Chain transfer rate constant

M] = monomer concentration [l] = initiator concentration [R ] = radical concentration f = initiator efficiency k-j g = chain transfer rate "constant" ktr = termination (combination or disproportionation) rate constant... 

The greater the rate constant for chain transfer, the lower the molecular weight of the polymer. One way to affect the rate constants is by changing the temperature. In general, the chain transfer rate constant is much more sensitive to temperature effects, increasing dramatically as the temperature is increased. For these reasons, there is an inverse correlation between temperature and molecular weight of polyvinyl chloride as shown in Fig. 22.3. 

Chain-transfer constants, 25 571t Chain-transfer rate constants, 19 832 Chain-transfer rates, 19 839 Chain transfer to solvent (CTS), 23 385 Chalcanthite, 7 772 Chalcogenide glasses, 12 575, 584 semiconductivity in, 12 587 Chalcogenides acidic, 12 190-191 gallium, 12 359 in photocatalysis, 19 75 plutonium, 19 691 zirconium, 26 641... 

Thus an increase of polymerization rates by (k /k T and molecular weight by k3/k3p should be observed when trideuterovinyl acetate is used (k3J) is the chain transfer rate constant for trideuterovinyl acetate). 

Hence a plot of jDP vs. 1/[M] for polymer samples prepared with constant [A] should be a straight line having ktr,i/i/kp as the intercept and kt A [A]/fcj> + k,/kp) as the slope. Similarly when 1/ >P is plotted vs. [A] for polymer samples prepared with constant [M], a straight line should be obtained having (A r>l/ + ks/kp [M]) as the intercept and kt A/kp [M] as the slope. Consequently all chain transfer constants can be evaluated as ratios of chain transfer rate constants... 

The chain-transfer rate constants, ktr, are determined from the average molecular weight in a polymerizing system. The intercept of a Mayo plot of number-average molecular weight against [I]/Rp where [I] is the initiator concentration and i p is the polymerization rate (20). Extensive compilations of tr values are found in the open literature (21). Dodecyl mercaptan is used in the manufacture of commercial Neoprene W. The chain-transfer rate constant for dodecyl mercaptan with chloroprene was determined using the Mayo plot technique (22) as follows ... 

A Smith-Ewart case 1 behavior in 1) can be observed during emulsion polymerization of monomers with a high chain-transfer rate constant such as vinyl chloride because the monomer radicals have a high tendency to escape from the particles. Especially for vinyl chloride emulsion polsunerization, Ugelstad and Hansen (113) derived the following equation 18 to calculate n, which predicts dependence, h a... 

If a particularly low molecular mass polymer is required or if a reactor s pressure rating is too low to operate at very high temperatimes, an additional chain-transfer agent can be employed. This reduces the molecular mass by increasing the chain-transfer rate constant above that for monomer alone in the above equation. As a result DP decreases, depending on the quantity of chain transfer agent used. 

Scheme 8 Simplified mechanism of RITP. (A , radical from the initiator I2, molecular iodine M, monomer unit n, mean number degree of polymerization fex, degenerative chain transfer rate constant fep, propagation rate constant).

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