Thiols chain transfer constants

The molecular weight of a polymer can be controlled through the use of a chain-transfer agent, as well as by initiator concentration and type, monomer concentration, and solvent type and temperature. Chlorinated aUphatic compounds and thiols are particularly effective chain-transfer agents used for regulating the molecular weight of acryUc polymers (94). Chain-transfer constants (C at 60°C) for some typical agents for poly(methyl acrylate) are as follows (87) ... 

More recently Chaudhuri, and Hermans (15) made use of the larger chain transfer constant of thiol groups, which they introduced into the cellulose by reaction with ethylene sulfide which became more readily available through the synthesis described by Searles, and Lutz (16), and which is known to add to alcohols as follows ... 

Viscosity of the medium can also play a role in the kinetics due to the importance of diffusion in the observed rate constants. In the bulk radical polymerization of 2-phenoxyethyl methacrylate, thiol chain-transfer reagents operate at rates close to those observed for MMA while the rate of CCT catalyzed by 9a is an order of magnitude slower (2 x 103 at 60 °C) than that of MMA.5 The thiol reactions involve a chemically controlled hydrogen transfer event, whereas the reaction of methacrylate radicals with cobalt are diffusion controlled. The higher bulk viscosity of the 2-phenoxyethyl methacrylate has a significant influence on the transfer rate. 

Mayo defined a chain transfer constant that could be empirically determined by Equation 1.22. P is the degree of polymerization, S and M are concentrations of solvent and monomer respectively, and Pq is the degree of polymerization in the absence of solvent. Thiols are particularly effective (albeit stoichiometricaUy as in 1.20 and 1.21) reagents for chain transfer [66]. 

Similarly, David et al carried out the thiol-ene radical coupling of vinylphosphonic acid in the presence of sulfanylacetic acid in water at 70 °C (Scheme 3.4). They showed that a low amount of the thiol compound was required, according to its very high chain transfer constant, and this reaction returned good yields. 

Chain-Transfsr Agents. Radical chain-transfer agents, eg, various thiols, have been applied over several decades to control the molecular weight of the polymerization or to terminate the growing polymer chain, eg, in the case of telomerization (38). Recently, we have observed that the chain-transfer constants of the water-soluble A(-acetyl-L-cystein to RAMEB-complexed monomers, eg, methyl methacrylate and styrene, are three times higher in water than in the case of a mixture ofiV,A( -dimethylformamide and water. 

Thiols may be used as transfer agents in a wide variety of free radical polymerization processes. Scheme 1.12 shows the general reaction mechanism for this class of transfer agents. Nucleophilic radicals react more readily with thiols than electrophilic radicals, so transfer coefficients are higher for vinyl esters and styrene than for acrylates and methacrylates. Aromatic thiols react more readily than aliphatic ones, i.e., the chain transfer constant is higher, but they also show a stronger retardation effect as the resulting S-centered radicals are less prone for monomer addition due to their increased stability. The product of the transfer reaction is a thiyl radical, which is electrophilic and will react preferably with the more electron rich monomer in copolymerizations. 

The thiol ( -dodecyl mercaptan) used ia this recipe played a prominent role ia the quaUty control of the product. Such thiols are known as chain-transfer agents and help control the molecular weight of the SBR by means of the foUowiag reaction where M = monomer, eg, butadiene or styrene R(M) = growing free-radical chain k = propagation-rate constant = transfer-rate constant and k = initiation-rate constant. 

Thus the thiol 0 2 25511 is capable of terminating a growiug chain and also initiating a new chain. If the initiation-rate constant, k is not much slower than the propagation-rate constant, the net result is the growth of a new chain without any effect on the overall polymerization rate (retardation). That represents a tme chain transfer, ie, no effect on the rate but a substantial decrease iu molecular weight (12). 

Traditionally thiols or mercaptans are perhaps the most commonly used transfer agents in radical polymerization. They undergo facile reaction with propagating (and other) radicals with transfer of a hydrogen atom and form a saturated chain end and a thiyl radical (Scheme 6.6). Some typical transfer constants are presented in Table 6.2. The values of the transfer constants depend markedly on the particular monomer and can depend on reaction conditions.4"1 44... 

Triphenylinethyl terminated polymers (41) are formed in polymerizations conducted in the presence of triphenylmethyl thiol (40).9 5 Transfer constants for 40 are similar to other thiols (17.8 for S, 0.7 for MM A, compare Section 6.2.2.1). When the polymers (41) are heated in the presence of added monomer it is presumed that the S-CPh bond is cleaved and triphenylmethyl-mediated polymerization according to Scheme 9.11 can then ensue to yield chain extended or block polymers (42). 

The chain length of the polymer formed is proportional to the transfer constant which is the ratio of the specific rate of radical transfer to the specific rate of chain propagation . Wall and Brown measured the isotope effect fet(H)/ t(D) of (he chain transfer step in the butanethiol-S-dj mediated polymerization of styrene. A value of 4, somewhat less than the predicted value of about 6, was obtained. The low kinetic isotope effect indicated that either the loss of zero point energy of the S—H bond had been compensated by the formation of unusually strong bonds or that the reaction was complicated by the abstraction of butyl hydrogens as well as thiol hydrogen. Data such as these can often aid in the search for more efficient transfer agents. 

The majority of radical reactions of interest to synthetic chemists are chain processes [3,4]. For those readers who are not familiar with this chemistry, some general aspects of radical chain reactions are discussed here. Scheme 1 represents the simple addition of a thiol to a carbon-carbon double bond as an example of a chain process. Thus, RS radicals, generated by some initiation processes, undergo a series of propagation steps generating fresh radicals. The chain reactions are terminated by radical combination or disproportionation. In order to have an efficient chain process, the rate of chain transfer steps must be greater than that of chain termination steps. Since the termination rate constants in the liquid phase are controlled by diffusion (i.e. 10 M s ) and radical... 

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