Reversible Addition-Fragmentation Chain Transfer Polymerization

Another type of “living”/controlled radical polymerization involves reversible addition-fragmentation chain transfer. It was named, therefore, RAFT polymerization. Great versatility and effectiveness was shown for the process [274]. The process is said to be compatible with a very wide range of monomers including functional monomers containing such functional groups as acids, acid salts, and hydroxyl or tertiary amine groups. The conditions of polymerization are those used in conventional free-radical polymerizations. They can be carried out in bulk, solution, emulsion and suspension (see Sect. 3.16). The usual azo or peroxide initiators are employed [274]. The reaction was originally illustrated as follows [274]:

The RAFT process depends upon rapid addition—fragmentation equilibrium reaction between propagating (Pn•) as well as intermediate radicals, and chain activity and dormancy, as shown below in the reaction scheme. The concentrations of each of the species within the equilibrium is dependent on the relative rate coefficients for addition of a propagating radical to the RAFT agent (Kadd) and fragmentation of the formed intermediate radical (Kfrag). This equilibrium applies correctly only for polymeric chains that are present in significant concentrations after an initialization period. During the initiation period there are mainly shorter chains present. The important part of this equilibrium is the relatively stable radical intermediates. It was reported that RAFT-mediated polymerization reactions typically contain anomalies, such as an “inhibition” period and rate retardation. The rate retardations or reductions in the polymerization rates apparently occur in the presence of RAFT agents, and are not observed when RAFT agents are absent. Examples are Di thiobenzoate-mediated polymerization reactions [275]. Tonge and coworkers [276] investigated the reactions of short-chain species during the initial period of cumyldithiobenzoate mediated polymerization of styrene at 84C. Using electronspin resonance and hydrogen and carbon NMR spectroscopies they were able to demonstrate that the reactions are very specific during the initial stages. There is a strong preference to add single monomer species. This is followed by fragmentation and release of shorter radicals prior to formation of longer chains. The effectiveness RAFT agents were investigated by Moad and coworkers [277]. These RAFT agents, such as thiocarbonylthio compounds, depend in effectiveness on the nature of the group, Z and R (shown below) that modify the reactivity of the thiocarbonyl group toward free radical addition. R is the free radical leaving group [277]:

These RAFT agents are based on the structure [S=C(Ph)S-R] [277] They found that the effec- tiveness of these agents also depends upon the nature of the monomer and on the polymerization conditions. For the polymerization of styrene, methyl methacrylate, butyl acrylate, and methyl acrylate at 60°C, the effectiveness of R decreases in the following order [277]:

R=C(alkyl),CN~C(CH3)2Ar>C(CH3),(C=O)O(alkyl)>C(CH3),(C=O)NH(alkyl)>C(CH3) 2CH2C(CH3)3>C(CH3)HPh>C(CH3)3 ~ CH2Ph In addition, among the above compounds, only when R=C(CH3)2CN or C(CH3)2Ph did these thiocarbonylthio compounds yield polymers with narrower polydispersities in batch polymerizations. Also, only these compounds allowed molecular weight control that may be expected from a living polymerization. The reaction mechanism was proposed by Moad and coworkers as follows [277]:

Moad and coworkers concluded [277] that a major factor that determines the transfer coefficient of Di thiobenzoate derivatives is the way the intermediate 3 (see the above in equation) partitions between starting materials and products. This in turn is determined by the relative ability of the leaving group R. and by the propagating radical. Steric factors and radical stability of R. are also important. They conclude that more stable, more electrophilic, and bulkier radicals are better leaving groups. The partitioning of R. between the monomers (to reinitiate) or by adding to polymeric RAFT can also have a significant effect on the rate of consumption of RAFT agent [277].

Vana and coworkers [278] studied conversion vs. time and molecular weight distributions in conjunction with the kinetic scheme for the RAPT process. In particular, conditions leading to inhibition and rate retardation were examined to act as a guide to optimizing the reaction. They demonstrated that there is an inhibition period of considerable length. It is induced by either slow fragmentation of the intermediate RAFT radicals appearing in the preequilibrium or is due to slow reinitiation of the leaving group radicals from the initial RAFT agent. The absolute values of the rate coefficients governing the core equilibrium of the RAFT process (at a fixed value of the equilibrium constant) are found to be crucial in controlling the polydispersity of the resulting M/M values. Higher interchange frequency effects narrower distributions. They also demonstrated that the size of the rate coefficient controlling the addition reaction of propagating radicals to polymer-RAFT agent, Kß, is mainly responsible for optimizing the control of the polymerization. The fragmentation rate coefficient, K-p, of the macro RAFT intermediate radical, on the other hand, may be varied over orders of magnitude without affecting the amount of control exerted over the polymerization. Based on the basic RAFT mechanism, shown above, its value mainly governs the extent of rate retardation in RAFT polymerizations [278].

Calitz, Tonge, and Sanderson reported the results of a study of RAFT polymerization by means of electron spin resonance spectroscopy [276]. They observed intermediate radical signals that were not consistent with current RAFT theory [276].

Sawamoto and coworkers reported obtaining simultaneous control of molecular weight and steric structure in RAFT polymerization of N-isopropylacrylamide by addition of rare earth metal, Y(O- tetrafluoromethane sulfonate)3, Lewis acid. The M/M ratio of the products ranged between 1.4-1.9 and the isotactic content was 80-84% [277].

Go to etal. [279] developed a process that they describe as reversible living chain transfer radical polymerization [278], where they us Ge, Sn, P, and N compounds iodides in the iodide mediated polymerizations. In this process, a compound such as Gel, is a chain transferring agent and the polymer-iodide is catalytically activated via a RFT process. They proposed that the new reversible activation process be referred to as RTCP [279]. The process can be illustrated by them as follows [279]:

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هل توقيت نشاطك اليومي يؤثر على صحة دماغك؟.. العلماء يجيبون

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المزيد

مواضيع ذات صلة

Special Types of Controlled/Living Polymerizations

Combinations of Click Chemistry and ATP as Well as ATP and RAFT Polymerizations

Nitroxide-Mediated Radical Polymerizations

Atom Transfer Radical Polymerizations

Controlled/“Living” Free-Radical Polymerization

Steric Control in Free-Radical Polymerization

The rmal Polymerization

Inhibition and Retardation

The Termination Reaction

Ring Forming Polymerization

Polymerization of Monomers with Multiple Double Bonds

Methods for Measuring Molecular Weights of Polymers