Biomedical Engineering Reference
In-Depth Information
10.19
Copolymerization Followed by Partial Degradation
Chemical reactions capable of cleaving the bonds that hold together the repeat
units are called degradation reactions. At completion, the degradation process yields
methane, propane, methanol, butanol, CO
2
or other small molecules. When the
reaction products are a mixture of dimers, trimers, tetramers and other short
A
m
B
n
oligomers, the degradation is not complete, and we refer this reaction as
partial
degradation
. The degradation process is referred as nonselective when A units and
B units are attacked indiscriminately. The degradation is referred to as totallyselec-
tive when only one of the two units (A or B) is attacked and the other unit remains
untouched by the degradation process. The sequence of the resulting copolymer de-
pends on the reaction used to synthetize the copolymer and also on the partial degra-
dation reaction [
73
]. The molar fraction,
I
A
m
B
n
, of the oligomer
A
m
B
n
is given by:
I
A
m
B
n
D ˚
1
.s; c
A
/;
(10.41)
where
˚
1
denotes generic dependence. Formulas for very short oligomers (dimers
and trimers) were derived long ago, and they turned out to be very useful for the
experimental work on gas chromatography (in fact dimers and trimers can be easily
brought in the gas phase). Formulas for longer oligomers were derived by our group
in the early 1990s. The reason is connected with the fact that we used mass spec-
trometers for sequence determination, and many MS instruments (at that time) could
not produce and detect ions above a certain mass limit. Using partial degradation,
we were able to reduce the chain length below the mass limit. For instance, using
(
10.41
) with
, we sequenced by MS an acrylonitrile-styrene
copolymer subjected to partial degradation by pyrolysis and a styrene-butadiene
subjected to partial degradation by ozonolysis. An entire collection of formulas has
been published [
73
].
s D 5; 6; 7; 8; 9; 10
10.20
Copolymerization with Sequence Constrains
In some copolymerization and terpolymerization reactions, the sequence AA or BB
or ABB is not formed. For instance, maleic anhydride (an industrially important
monomer) does not homopropagate (at least under usual free-radical conditions).
However, if a second monomer is added, the copolymerization reaction produces a
random copolymer. It is actually a pseudo-random copolymer, which differs from
a random one, due to the systematic absence of two consecutive maleic anhy-
dride units. The equations which define the sequence distribution are always the
same: the terminal model equation and the penultimate model equations. Simply,
some parameters are zero. Some authors obtained a pseudo-random copolymer
reacting sulphur dioxide with styrene, and they succeeded in performing NMR se-
quencing of the copolymer using an ultra-simplified pen-penultimate model [
74
].
The simplification was based on the fact that sulphur dioxide cannot homopoly-
merize. Braun et al. analysed by NMR the products of the terpolymerization of
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