAFT polymerizations, as previously proposed by Schork et al [60]. The primary nature of the leaving group and high molecular weights obtained during the polymerization suggest that the RAFT agent did not undergo addition-fragmentation chain transfer, although reversible addition to the C=S double bond may have taken place, causing the observed retardation.…”

“…Lower levels of branching have been observed in controlled radical polymerizations, for example 0.045 mol % in CoMRP at 40 °C [52], although this may simply be a result of the lower temperature. Reduction in the degree of branching has been observed in controlled radical polymerizations of butyl acrylate, attributed either to the narrower chain length distribution obtained in controlled polymerizations (fewer reactive short chain radicals) [59] or to reduced conformational freedom of the chain end immediately after activation of the dormant polymer [60]. While these effects should apply equally to vinyl esters, no experimental evidence for reduction of branching in RAFT polymerizations of vinyl esters has been published.…”

Xanthate-Mediated Copolymerization of Vinyl Monomers for Amphiphilic and Double-Hydrophilic Block Copolymers with Poly(ethylene glycol)

“…22 Chain branching is less likely to occur to a significant extent in the presence of a CTA. 23 This translates experimentally to the absence of the copolymer signal (Figure 8b), where X1′ was added to the polymerization mixture. These experiments confirmed that the EG 75 -OH mostly remained unmodified but did not inhibit either xanthate-mediated or conventional free-radical polymerization of NVP.…”

Poly(vinyl ester) Block Copolymers Synthesized by Reversible Addition−Fragmentation Chain Transfer Polymerizations

“…1 H NMR spectra were recorded on both Varian Unity Inova 500 and Bruker AC+ 300 spectrometers and were referenced relative to the residual protiated solvent peaks in the samples. 13 C NMR spectra were obtained at 125 MHz using the Varian Unity Inova 500 spectrometer. Elemental analysis was performed at Columbia Analytics Laboratory (Tuscon, AZ).…”

“…8,13 Access to new degradable vinyl ester homopolymers and block copolymers with tunable properties requires the development and optimization of controlled polymerization methodologies for this class of monomers. Advances in this arena may enable the generation of new, degradable pressure-sensitive adhesives and thermoplastic elastomers, based on design principles discovered in the development of traditional styrenic and diene derived thermoplastic elastomers.…”