Chemistry Reference
In-Depth Information
Monomers that yield radicals in which the unpaired electron is extensively
delocalized have ground state structures that are themselves resonance stabilized.
The important factor is the relative stability of the product radical, however,
because a single electron is more easily delocalized than one in a C
C double
bond. Thus, resonance stabilization causes an increase in monomer reactivity and
a decrease in reactivity of the resulting polymer radical. Styrene is more reactive
toward polymerization than vinyl acetate, for example, and the propagation rate
in the former polymerization is much slower than in the radical synthesis of poly
(vinyl acetate).
Nonpolar radicals follow a consistent pattern, in which the basic reactivity
decreases with more extensive delocalization of the unpaired electron. Efficient
copolymerization between nonpolar monomers occurs only when both or neither
is resonance stabilized. The double bonds in styrene and butadiene are conjugated
and the unpaired electrons are extensively delocalized in both corresponding radi-
cals. These monomers copolymerize but neither reacts with vinyl chloride to any
appreciable extent, because the addition of the latter monomer to the styryl or
butadiene radical is energetically unprofitable. Similarly, the chain transfer con-
stants of cyclohexane and of toluene are several hundredfold greater in
vinyl acetate than in styrene polymerizations. Also, since abstraction of a hydro-
gen atom from the side group of isopropylbenzene produces a more stable radical
than that resulting from the same reaction on the methyl group of toluene, the
former
Q
is a more generally reactive chain transfer agent
in free-radical
polymerizations.
Basic reactivity is controlled by the extent of delocalization of the unpaired
electron. When the radical is significantly polar, however, its behavior during a
propagation reaction is no longer exclusively determined by the potential for reso-
nance stabilization.
9.10.2
Polar Effects
Free radicals and vinyl monomers are neutral, but variations in the reactivities of
both species can be rationalized and predicted by considering that the transition
states in their reactions may have some polar character. Appropriate substituents
may facilitate or hinder a particular reaction because of their influence on the
polarity of the reaction site.
An example from micromolecular chemistry involves the selectivity of hydro-
gen and chlorine atoms in abstraction of hydrogen from propionic acid (9-1).
Hydrogen atoms attack at carbon atom 2 more rapidly than at carbon 3, in accor-
dance with the relative strengths of the
3
CH
3
2
1
CH
2
COOH
9-1
