Living Radical Polymerization by Reversible Addition−Fragmentation Chain Transfer in Ionically Stabilized Miniemulsions

Abstract

In theory, a miniemulsion should be an ideal environment for “living” radical polymerization via the reversible addition−fragmentation chain transfer process (RAFT). Compartmentalization minimizes radical−radical termination events, and droplet nucleation eliminates the mass transfer limitation found in conventional “living” emulsion polymerizations. In practice, however, several phenomena were observed when using the RAFT technique, indicating a deviation from this idealized theory when the miniemulsion was stabilized by an ionic surfactant. Inefficient droplet nucleation, a steadily rising polydispersity over the reaction, and the appearance of a separate organic phase after initiation were all indications of particle instability. A distinct difference between standard polymerizations and those that involve highly active RAFT agents is the fact that in RAFT polymerization there is a time interval early in the reaction where oligomers dominate the molecular weight distribution. The presence of large quantities of oligomers is postulated to be the culprit behind the destabilization observed through a detrimental interaction with the ionic surfactant of the miniemulsion. Conductivity measurements verified the increase of free surfactant in the aqueous phase over the course of reaction. Despite this, results showed clear indication of “living” character with a linear evolution of molecular weight until roughly 40% monomer conversion, after which the molecular weight showed contributions from initiator-derived chains.

Living Radical Polymerization by the RAFT Process

“…[9,68,82,102,[185][186][187][188][189][190][191][192][193][194][195] We showed that RAFT in miniemulsion can be used to produce polystyrene of narrow polydispersity in a batch process. [9] Some retardation is observed with dithiobenzoate RAFT agents.…”

Controlled/Living Radical Polymerization in Aqueous Miniemulsion

“…RAFT miniemulsion polymerization has been recently reviewed by Zetterlund and co-workers [10] and by Cunningham [7]. In addition, numerous papers have appeared reporting on the RAFT miniemulsion of a large range of monomers such as: styrene [126][127][128][129][130][131][132][133][134] [ [135][136][137][138][139][140][141][142][143][144],4-acetoxystyrene [145],MMA [135,138,142,[146][147][148],EHMA [129,136], nBMA [129,136,147,[149][150][151], nBA [127,128,141,152,153], MAA [153], fluorinated alkyl methacrylates [149,151], acrylamide (inverse RAFT miniemulsion) [125], VAc [154,…”

RAFT mediated polysaccharide copolymers

“…However, premature chain termination does occur when radicals react in a way that prevents further propagation. 16 The most common termination reactions are known as coupling (combination) and disproportionation. Another mechanism of termination is chain transfer by hydrogen abstraction from an initiator, monomer, polymer, or solvent molecule.…”

“…5 Many of the techniques, such as the catalytic polymerization of ethylene and propylene, require specialised equipment and are extremely sensitive to impurities. 16,17 This can increase production costs substantially. One option which is more feasible is the free radical polymerization technique.…”