Chemistry Reference
In-Depth Information
type of species are dormant this was simplified further to
P
w
=P
n
1
þ
1
=b
. Generally, the molecular
weight distributions are narrower if the monomer is added slowly [
400
].
2
2
Y ¼ a
ð
1
aÞð
1
lÞ=½a þ lð
1
aÞ
; Y<
1
-heterocyclic carbenes are potent catalysts for group transfer
polymerization of acrylic esters. The reaction is illustrated as follows:
Scholten et al. [
401
] reported that
N
N
N
R
R
R
Si
H
O
O
+
n
R
O
O
O
O
O
O
O
O
or
N
N
0.1-0.5 moles of catalyst are used in tetrahydrofuran. The reaction is quenched with methyl
alcohol. The product had a narrow molecular weight distribution.
4.10 Configurational Statistics and the Propagation Mechanism
in Chain-Growth Polymerization
Analyses of polymers to determine stereosequence distributions and understand the propagation
mechanism can be carried out with NMR spectroscopy aided by statistical propagation models
[
222
,
402
,
403
]. A detailed discussion of the subject is beyond this topic. The following is a brief
explanation of the concepts. The Bernoulli, Markov, Colman-Fox, models describe propagation
reactions with chain end control over monomer placement. The Bernoulli model assumes that the
last monomer unit in the propagating chain end determines the stereochemistry of the polymer. No
consideration is given to the penultimate unit or other units further back. In such an event, two modes
of propagation are possible, meso and racemic:
P
m
P
r
The statistical probabilities,
P
m
and
P
r
, which are called
conditional probabilities
or
transitions
,of
forming meso or racemic dyads are defined by the following equations:
P
m
¼ R
m
=ðR
m
þ R
r
Þ
P
r
¼ R
r
=ðR
m
þ R
r
Þ
P
m
þ P
r
¼
1
