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for the growth of the chain and that secondary insertion leads to chain interrup-
tion due to rapid
-hydrogen elimination [95, 97].
Systems modified by hybrid P-N ligands (
68
-
75
) were also found to be active
for the copolymerization of styrene. In contrast to the N-N ligand [8, 9], their pro-
ductivity is increased by increasing the pressure of carbon monoxide. In the case
of the phosphine-dihydrooxazole ligands (
68
-
72
) the geometry of the ligand is
very important for the steric control. The presence of only one substituent on po-
sition 4 of the dihydrooxazole ring (e.g.,
69
) is essential to achieve isotactic copoly-
merization [98]. Chirality associated with the presence of two different substitu-
ents in that position (Scheme 8.14,
72
) is not sufficient to cause efficient
l
-enantio-
face discrimination [99]. On the basis of these results, the model for styrene coor-
dination (
76
) reported in Scheme 8.15 was assumed.
Remarkably, pyridine-dihydrooxazole ligands with the same chiral moiety as the
previous ligands (compare
59
and
60
with
68
and
69
) produce prevailingly syndio-
tactic copolymers. The sign of the low optical activity of those copolymers, how-
ever, is consistent with the presence of small amounts of
l
-dyads, for which the
same enantioface, as in the case of the corresponding P-N ligands, is prevailingly
inserted (compare
78
vs.
76
in Scheme 8.15); however, intermediates
77
and
77
alternatively, and not
78
, are prevailingly responsible for chain growth. Model
studies on alkyl and acetyl complexes of these ligands are in keeping with this in-
terpretation [100, 101]. It is remarkable that regioirregular styrene copolymers
have become accessible by increasing the size of the arene substituents on phos-
phorus in the phosphine-dihydrooxazole ligands. According to the proposed
model (Scheme 8.15), steric repulsion by those substituents and the growing
chain would make a possible intermediate, similar to
76
, unfavorable, and would
cause a change in the olefin coordination site as in
79
and
79
. Again, steric repul-
sion between substituents on the double bond and on the arene causes a switch
in the preferred regiochemistry towards
79
and
79
.
Scheme 8.15
Model for enantioface selection dur-
ing styrene-carbon monoxide copolymerization
using diphenylphosphine- (
76
) or pyridine-dihydro-
oxazole (
77,77
,
78
) chelate ligands and for inver-
sion of regiochemistry of the insertion (
79
,
79
) for
sterically hindered diarylphosphine-dihydrooxazole
ligands (GPC=growing polymer chain).
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