Chemistry Reference
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2.2
10
3
mol substrate per mol catalyst at 50
activity (turnover number
Cin
23 h) [32]. The same system was recently used for functionalized olefins such as
methyl
N
-(3-butenyl)carbamate, phenyl
N
-(2-propenyl)amide [112] and some vinyl
epoxides [113]. Highly isotactic enantioselective copolymerization, however, with
low catalytic activity (turnover frequency up to
37 mol (mol h)
-1
at 45
C), with
propene, 1-heptene or 1-octene as the substrate, was achieved with a catalytic sys-
tem modified by Ddppi (
67
, Scheme 8.14) [34, 114]. Using the same system, func-
tionalized olefins such as
-undecenyl alcohol, ethyl and butyl acrylate were also
copolymerized [34, 115]. The most active class of ligands identified so far for pro-
pene is that of the
C
1
-symmetric ferrocenylphosphines
87
-
91
[116, 117]. Central
and planar chirality, large substituents on the phosphorus bound to the asym-
metric carbon atom, and aryl groups on other phosphorus atom containing elec-
tron withdrawing substituents (see ligand
91
) are all essential for high catalytic ac-
tivity and regularity of the copolymers [117]. The first investigated ligand, josiphos
(
90
), gave good results in the copolymerization of other olefins such as 1-butene,
4-methyl-1-pentene and various allylbenzenes [118].
As mentioned above for dppp
6
, diphenylphosphino-disubstituted chelate di-
phosphine ligands usually copolymerize propene to regio- and stereoirregular
materials [104]. A notable exception was found in the 1,2-bis[(diphenylphosphino)
methyl)]benzene ligand
92
, for which irregularity is quite low [119]. This irregular-
ity was further reduced when the aryl substituents contained electron withdraw-
ing groups, such as
93
. According to the analysis of the low-molecular weight by-
products, which are formed under conditions similar to those applied for the co-
polymerization reactions, ligands
93
and
94
insert propene with essentially re-
versed regiochemistry [119]. However, the copolymers formed with the two catalyt-
ic systems are quite similar with respect to stereochemistry (prevailingly isotactic)
and regiochemistry (about 98% content of the h-t enchainment).
It is very interesting that the stereochemistry of the ligand (
C
2
vs.
C
s
-symmetry,
95
and
96
, Scheme 8.16) does not affect the stereochemistry of the produced co-
polymer, which is regioregular and isotactic in both cases; this is in sharp contrast
to the behavior of zirconocene catalysts having the same symmetry for olefin poly-
merization [120] and to the trend observed in the polymerization of propene by
nickel-diimine catalysts [121]. However, the reactivity of the meso-ligand
96
is
about 84
that of the rac-ligand
95
for propene, whereas the reactivity ratio is
only 2.4 when ethene is used as the substrate. Similar but weaker effects were ob-
served for diastereomeric
13
and
14
and for their
o
-anisyl-substituted homologues,
for which, however, the copolymerization lacks regioselectivity. There is clearly an
important steric contribution to the difference in reactivity due to the fact that
propene has two enantiofaces. These results were rationalized by assuming the
formation (Scheme 8.17) of the intermediate responsible for the growth of the
chain (i.e., the intermediate for which there is a
l
-topicity between the growing
chain and the coordinated olefin) in low concentration for the racemic ligand, pos-
sibly because of steric reasons [122].
Analogous to the case of styrene, the terpolymerization reactions of propene with
ethene were carried out to identify the factors responsible for enantioface discrimi-
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