Free-Radical Chain-Growth Polymerization - Principles of Polymer Chemistry - page 67

Chapter 3
Free-Radical Chain-Growth Polymerization

3.1 Free-Radical Chain-Growth Polymerization Process

Polymerizations by free-radical mechanism are typical free-radical reactions. That is to say, there is
an initiation , when the radicals are formed, a propagation , when the products are developed, and a
termination , when the free-radical chain reactions end. In the polymerizations, the propagations are
usually chain reactions. A series of very rapid repetitive steps follow each single act of initiation,
leading to the addition of thousands of monomers.
This process of polymerization of vinyl monomers takes place at the expense of the double bonds,

-C-C-. Table 3.1 illustrates the steps in this process.
Formation of initiating radicals is the rate-determining step in the initiation reaction. The formation
of initiating radicals can result, as shown in Table 3.1 , from cleavages of compounds, such as peroxides,
or from other sources. Actually, many reactions lead to formations of free radicals. The initiating
radicals, however, must be energetic enough to react with the vinyl compounds. A linear correlation
exists between the affinities of some radicals for vinyl monomers and the energy (calculated) required to
localize a

C ¼ C- !

-carbon of the monomer [ 1 ]. By comparison to other steps in the
polymerization process, initiation is a slow step and requires high energy of activation.

p electron at the b

3.1.1 Kinetic Relationships in Free-Radical Polymerizations

A kinetic scheme for typical free-radical polymerizations is pictured as follows [ 2 ]; the rate of
propagation

R p is equal to the rate of polymerization R pol , as all the monomer molecules (except

one) are consumed during this step.

Initiation d ½ R = d t ¼ R i ¼ 2 k d ½ I

k d 2R

I !

d[R ]/d t ¼ k I ½ R ½ M

k i RM

R þ M !

R P ¼ k P P RM n Propagation ½ ½ M

k p

RM þ nM ! P n þ 1

Termination

2

k t

d[P ]/d t ¼ 2 k t ½ P

P n þ 1 þ P m þ 1 ! polymer

Transfer

k tr

P n þ S ! P n þ S

S þ nM ! SM n

In the above shown kinetic scheme, M stands for the monomer concentration, I is the concentration
of the initiator, and [R￿] and [P￿] mean the concentration of primary and polymer radicals,