Page 263 - Carrahers_Polymer_Chemistry,_Eighth_Edition

226 Carraher’s Polymer Chemistry

The reactivity ratios are determined by an analysis of the change in the composition of the feed
during the early stages of polymerization. Typical free radical chain copolymerization reactivity
ratios are given in Table 7.1. The copolymer composition and type can be predicted by looking at
the values of r and r . If the value of r is greater than 1, then M tends to react with itself produc-
1 2 1 . 1
ing homopolymers, or block copolymers in M . When r is greater than 1 and r less than 1, then a
1 1 2
copolymer is formed with blocks in M (A; Equation 7.20).
1
AAAAAABAAAAAAABAAAAAABAAAAAAABAAAAAAAABABAAAAAAA (7.20)

If both r and r are greater than 1, then a copolymer is formed with blocks of M (A) and blocks
1 2 1
of M (B) (Equation 7.21).
2
AAAAAABBBBBAAAAAABBBBBBBBAAAAAABBBBBBAAAAABBBAAAA (7.21)
The length of each block will be some statistical average with the statistical length dependent on
how large the reactivity ratios are. In general, the larger the value of the reactivity ratio the larger
the block will be.
Preference for reaction with the unlike monomer occurs when r is less than one. When r and
1 1
r are approximately equal to 1, the conditions are said to be ideal, with a random (not alternating)
2
copolymer produced, in accordance with the Wall equation. Thus, a random copolymer (ideal copol-

ymer) would be produced when chlorotrifluoroethylene is copolymerized with tetrafl uoroethylene
(Table 7.1).
When r and r are both approximately zero, as in the case with the copolymerization of maleic
1 2
anhydride and styrene, an alternating copolymer is produced (Equation 7.22). In general, there will
be a tendency toward alternation when the product r r approaches zero.
1 2
ABABABABABABABABABABABABABABABABABABABA (7.22)

In contrast, as noted above, if the values of r and r are similar and the product r r approaches
1 2 1 2
1, tendency will be to produce random copolymers (Equation 7.23).
ABABBABAABABBAABABBBABAABAABBAABABBAABABBABAAABABBA (7.23)

In batch, reactions where the yield is high and no additional monomer is added at the end of the
reaction, the average content of each monomer unit in the total copolymer formed will necessarily
be directly related to the initial ratio of the two monomers. Thus, the copolymer content will vary

depending on when the particular chains are formed but the final overall product will have an aver-
age of units in it reflective of the initial monomer concentrations.

The values for r and r are used to help guide the ratio of reactants necessary to achieve a
1 2

specific copolymer composition. When a desired copolymer composition is required and either
reactivity ratio is unlike and not near zero, continuous flow processes or batch processes, where

monomer is continuously added, are employed. Considering the reaction between VAc and VC,
r is 0.23 meaning growing chains with VAc radicals at their ends have a greater tendency to
1
react with a VC monomer than a VAc monomer. The value for r is 1.68 meaning that there is
2
a tendency for growing chains with VC radical ends to add to VC monomer rather than VAc
monomer. Thus, given both monomers present in equal molar amounts, the beginning polymer
formed will be rich in VC-derived units. As the amount of VC is depleted, the forming copol-
ymer chains will have greater amounts of VAc until all of the VC is almost used up and chains
with almost only VAc units will be formed. If a copolymer composition that favors a copolymer

rich in VC is desired, then the concentration of VC will have to be maintained sufficient to give

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