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766 Polymers, Synthesis

the rate of polymerization can proceed basically as TABLE VIII Chain Transfer Constants of Various
before. Agents to Styrene at 333 K
There are many types of chain transfer steps, such as Agent 10 C S
4
transfer to monomer, to a solvent, to an initiator, to a poly-
mer, or even to a modifier. In all cases, the molecular Benzene 0.023
weight will be decreased. The quantitative decrease will n-Heptane 0.42
bedependentonthereactivityofthegrowingmacroradical sec-Butylbenzene 6.22
with the small-molecule species. The extent to which this m-Cresol 11.0
happens is very dependent on the structure of the agent in CCI 4 90
question and can be roughly related to its reactivity with CBr 4 22,000
a small-molecule radical species. Some typical data are n-Butylmercaptan 210,000
shown in Table VIII. An aromatic molecule, such as ben-
zene, would not be expected to interact particularly effec- larweightorthenumber-averagedegreeofpolymerization
tively with radicals and, indeed, the chain transfer constant versus this ratio.
here is rather small. On the other hand, molecules such as An overview of the kinetics of free-radical polymer-
carbon tetrachloride or certainly mercaptans have enor- izations that takes into account chain transfer is provided
mous activity with macroradicals and are very efficient in Scheme 16. Chain transfer to the polymer chain pro-
chain transfer agents. One can assess the effectiveness of duces a particular type of structure, namely, a branched
a chain transfer agent by conducting polymerizations at macromolecule.
various ratios of the chain transfer agent to the monomer To this point, we have not discussed any detailed aspect
and plotting the reciprocal of the number-average molecu- of the polymerization process itself. There are essentially

SCHEME 16 Overview of free-radical kinetics.