Addition polymerization
Addition polymerization
Addition
polymerization occurs between molecules containing double or triple bonds; but
in certain cases it can also occur between bifunctional compounds that result
from the opening or ring structures. There is no liberation of small molecules
during addition polymerization.
A very
important group of olefinic compounds that undergo addition polymerization is
of the type CH2=CHY, where Y may be H, X, CO2R, CN, etc.,
nCH2=CHY → (—CH2CHY—)n
There are
three possible ways in which this polymerization can occur:
- Head to tail: —CH2CHY—CH2CHY—
- Head to head and tail to tail: –CHYCH2–CH2CHY–CHYCH2–CH2CHY–
- A random arrangement involving (i) & (ii)
Experimental work indicates that (1) is
favoured.
Most
polymerizations are carried out in the presence of catalysts, and
polymerization of alkenes can be accelerated by ionic-type catalysts or radical
type catalysts. Both types of reaction consist of a number of steps which
follow one another consecutively and rapidly, and take place in three principal
stages.
- The initiation or activation.
- The growth or propagation
- The termination or cessation.
The ionic
mechanism of
catalysis : Ionic catalysts are usually Lewis acids, e.g., H2SO4,
HF, AlCl3, BF3, etc. The ionic mechanism may be
illustrated by the polymerization of isobutene in the presence of sulphuric
acid.
This
polymerization, carried out in the presence of a small amount of butadiene,
produces butyl rubber. The most important ionic catalysts are those introduced
by Ziegler and Natta.
The
free-radical mechanism of catalysis : Addition polymerization may take place by chain reactions
brought about by catalysts that are known to generate free radicals. The most
widely used catalysts are the organic and inorganic peroxides and the salts of
the per-acids, e.g., benzoyl peroxide, acetyl peroxide,
hydrogen peroxide, potassium perborate, etc.
This type of
mechanism is illustrated for polymerizations in the presence of organic
peroxides.
(i)
(RCO)2O2 → 2RCO2. → 2R.
+ 2CO2
R. + CH2=CH2 → R-CH2-CH2.
(ii) R – CH2‒CH2.
+ nCH2=CH2 → R-(CH2-CH2)n-CH2‒CH2.
(ii) The
termination reaction may take place in various ways. One obvious way is by the
combination of two growing chains, i.e.,
2R—(CH2‒CH2)n. → R-(CH2-CH2)2n-R
On the other hand, termination may also occur by
disproportionation :
2R‒(CH2-CH2)n-CH2-CH2. → R-(CH2-CH2)n-CH2CH3
+R—(CH2-CH2)n‒CH=CH2
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