Chemistry Reference
In-Depth Information
constitution of most vinyl polymers because of the influence of resonance and ste-
ric effects.
H
H
H
H
H
CH
2
C
CH
2
C
CH
2
C
C
CH
2
CH
2
C
F
F
F
F
F
Head−to−Tail
Head−to−Head
Tail−to−Tail
1-57
Vinyl monomers polymerize by attack of an active center (1-58) on the double
bond.
Equation (1-14)
represents head-to-tail enchainment:
Y
Y
Y
Y
CH
2
C
+ CH
2
C
CH
2
C
CH
2
C
(1-14)
X
X
X
X
1-58
1-59
while
Eq. (1-15)
shows the sequence of events in head-to-head,
tail-to-tail
polymerization:
Y
Y
Y
Y
CH
2
C
+ CH
2
C
CH
2
C
C
CH
2
(1-15)
X
X
X
X
1-60
carbon bond (Chapter 8).
In any event the propagating species 1-59 will be more stable than its counterpart
1-60 if the unpaired electron or ionic charge can be delocalized across either or
both substituents X and Y.WhenX and/or Y is bulky there will be more steric hin-
drance to approach of the two substituted C atoms than in attack of the active cen-
ter on the methylene C as in reaction (1-14). Poly(vinyl fluoride) contains some
head-to-head linkages because the F atoms are relatively small and do not contrib-
ute significantly to the resonance stabilization of the growing macroradical.
Positional isomerism is not generally an important issue in syntheses of poly-
mers with backbones that do not consist exclusively of enchained carbons. This is
because the monomers that form macromolecules such as poly(ethylene tere-
phthalate) (1-5) or nylon-6,6 (1-6) are chosen so as to produce symmetrical poly-
meric structures that facilitate the crystallization needed for many applications
of these particular polymers. Positional isomerism can be introduced into such
macromolecules by using unsymmetrical monomers like 1,2-propylene glycol
The active center may be a free-radical, ion, or metal
