Page 560 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Reactions on Polymers 523

extremely high molecular weight chains. Copolymers containing sodium acrylate and acrylamide
groups are used in tertiary oil recovery and in water purifi cation.

16.3 DEGRADATION

Here, the term degradation includes any change, decrease, in polymer property because of the
impact of environmental factors, namely light, heat, mechanical, and chemicals. Seven polymers
represent the majority of the synthetic polymers. These are the various polyethylenes (PE), poly-
propylene (PP), nylons, poly(ethylene terephthalate) (PET), PS, poly(vinyl chloride) (PVC), and PC.
Each of these has their own particular mode of degradation. Even so, there are some common gen-
eralities for the condensation (PET, PC, nylons) polymers that contain a noncarbon in the backbone
and vinyl (PE, PP, PS, PVC) polymers that contain only a carbon backbone.
Some reactions on polymers are intended and give a material with different desired properties.
The (positive) modification of polymers is an area of vigorous activity. Other reactions on polymers

are unintended and generally result in a material with unfavorable properties. Included in the latter
are a whole host of polymer degradation reactions. Some of these degradation reactions are cov-
ered elsewhere. Here we will focus on some general concepts. Degradation can be promoted by
many means and any combination of means. The major means of polymer degradation are given in
Table 16.1.
Backbone chain scission degradation can be divided as occurring via depolymerization, random
chain breakage, weak-link or preferential site degradation, or some combination of these general
routes. In depolymerization, monomer is split off from an activated end group. This is the opposite
of the addition polymerization and is often referred to as “unzipping.”

–M –M –M –M
P–MMMMMMMM → P–MMMMMMM → P–MMMMMMM → P–MMMMM → etc. (16.3)

Chain scission is similar to the opposite of stepwise polycondensation where units are split apart
in a random manner.
P − P → P + P (16.4)
m n m n

Depolymerization can result in backbone degradation and/or in the formation of cyclic or other
products. The thermal degradation of PVA and PVC occurs with the splitting-out of water or HCl

TABLE 16.1
Major Synthetic Polymer Degradative Agents

Degradation Agent (Most) Susceptible Polymer Types Examples
Acids and bases Heterochain polymers Polyesters, polyurethanes
Moisture Heterochain polymers Polyesters, nylons, polyurethanes
High-energy radiation Aliphatic polymers with quaternary Polypropylene, LDPE, PMMA,
carbons poly(alpha-methylstyrene)
Ozone Unsaturated polymers Polybutadienes, polyisoprene
Organic liquids/vapors Amorphous polymers
Biodegradation Heterochain polymers Polyesters, nylons, polyurethanes
Heat Vinyl polymers PVC, poly(alpha-methylstyrene)
Mechanical (applied Polymers below T g
stresses)

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