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.

In situ 1H nuclear magnetic resonance (NMR) spectroscopy has been used to directly investigate the processes that occur during the early stages (typically the first few monomer addition steps) of an AIBN-initiated reversible addition−fragmentation chain transfer polymerization of styrene in the presence of the RAFT agent cyanoisopropyl dithiobenzoate at 70 and 84 °C. The change in concentration of important dithiobenzoate species as a function of time has been investigated. It was found that the reaction was extremely selective during the period of consumption of the initial RAFT agent (defined as the initialization period), with almost no production of RAFT-capped chains of degree of polymerization greater than unity until all of the cyanoisopropyl dithiobenzoate was converted to its single monomer adduct. The rate-determining step for this process was found to be the addition (propagation) of the cyanoisopropyl radicals to styrene. During the period where the initial RAFT agent was consumed, fragmentation of formed intermediate radicals strongly favored the production of the tertiary cyanoisopropyl radicals, which were the only significant propagating species during that period. This led to a greater rate of propagation during that period, since the propagation rate coefficient for the cyanoisopropyl radical is greater than that of polystyryl radicals. It was found that inhibition effects can occur in the presence of RAFT agents in homogeneous media when the k p for initiator fragments is smaller than for long chain radicals, which is a result of this aspect of the RAFT mechanism.

In situ 1H nuclear magnetic resonance spectroscopy was used to directly investigate the processes that occur during the early stages (the first few monomer addition steps) of azobis(isobutyronitrile)-initiated reversible addition−fragmentation chain transfer (RAFT) polymerizations of styrene in the presence of cumyl dithiobenzoate at 70 and 84 °C. The change in concentration of important dithiobenzoate species and monomer as a function of time was investigated. The predominant type of growing chain under the reaction conditions carries a cumyl end group. The initialization period (the period during which the initial RAFT agent is consumed) in the presence of cumyl dithiobenzoate in homogeneous media was significantly longer than for equivalent reactions using cyanoisopropyl dithiobenzoate as RAFT agent, and the rate of monomer conversion was correspondingly slower. Very strong fragmentation selectivity of the formed intermediate radicals (to form the tertiary propagating radical) was observed during the initialization period. The rate-determining step for the initialization process was the addition (propagation) of the initiator-derived and cumyl radicals to styrene, to form the corresponding single-monomer adducts. The greater length of this period with respect to the same reaction using cyanoisopropyl dithiobenzoate as RAFT agent is suggested to be a result of slower propagation due to a smaller addition rate coefficient of the cumyl radical (which was found to be the dominant propagation process during initialization) to styrene, than for the cyanoisopropyl radical, and to a higher average termination rate for the cumyl radicals than for the cyanoisopropyl radicals. The probable (small) difference in intermediate radical concentration is considered to be a less significant contributor to the length of the period.

Two xanthate end-functional poly(ethylene glycol)s (PEGs) were tested as macromolecular chaintransfer agents (macroCTA) in the reversible addition-fragmentation transfer-mediated polymerization of vinyl acetate (VAc) and N-vinylpyrrolidone. The macroCTA leaving group played a determining role in the preparation of the block copolymers. PEG-b-PVAc and PEG-b-PVP diblock copolymers were obtained when the macroCTA had a propionyl ester leaving group, whereas under the same experimental conditions the macroCTA with a phenylacetyl ester leaving group inhibited the polymerization. In situ 1 H NMR spectroscopy polymerizations were performed with low molecular weight xanthate analogues to investigate the cause of inhibition. Block copolymers were prepared with the macroCTA which did not inhibit the polymerization and were characterized via size exclusion chromatography, high-performance liquid chromatography, and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The ability to produce narrowly distributed (PDI < 1.4) block copolymers end capped with a xanthate moiety with little to no homopolymer contaminant is presented. IntroductionPoly(ethylene glycol) (PEG) is widely used in the pharmaceutical and biomedical fields. It is a nonionic polymer, soluble in water and most common organic solvents. The incorporation of a PEG segment in a macromolecule modulates its solution properties. Many synthetic pathways are available for the preparation of block copolymers comprising a PEG block. Each block can be prepared separately and connected by postpolymerization coupling of functional end groups. 1 Commercially available PEGs prepared via anionic polymerization can be found with one or two hydroxyl end functionalities, which enable almost unlimited chemical modification 2 and the preparation of di-, tri-, or multiblock copolymers. The main prerequisite for this approach is that the second polymeric block must be quantitatively end functionalized. Thus, this method has been used mostly to prepare biodegradable block copolymers of PEG with a second block obtained via polycondensation or ring opening polymerization. 3 Poly(N-vinylpyrrolidone) (PVP) and poly(vinyl acetate) (PVAc) are typical examples of widely used industrial polymers that can only be prepared via free-radical polymerization. Conventional free-radical polymerization does not normally provide end functionality because of transfer and termination reactions. By taking advantage of transfer reactions, however, Ranucci et al. synthesized a variety of low molecular weight PVPs bearing chain-end functionality. 4 Another synthetic approach consists of growing a second block from an endfunctional PEG precursor. By selecting a macromolecular precursor bearing a functional group capable of controlling the polymerization of the second comonomer, it is possible to not only obtain block copolymers but also control the molecular weight distribution of the blocks. The recent development of