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Scheme 8.12
Various palladium catalyst pre-
cursors modified with nitrogen ligands for
carbon monoxide-styrene copolymerization
(prevailingly syndiotactic (
51-60
), atactic (
61
),
and isotactic (
62-65
)).
shown, however, low diastereoselectivity for the second styrene insertion. Analo-
gously, 2-(1-pyrazolyl)pyridine
54
or the corresponding pyrimidine ligand gave a
prevailingly syndiotactic copolymer (
85%
u
-diads) [86]. The behavior of the pyri-
dine-imidazoline systems using 4-
tert
-butylstyrene as the substrate is more unusu-
al [87]. Whereas
55
, in spite of their chirality, gave copolymers with
u
-diads ex-
ceeding 82 to 85%, the analogous systems
61
gave largely atactic materials with a
content of
l
-diads in the range 52 to 65%. It is noteworthy, however, that the ge-
ometry of the alkyl catalyst precursors
55
and
61
differs. The binuclear pyridine-
imine complexes (
56
and
57
) both give a prevailingly syndiotactic styrene copoly-
mer [88]. The low optical activity of the material prepared with (
R
,
R
)-
57
is consis-
tent, however, with a larger concentration of
l
-diads with respect to
56
, as also
shown by NMR spectroscopy on the two copolymers.
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