Ab initio Emulsion Polymerization by RAFT (Reversible Addition–Fragmentation Chain Transfer) through the Addition of Cyclodextrins

Abstract

Dedicated to Professor Giambattista Consiglio on the occasion of his 65th birthday A novel process to produce homo-and copolymers by RAFT polymerization in emulsion is presented. It is known that RAFT-controlled radical polymerization can be conducted in emulsion polymerization without disturbing the radical segregation characteristic of this process, thus leading to polymerization rates identical to those encountered in the corresponding nonliving systems. However, RAFT agents are often characterized by very low water solubility and, therefore, they diffuse very slowly from the monomer droplets, where they are initially solubilized, to the reaction loci, i.e., the polymer particles. Accordingly, when used in emulsion polymerization, they are practically excluded from the reaction. In this work, we show that cyclodextrins, well-known for their ability to form water-soluble complexes with hydrophobic molecules, facilitate the transport across the H 2 O phase of the RAFT agent to the polymer particles. Accordingly, chains grow through the entire process in a controlled way. This leads to the production of low-polydispersity polymers with well-defined structure and end functionalities as well as to the possibility of synthesizing block copolymers by a radical mechanism.1. Introduction. -Despite the recent progresses in living radical polymerization (LRP) by RAFT (reversible addition-fragmentation chain transfer), especially in terms of new RAFT agents and produced polymers [1 -3], its application to emulsion polymerization is far from being assessed. On one hand, it has been shown, based on kinetic arguments, that this is the only living mechanism that allows realizing a radical-segregated system where the polymerization rates for the living and the equivalent nonliving processes are identical [4 -6]. On the other hand, the application to conventional emulsion polymerization, i.e., the so-called ab initio polymerization, did not enjoy much success, and good results have been obtained only in mini-emulsion poly-Mini-emulsion polymerization is generally regarded as more complex than conventional emulsion polymerization, which remains the preferred way to carry out industrial polymerizations. However, the application of RAFT to ab initio emulsion polymerization is not straightforward, mainly because of the heterogeneous nature of the process and, specifically, of the partitioning of the RAFT agent among the three phases present in the system: the monomer droplets, the H 2 O phase, and the polymer particles. In this respect, to best achieve satisfactory living conditions in ab initio emulsion polymerization, the RAFT agent must satisfy three main requirements: i) it has to be fully trans-

Macromolecular design via reversible addition–fragmentation chain transfer (RAFT)/xanthates (MADIX) polymerization

“…Ferguson and coworkers139, 216 developed further the idea of surface‐active RAFT oligomers by producing in situ low‐molecular‐weight amphiphilic block copolymers of poly(acrylic acid‐ b ‐butyl acrylate) that can self‐assemble in micelles, which in turn are used as surface‐active CTAs controlling the polymerization of hydrophobic monomers. Another elegant solution to the problem of the transfer of the CTA in the aqueous phase was proposed by Apostolovic et al,227 who used cyclodextrins to encapsulate the hydrophobic CTA and facilitate its transport across the water phase to the polymer particles. RAFT/MADIX‐mediated emulsion polymerization has also been used to produce latexes71, 212, 215, 228 and core–shell particles 213, 216.…”

Reversible addition–fragmentation chain transfer polymerization of methyl methacrylate in emulsion

“…More recently, the research attention of the CLRP processes has been directed to emulsion14–17 and miniemulsion18–20 polymerization systems, which are commercially viable methods for implementing CLRP polymerization and are environmentally benign. However, few successes with CLRP have been reported for an ab initio emulsion system 21, 22. When SFRP, ATRP, and RAFT polymerizations are carried out in an ab initio emulsion system, colloidal instability is the major problem 17, 23.…”

“…These problems have been solved with miniemulsion polymerization, a modified emulsion polymerization, with careful design of the system 24. On the other hand, seeded emulsion polymerization,25 an amphiphilic RAFT agent,26 β‐cyclodextrin in methyl methacrylate (MMA) RAFT emulsion polymerization,21 a fluorinated xanthate agent in the emulsion polymerization of styrene,22 and a special two‐step process for SFRP,27 in which a very small portion of a monomer and water‐soluble nitroxide is used first to synthesize living seed latex, have all been reported to be successful.…”

Block Copolymers From Living Emulsion Polymerization: Reactor Operating Strategies and Blocking Efficiency

“…UV, RI, and viscosity detectors were used in combination to determine whether all polymer chains have the RAFT agent moiety (uniform distribution of RAFT agent within particles). The UV wavelength set at 300 nm is able to detect the RAFT moiety [SC(OEt)S] as it strongly absorbs UV at this wavelength 20, 21. The UV detector is able to detect only polymeric species carrying a RAFT moiety, while the concentration sensitive RI/Visco detectors detect polymers with and without the RAFT moiety.…”