Identification of a Transient but Key Motif in the Living Coordinative Chain Transfer Cyclocopolymerization of Ethylene with Butadiene

Copolymers of ethylene
and butadiene were prepared using the ansa-bisfluorenyl
complex Me2Si­(C13H8)2NdR
in combination with dialkylmagnesium
as a chain transfer agent. Thorough kinetic studies and computational
mechanistic investigations of this copolymerization reaction were
performed. In combination with detailed analyses of the polymer microstructure
and chain ends, these studies demonstrate that the title copolymerization
operates according to a living coordinative chain transfer copolymerization
of ethylene and butadiene. In addition to the formation of the previously
described 1,2-cyclohexane inner chain cyclic motif, the presence of
bicyclic 1,5-decalin units via the formation of transient (vinylcyclohexyl)­methyl
chain ends is discussed in the present communication. The nonaccumulation
of the vinylcyclohexane motif within the chains is explained by the
reversibility of its formation, as interpreted with the help of DFT
calculations, or by its rapid conversion into decalin motif after
one ethylene insertion. Finally, this study also illustrates the ability
of the fluorenyl ligand to adjust its binding mode on demand in order
to avoid inhibition of the catalyst.