Page 132 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polycondensation Polymers 95

The formation of polyesters from a dialcohol (diol) and a dicarboxylic acid (diacid) will be used
to illustrate the stepwise kinetic process. Polymer formation begins with one diol molecule reacting
with one diacid forming one repeat unit of the eventual polyester (4.3).

O O O O
− − − − − − − − (4.3)
HO−C−R−C−OH + HO−R −OH HO−C−R−C−O−R −OH + H O
2
This ester-containing unit can now react with either an alcohol or an acid group producing chains
ending with either two active alcohol functional groups or two active acid groups (4.4)

O
+ HO−R −OH HO−R −O−C(O)−R−C(O)−O−R −OH + H 2
HO−C(O)−R−C(O)−O−R −OH
+ HO−C(O)−R−C(O)−OH HO−C(O)−R−C(O)−O−R −O−C(O)− (4.4)
RC(O)−OH + H O
2

The chain with two alcohol ends can now condense with a molecule containing an acid end
and the chain with two acid ends can now condense with a molecule containing an alcohol group
resulting in molecules that contain one acid and one alcohol active group. This reaction continues
through the reaction matrix whenever molecules with the correct functionality, necessary energy of
activation, and correct geometry collide. The net effect is the formation of dimers, trimers, and so
on, until polymer is formed. The monomer concentration is low to nonexistent during most of the
polymerization.

HO–R –O–C(O)–R–C(O)–O–R –OH + HO–C(O)–R–C(O)–OH
(4.5)
HO–R –O–C(O)–R–C(O)–O–R –O–C(O)–R–C(O)–OH + H 2 O

HO–C(O)–R–C(O)–R –O–C(O)–RC(O)–OH + HO–R –OH
(4.6)
HO–C(O)–R–C(O)–R –O–C(O)–RC(O)–O–R –OH + H 2 O

The reactants are consumed with few long chains formed until the reaction progresses toward
total reaction of the chains with themselves. Thus, polymer formation occurs one step at a time,
hence the name “stepwise” kinetics.
Chain-growth reactions require initiation to begin chain growth. We will look at the free radical
growth forming polystyrene to illustrate chain growth. Here, the initiation of a styrene molecule,
R, will illustrate the chain-growth process. The initiator reacts with a styrene monomer creating a
free radical active chain end. This free radical chain end then reacts with another styrene monomer
that, in turn, reacts with another styrene monomer, and so on until termination stops chain growth
with the formation of a polystyrene polymer chain (Figure 4.3). Polystyrene chains are formed from
the beginning of the polymerization process with growth occurring through formation of polymer

chains. As the first chain is formed (Figure 4.3; middle), only polymer and monomer are present.
As additional polymer chains are formed (Figure 4.3; right), the polymer mix still consists of only
polymer chains and unreacted monomer. This is shown in the following sequences.

R
• •
R + H C CH
2
(4.7)

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