Intermediate Complexes in the Octacarbonyl Dicobalt-Initiated Living Polymerization of 3-Methyl-1,2-butadiene

In the reaction of octacarbonyl dicobalt with
3-methyl-1,2-butadiene at room temperature dinuclear
η3-allyl-type complexes are formed which contain 2 + n
(n = 0, 1, 2, 3, ···) five-carbon units depending on
the applied
molar ratio. These complexes are the individual compounds in the
octacarbonyl dicobalt-initiated living polymerization
of 3-methyl-1,2-butadiene. The first three members in this series
of complexes containing two (1), three (2),
and
four (3) five-carbon units have been isolated and
characterized by IR, Raman, 1H, and 13C NMR
spectroscopies,
molecular weight, and for 1 and 2 also by
single-crystal X-ray diffraction. The first two molecules of
3-methyl-1,2-butadiene establish the formation of two
(η3-3,3-dimethylallyl)cobalt tricarbonyl parts which
are linked together
at the central allylic carbon by a carbonyl group. By the addition
of 3-methyl-1,2-butadiene in excess, the monomer
inserts into the unsubstituted allylic carbon−cobalt bond in a
1,2-fashion, pushing the cobalt atoms further apart.
The X-ray structures of 1 and 2 disclose
that the 1,2-polymer chain grows in a helical manner. The rate of
the
insertion of the monomer is first order with respect of both the
3-methyl-1,2-butadiene and the dicobalt complex and
is negative first order with respect of carbon monoxide. The
observed rate constants of the formation of 1−3
at 30
°C are 14.3 × 10-4, 1.75 ×
10-4, and 0.79 ×
10-4 s-1,
reproducible to within 5, 10, and 7%, respectively.