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:
  1. Head to tail:  —CH2CHY—CH2CHY—
  2. Head to head and tail to tail:   –CHYCH2–CH2CHY–CHYCH2–CH2CHY–
  3. 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.

  1. The initiation or activation.
  2. The growth or propagation
  3. 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.

Addition polymerization

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