Page 134 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polycondensation Polymers 97

T 3

T 2

DP
T 1

where T > T > T
1 2 3

Reaction time

FIGURE 4.4 Idealized average molecular weight of formed polymer as a function of reaction time and tem-
perature, T, for chain-type polymerization.

TABLE 4. 1
Comparison between Stepwise and Chain-wise Polymerizations
Chain Step
Growth occurs by addition of one unit at a time to the active Any two unlike molecular units can react
growing chain end
Monomer concentrations decrease steadily throughout the Monomer disappears early in the reaction
polymerization
Polymer chains are formed from the beginning of the Polymer chain length increases steadily during the
polymerization and throughout the process polymerization
Average chain length for reacted species remains Average molecular weight for the reaction (for the
approximately constant throughout the polymerization reacted species) increases throughout the
polymerization
As reaction time increases polymer yield increases, but High “extents” of reaction are required to obtain high
molecular weight remains about the same chain lengths
Reaction mixture contains almost only unreacted monomer, Reaction system contains various stages, chain lengths,
polymer, and very little growing polymer chains of product present in a calculable distribution

Most addition polymers are formed from polymerizations exhibiting chain-growth kinetics. This
includes the typical polymerizations, via free radical or some ionic mode, of the vast majority of vinyl
monomers such as vinyl chloride, ethylene, styrene, propylene, methyl methacrylate, and vinyl acetate.
By comparison, most condensation polymers are formed from systems exhibiting stepwise kinetics.
Industrially, this includes the formation of polyesters and polyamides (nylons). Thus, there exists a
large overlap between the terms stepwise kinetics and condensation polymers and chain-wise kinetics
and addition (or vinyl) polymers. A comparison of the two types of systems is given in Table 4.1.
Even so, there is not a total overlap between the various characteristics of vinyl-chain kinetics
and condensation-step kinetics. Following are examples illustrating the lack of adherence to this
overlap.

1. The formation of polyurethanes (PU) and polyureas typically occur through stepwise
kinetics with the polymer backbones clearly containing noncarbon atoms. Yet, no byprod-
uct is released through the condensation process because condensation occurs through an
internal rearrangement and shift of the hydrogen—neither steps resulting in expulsion of a
byproduct.

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