ABSTRACT:Investigations into the kinetics and mechanism of dithiobenzoate-mediated Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerizations, which exhibit nonideal kinetic behavior, such as induction periods and rate retardation, are comprehensively reviewed. The appreciable uncertainty in the rate coefficients associated with the RAFT equilibrium is discussed and methods for obtaining RAFT-specific rate coefficients are detailed. In addition, mechanistic studies are presented, which target the elucidation of the fundamental cause of rate retarding effects.

The rate coefficients for activation (ka0 app) of two alkyl halides, methyl 2-bromopropionate, MBrP, and Brcapped\ud poly(methyl acrylate), by Cu0 were determined under various conditions. The value of ka0 app was studied in two solvents, dimethyl sulfoxide (DMSO) and acetonitrile (MeCN), and their mixtures with methyl acrylate (MA), using tris[2-(dimethylami no)ethyl]amine (Me6TREN) and tris(2-pyridylmethyl)amine (TPMA) as ligands. The experiments showed that the rate of activation increased with the surface area of Cu0 but was typically not affected by the ratio of ligand to initiator, if a sufficient amount of ligand was present. The choice of solvent and presence of monomer/polymer had a small influence on ka0 app. The activation rate coefficient of MBrP was ka0 app= 1.8 × 10−4 cm s−1 with Me6TREN as the ligand in DMSO at 25 °C while the activation rate coefficient of Br-capped poly(methyl acrylate) by Cu0 was slightly lower, ka0 app = 1.0 × 10−4 cm s−1, as measured in a polymerization of MA in MA/DMSO = 1/1 (v/v) with Me6TREN. On the basis of the measured rate coefficients, the activation rate of MBrP by 1 mM CuIBr/Me6TREN (ka1app = 3.2 × 102 M−1 s−1) is similar to the activation rate by 2 km Cu0 wire with diameter of 0.25 mm in 7 mL of DMSO. Thus, under typical conditions, conducted in the presence of ca. 1 cm Cu0 wire, alkyl halides are predominantly activated by CuI species. Consequently, Cu0 acts as a supplemental activator\ud and also as a reducing agent (SARA) because comproportionation dominates disproportionation, for the polymerization of MA in DMSO. These results support the SARA ATRP mechanism rather than the proposed single electron transfer−living radical polymerization (SET-LRP) process, which requires exclusive activation by Cu0 and instantaneous disproportionation of CuI

Pulsed laser polymerization was used in conjunction with aqueous-phase size exclusion chromatography with multi-angle laser light scattering detection to determine the propagation rate coefficient (k p ) for the water-soluble monomer acrylamide. The influence of the monomer concentration was investigated from 0.3 to 2.8 M, and k p decreased with increasing monomer concentration. These data and data for acrylic acid in water were consistent with this decrease being caused by the depletion of the monomer concentration by dimer formation in water. Two photoinitiators, uranyl nitrate and 2,2Ј-azobis(2-amidinopropane) (V-50), were used; k p was dependent on their concentrations. The concentration dependence of k p was ascribed to a combination of solvent effects arising from association (thermodynamic effects) and changes in the free energy of activation (effects of the solvent on the structure of the reactant and transition state). Arrhenius parameters for k p (M Ϫ1 s Ϫ1 ) ϭ 10 7.2 exp(Ϫ13.4 kJ mol Ϫ1 /RT) and k p (M Ϫ1 s Ϫ1 ) ϭ 10 7.1 exp(Ϫ12.9 kJ mol Ϫ1 /RT) were obtained for 0.002 M uranyl nitrate and V-50, respectively, with a monomer concentration of 0.32 M.

A novel approach to conducting controlled free radical polymerization in water-borne organic dispersions using reversible addition-fragmentation chain transfer (RAFT) has been studied. The novel approach in this study focused on eliminating monomer and oligomer transport and comprised two fundamental steps: the synthesis of dithiobenzoate-end-capped styrene oligomers in bulk followed by emulsification of these oligomers to yield a polymerizable water-borne dispersion. Dithioesters that act as chain transfer agents in the RAFT process were synthesized in situ. The free radical polymerization of the dithiobenzoate-end-capped styrene oligomers in the water-borne organic dispersion proceeded in a controlled manner; molar mass increased in a linear fashion with increasing conversion, while polydispersities remained low. The familiar red layer formation associated with RAFT polymerization in conventional emulsions was not observed under these conditions. The effects of changing the costabilizer (hydrophobe) and the degree of polymerization of the emulsified oligomers were investigated. Better control was achieved with a less hydrophilic costabilizer and for the shorter of the oligomers tested.