Termination, by coupling and

Results of Melville and Bickeps have been recalculated assuming termination by coupling and using the present notation. 

In copolymerization, several different combinations of initiation and termination mechanisms are possible, giving rise to a variety of different polymerization rate equations. Only two cases will be singled out here free-radical copolymerization with termination by coupling, and ionic polymerization with termination by chain transfer to a deactivating agent or impurity. For other combinations, the derivation of rate equations follows along the same lines. 

Not being aware of the earlier work, the present author first noticed the phenomenon in 1981. Geiger and Huber10 had photolyzed dimethylnitrosamine in the gas phase at 1 Torr and under 100 Torr N2 buffer. This compound fragments from the first excited singlet state into dimethylaminyl radicals and nitrous oxide NO with unity quantum yield, but neither photoproducts nor a decrease of the initial compound pressure were observed. Even after 20 h photolysis the back-reaction was complete to more than 99.9% (Scheme 6). This seemed quite puzzling because sterically unhindered aminyl radicals are transient and readily self-terminate by coupling and disproportionation. 

Let X fraction of chain radicals terminate by coupling and (1 — x) fraction by disproportionation. Therefore 1 mol of chain radicals give rise to 0.5a5 + (1 — x) or (1 — 0.5x) mol polymer molecules. Hence... 

Problem 6.21 A vinyl monomer of molecular weight 132 is polymerized by a free-radical initiator in the presence of dodecyl mercaptan (C12H25SH). The rate of polymerization is not depressed by the mercaptan. The purified polymer has a sulfur content of 0.02% (w/w) and its DP is 450. If 80% of the kinetic chains are terminated by coupling and 20% by disproportionation, what should be the extent of terminal unsaturation of the chains ... 

Only K and fkd depend on temperature. From Eq. (6.102), assuming termination by coupling and considering initial state,... 

Ziegler-Natta polymerization is well known to involve a two-stage process [148, 149]. In the first stage, an aluminum alkyl (such as trialkyl aluminum) is reacted with TiCU in order to produce active jS-TiCls. The alkyl radicals, which are also produced in this reaction, are terminated by coupling and create inert products. Subsequent alkylation of -TiCb then occurs to generate the titanium species that is capable of initiating the polymerization of olefins such as ethylene (Scheme 11.37). 

Processes in which macromolecules are produced are termination by coupling and/or disproportion, and transfer to other molecule, i.e. monomer. In this case it is assumed that there is no transfer to other molecules. Since it is not known which termination mechanism (coupling or disproportion) is in the majority, and since only one termination rate constant can be calculated, two extreme cases are considered. The calculated termination rate constant will be assumed to be equal to the termination rate constant by coupling and the termination rate constant by disproportion, as presented below. 

Instantaneous initiation, monomer varies, either no transfer occurs or transfer to monomer, solvenf or polymer is allowed termination by coupling and varies with chain length... 

If half the kinetic chains are terminated by coupling and half by disproportionation, what is the expected value of C ... 

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