Chain-transfer agents for polymers

Mention may finally be made of graft polymers derived from natural rubber which have been the subject of intensive investigation but which have not achieved commercial significance. It has been found that natural rubber is an efficient chain transfer agent for free-radical polymerisation and that grafting appears to occur by the mechanism shown in Figure 30.8. 

Polymer propagation stops with the addition of a chain transfer agent. For example, carbon tetrachloride can serve as a chain transfer agent ... 

To avoid these stability problems, it is necessary to minimize the proportion of chains that terminate by radical-radical reaction. One way of achieving this is to conduct the polymerization in the presence of an appropriate chain transfer agent. For example, if polymerization is performed in the presence of a H-donor chain transfer agent, conditions can be chosen such that most chains terminate by hydrogen-atom transfer. Bagby et al.iA examined the thermal stability of PMMA formed with dodecanethiol. These polymer chains will then possess, more... 

Simplest of the techniques requiring only monomer and monomer-soluble initiator, and perhaps a chain-transfer agent for molecular weight control. Characterized, on the positive side, by high polymer yield per volume of reaction, easy polymer recovery. Difficulty of removing unreacted monomer and heat control are negative features. Examples of polymers produced by bulk polymerization include poly(methyl methacrylate), polystyrene, and low-density (high pressure) polyethylene. 

The hydroxide ion is usually not sufficiently nucleophilic to reinitiate polymerization and the kinetic chain is broken. Water has an especially negative effect on polymerization, since it is an active chain-transfer agent. For example, C s is approximately 10 in the polymerization of styrene at 25°C with sodium naphthalene [Szwarc, 1960], and the presence of even small concentrations of water can greatly limit the polymer molecular weight and polymerization rate. The adventitious presence of other proton donors may not be as much of a problem. Ethanol has a transfer constant of about 10-3. Its presence in small amounts would not prevent the formation of high polymer because transfer would be slow, although the polymer would not be living. 

In all cases, chain transfer may occur to monomer, solvent, polymer, or a chain transfer agent. For this to be a transfer reaction rather than termination, the molecule generated (A ) must be reactive enough to reinitiate polymerization [Eq. (10)]. 

Tlie rubber latex is usually produced in batch reactors. The rubber can be polybutadiene [9003-17-2] or a copolymer of 1,3-butadiene [106-99-0] and either acrylonitrile [107-13-1] or styrene [100-42-5]. The latex normally has a polymer content of approximately 30 to 50% most of the remainder is water. In addition to the monomers, the polymerization ingredients include an emulsifier, a polymerization initiator, and usually a chain-transfer agent for molecular weight control. 

PMMA macromonomers have been prepared by several methods. The most common involves the use of thioglycolic acid, as a chain transfer agent for the preparation of polymers with carboxyl end groups, followed by the reaction with glycidyl methacrylate (GMA) to introduce a methacrylate terminal group [8] (Scheme 42). 

Athey, Mosher and Weston (19. 20) studied the use of S3ra net-rical difunctional chain transfer agents for synthesis of tele-chelic polymers. Dithiodiglycolic acid (DTDGA, II) was selected in the present work as a chain transfer agent to Increase the functionality of the PnBA. This particular chain transfer agent cleaves at the sulphur-sulphur bond in the presence of a free radical as shown below ... 

Li, C., and B.C. Benicewicz. 2005. a-Cyanobenzyl dithioester reversible addition-fragmentation chain-transfer agents for controlled radical polymerizations. Journal of Polymer Science Part A Polymer Chemistry 43(7) 1535-1543. 

POly(acryliC acid). Poly(acrylic acid) (PAA) can be prepared by direct tree-radical polymerization of 2 in aqueous solution (6) or by precipitation polymerization in benzene. Alcohols and mercaptans are commonly used chain-transfer agents for regulating polymer molecular weight. Alternatively, PAA can be prepared by hydrolysis of poly(alkyl acrylates) (6) (Fig. 29). 

---