Monomer chain transfer constant

The monomer chain-transfer constants are generally small for most monomers—in the range 10 5 to 10-4 (Table 3-4). Chain transfer to monomer places the upper limit to the polymer molecular weight that can be obtained, assuming the absence of all other transfer reactions. Transfer to monomer does not, however, prevent the synthesis of polymers of sufficiently high molecular weight to be of practical importance. Cm is generally low because the reaction... 

Problem 6.22 Vinyl acetate has a relatively high monomer chain transfer constant (2x10 at 60°C). What is the upper limit of molecular weight of poly(vinyl acetate) made by radical polymerization at 60°C ... 

The monomer chain transfer constants are generally small for most... 

The relative rates of transfer and propagation are given by the ratio ktr,u/kp, which is the monomer chain transfer constant Cm- The value of... 

Problem 6.19 Vinyl acetate has a relatively high monomer chain transfer constant (2x 10 ... 

Table 6.5 Some Values of Monomer Chain Transfer Constants...

The ratio ktr,ulkp defines the monomer chain transfer constant Cm and, as in the case of free-radical polymerization (Section 6.8.1), its value determines the polymer molecular weight, in the absence of other chain termination processes. 

Monomer chain transfer constants are generally less than 1Reaction (6-79) involves breaking the strong vinyl C—H bond and the products of reaction (6-80) are not appreciably more stable than the reactants. 

The chain transfer constants of various monomers at 60°C are presented in Table 3. The monomer chain transfer constants Cm are generally small 10 -10 " ) for most monomers because the reaction involves breaking the strong vinyl C-H bond ... 

Hence, a plot of the inverse number average degree of polymerization, Pn against the rate of polymerization Rp, - the rate of polymerization can be easily varied by the concentration of the initiator - yields the monomer chain transfer constant Cm as intercept of a linear plot. The value of Cm constitutes a limit for the maximum number average degree of polymerization, P , as transfer to monomer cannot be avoided. Methyl methacrylate, for instance, has a monomer chain transfer constant of about Cm = 5 x 10 at 60 °C, leading to a maximal chain length of about 20 000, whereas in a radical polymerization of vinyl acetate with Cm = 2 x 10 " at 60 °C, the limit is already reached at 5000. 

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