Living/controlled Polymerization Conducted in Aqueous Based Systems

Abstract

In the last decade processes known as living/controlled radical polymerizations (L/CRP) have been developed which permit the synthesis of high-value specialty polymers. Currently, the three processes that have demonstrated the most potential are: reverse addition fragmentation chain transfer polymerization (RAFT), atom transfer radical polymerization (ATRP) and stable free radical polymerization (SFRP). While each process has their strengths and weaknesses with regard to specific polymers and architecture, the viability of these systems to industrial scale production all lie in the ability to perform the polymerization in a water based system because of process, environmental and economic advantages. The most effective method of controlling the polymerization of vinyl acetate in bulk has been RAFT. We have developed a miniemulsion RAFT polymerization using the xanthate methyl (ethoxycarbonothioyl)sulfanyl acetate. The miniemulsion is stabilized with 3 wt% sodium lauryl sulfate, initiated with the azo-based water-soluble VA-060. The main focus of this research was adapting ATRP to a miniemulsion system. It was determined that ionic surfactants can be successfully employed in emulsion-based ATRP. The cationic surfactant cetyltrimethylammonium bromide provides excellent stability of the latex over a range of surfactant loadings (allowing the particle size to be easily manipulated), at temperatures up to 90 C, for a wide variety of ATRP formulations. A new method of initiation was developed for reverse ATRP, using the redox pair hydrogen peroxide/ascorbic acid. This nearly eliminated the induction period at the start of the polymerization, increased the polymerization rate 5 fold and, surprisingly, enabled the formation of well-controlled polymers with a number-average molecular (Mn) weight approaching 1 million (typically ATRP is limited to ~200 000). The ability to control the particle size and the number of polymer chains (through the target Mn) over a wide range of values allowed us to determine that ATRP is influenced by compartmentalization effects. The knowledge gained from our work in L/CRP was used to develop the surfactant-free SFRP of styrene. A multi-stage approach was adopted starting from dilute styrene/water solutions to favor the formation of the alkoxyamine and short chain SG1-oligomers (stage one) before the addition of the majority of the styrene (stage two).