Dialkenylmagnesium Compounds in Coordinative Chain Transfer Polymerization of Ethylene. Reversible Chain Transfer Agents and Tools To Probe Catalyst Selectivities toward Ring Formation

A range
of dialkenylmagnesium compounds ([CH2CH­(CH2)n]2Mg; n = 1–6) were synthesized and used as chain transfer agents
(CTA) with either (C5Me5)2NdCl2Li­(OEt2)2 (1) or [Me2Si­(C13H8)2Nd­(BH4)2Li­(thf)]2 (2) neodymium precursors
for the polymerization of ethylene. In all cases, the systems followed
a controlled coordinative chain transfer polymerization mechanism.
The intramolecular insertion of the vinyl group on the CTA in growing
chains is possible and led to the formation of cyclopentyl, cyclohexyl,
and possibly cycloheptyl chain ends. While the production of cyclopentyl-
or cyclohexyl-capped polyethylene chains can be quantitative (n = 2–5), the integrity of this double bond can also
be kept if n is higher than 6. In comparison to 1/CTA catalytic systems, 2/CTA catalytic systems
showed a higher propensity to produce cycloalkyl chain ends. This
was ascribed to the lower steric demand around the active site, as
shown by DFT calculations. In addition, the formation of bis­(cyclopentylmethyl)­magnesium
from dipentenylmagnesium using a catalytic amount of 2 was shown.