Chemistry Reference
In-Depth Information
by a chain mechanism, it is relevant to cover the salient features of step-growth
polymerizations as related to ADMET, although detailed discussions of step-
growth polymerization are covered in a variety of excellent texts [32].
The kinetics of step-growth polymerizations demand that monomers of high
purity be used in order to achieve high molecular weights [32]. The inclusion of
even very small amounts of monofunctional impurities, monoolefins in the case
of ADMET, has a detrimental effect on the polymerization as implied by manipu-
lation (Eq. (1) below) of the Carothers equation, in which
is the number aver-
age degree of polymerization (DP),
f
imp
is the amount of monoolefin impurity as
a fraction of the amount of diene monomer, and p is the extent of reaction as a
fraction of unity. Assuming an extent of reaction of 0.99, the presence of 0.5%
monoolefin impurity cuts
X
n
X
n
by 20%, from 100 to 80.2, and the presence of 1%
impurity cuts
by 33% to 67. This equation also underlines the requirement
of high extents of reaction. Assuming pure monomer (
f
imp
=0), slightly reducing
p
from 0.995 to 0.990 to 0.985 has the effect of reducing
X
n
from 200 to 100 to
67. This result underscores the necessity of removing the condensate molecule,
ethylene in the case of ADMET.
X
n
2
f
imp
X
n
1
2
f
imp
2
p
Given these rigorous demands for the success of a polycondensation reaction,
there are several reasons why olefin metathesis is particularly well suited for poly-
condensation. The first is that although olefin impurities are detrimental to AD-
MET, other more common synthetic impurities are not, unless they react with the
catalyst. Secondly, like most polycondensations, ADMET is a reversible process
and removal of the condensate will shift the equilibrium towards higher molecu-
lar weight polymer. The condensate molecule for ADMET, ethylene, is a gas at
room temperature and is thus readily removed from the reaction mixture to drive
the reaction to high conversion (Scheme 6.4).
In the case of polyesterification, the condensate is water or an alcohol, which
are somewhat more difficult substances to remove from the reaction. Removal of
ethylene may be accomplished by applying vacuum to the reaction mixture or by
passing an inert carrier gas over the reaction, although moderately high molecular
weights can be achieved at room pressure without active removal of ethylene. For
these reasons, high DP is attainable via ADMET, as demonstrated by a reported
DP of almost 830 for the polymerization of 1,9-decadiene by a tungsten-based cat-
alyst [31].
Scheme 6.4
Shifting ADMET equilibrium
towards high molecular weight polymer
by facile removal of ethylene.
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