Chain-Growth Termination and Re-initiation

Since its dependence on olefin concentration is the same as that observed for chain-growth by olefin insertion, p-H transfer to a monomer has the consequence that average chain lengths are not affected by changes in olefin concentration, since the latter cancels between the numerator and denominator of the expression Pn = v /vt = k [M]/k( [M] = k /kt. For P-H transfer to the metal, however, chain lengths increase with olefin concentration, since the latter affects only the numerator of the expression Pn = v /vt = k [M]/kt. 

Other processes also contribute to chain growth termination under special conditions. In particularly crowded catalysts, fi-methyl transfer to the metal centre can occur instead of p-H transfer. When other reaction paths are blocked, a-bond metathesis, i.e. transfer of an H atom from a monomer to the metal-bound alkyl C atom can release a polymer with a saturated chain end with formation of a new unsaturated metal-bound chain start. Saturated chain ends will also result when H2 gas is added to a catalyst system thus leading to the production of shortened polymer chains. Such an H2 addition will often also cause an increase in overall catalyst activity, since H2 will predominantly react with species - such as occasional 2,1-inserted units - which are rather 

Polymers with even narrower mass distributions, e.g. with PDI values close to 1, arise in living polymerization systems, in which no chain termination processes can occur at all, such that all chains remain bound to the metal centre from which they have started to grow at the same time. Living polymerizations, which offer useful opportunities, e.g. with regard to the production of block copolymers by exchange of one monomer for another, occur in anionic polymerizations of styrenes or butadienes such as are induced by simple lithium alkyls. For a-olefin polymerization catalysts of the type discussed above, living polymerizations are rare. These more elaborate catalysts can thus release a newly formed polymer chain within a time interval of typically less than one 

---