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Encyclopedia of Physical Science and Technology EN008C-602 July 25, 2001 20:31

886 Macromolecules, Structure

from extensive study of model hydrocarbons) are shown either vibrational or NMR—was not available, chemi-
here: cal methods had to be employed. Thus, to determine
the structure of poly(vinyl chloride) the Freund reaction
45.5-46.5 14.8 H CH 3
was employed. This employed metallic zinc and was be-
lieved to be specific for the removal of chlorine atoms in
H CH 3 H CH 3 H CH 3
, 1,3 positions:

H CH 2
37.9 37.5 CH 3
CH 2
CH CH
24.9 27.8 Zn CH CH ZnCl 2
H
H CH 3 CH 3 H CH 3 Cl Cl
, , , , ,

It thus would not proceed if the structure were head-to-
37.5 30.7 head: tail-to-tail. Since it did in fact remove most of the
chlorine from poly(vinyl chloride), it was concluded that
27.4
H CH 3 H CH 3 the polymer had a head-to-tail structure. NMR confirms

, , , , this conclusion.
A more subtle chemical approach has been applied to
H poly(vinyl alcohol). Here, it is assumed that the predom-
37.5 30.4 CH 3
inant structure is head-to-tail and the question posed was
27.4 the frequency of occurrence of the occasional inverted
H CH 3
unit. It is assumed that periodic acid reacts only with 1,2-

, , , , ,
glycol units and not with 1,3 units:
37.5 30.4
CH 2 CH CH 2 CH CH CH 2 CH 2 CH
H 27.4 30.0 H CH 3
CH 3
OH OH OH OH

, , , , , ,
HIO 4
CH 2 CHO OCHCH 2
The Greek letter designations indicate the position of
the nearest branch carbon. A plus sign indicates that the Inverted units in poly(vinyl alcohol) thus lead to chain
branch is at the δ or more distant position. Those methy- scission, the extent of which can be measured by the re-
lene carbons that are four or more carbons removed from duction of solution viscosity (Section II.F). It is concluded
a branch carbon give the same chemical shift (30.0 ppm). that 1–2% of head-to-head units are present, this mode of
Propylene inversions can be clearly identified through the propagation increasing with temperature.
presence of methylene sequences two and four carbons in Fluorine-substituted ethylenes are particularly subject
length. From these assignments and using a mathematical to the generation of inverted units, presumably because
model of chain growth that treats inverted propylene units fluorine atoms are relatively undemanding sterically. At
as a third monomer, a complete analysis of the statistical the same time, the physical properties of their polymers
distribution of sequences can be carried out. The extent may depend strongly on the presence of such units. The
of inversion is about 5%. A value of the product of reac- presence of 19 F offers an additional means for detailed
19
tivity ratios r 1 r 2 0.35 ± 0.04 was found. This corresponds study, as F chemical shifts are highly sensitive to struc-
to a propagation process not far from random but with a tural variables. Poly(vinyl fluoride), a commercial plas-
moderate tendency toward alternation of monomer units. tic having high resistance to weathering, has a substan-
tial proportion of head-to-head units. The 188-MHz 19 F
spectrum of a commercial polymer is shown in Fig. 29.
F. Regioregularity 19
The F H J-coupling multiplicity has been removed by
In the earlier days of polymer science it was not known proton irradiation as in 13 C spectroscopy (Section III.C).
whether vinyl polymers had head-to-tail or head-to-head: The resonance assignments, designated by capital letters,
tail-to-tail structures (Section I). Because spectroscopy— are:
A A B C D A
CH 2 CHF CH 2 CHF CH 2 CHF CHF CH 2 CH 2 CHF CH 2 CHF