Nickel Polymerization Catalysts with Ylide Steering Ligands - Late Transition Metal Polymerization Catalysis - page 14

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A mean degree of polymerization of 42 and a molecular weight M n of approxi-
mately 1200 g mol -1 can be estimated from the quantitative end group determina-
tion in the given example.
Gas chromatographic analysis identifies the oligomers in the reaction solution
as an homologous series of linear -olefins in the range C 4 to C 40 .

1.3.3.2 Short Chain-Branched Macromolecules
A conventional approach to the controlled formation of short-chain branches is
ethene copolymerization with co-monomers such as propene, butene(1), 4-methyl-
pentene(1), hexene(1) or octene(1). In the ethene/propene copolymerization exam-
ple given below an increased number of methyl groups compared with vinyl end
groups is consistent with a propene incorporation of approximately 6 mol% [Eq.
(13)], the observed lower DSC melt temperatures and lower densities are typical
for medium density (MDPE) and linear low density polyethylene (LLDPE).

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Ylide nickel-catalyzed ethylene polymerizations can also produce branched mac-
romolecules from ethylene alone (that is without adding a co-monomer) due to li-
gand effects, which induce a nonlinear specificity. This “self-branching” can be
achieved with special ylide ligand combinations and adequate reaction conditions,
which allow the insertion of olefins other than ethylene into Ni-H and/or Ni-C
bonds. Low ethylene pressure and high catalyst concentrations favor self-branch-
ing. With the catalyst shown in Eq. (14), almost equal quantities of vinylidene,
trans- vinylene and vinyl double bonds are formed. The excess methyl content ver-
sus the total number of double bonds per 1000 carbon atoms, and the appearance
of significant amounts of vinylidene groups, indicate a branched structure.

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The appearance of vinylidene double bonds may be interpreted by successive
chain growth, chain termination by

-H elimination with formation of -olefin,