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878 Macromolecules, Structure

do not necessarily appear as singlets if the sequence as a
whole lacks a twofold axis.
The numbers of observationally distinguishable config-
urational sequences, or n-ads, designated N(n), obey the
relationship

n 2 3 4 5 6 7 8 9
N(n)236 10 20 36 72 136

or in general
N(n) = 2 n −2 + 2 m −1 , (47)
where m = n /2if n is even and m = (n − 1)/2 if n is odd.
Discrimination of these longer sequences is unlikely to be
possible beyond n = 6 (hexads) or n = 7 (heptads). The
FIGURE 21 Infrared spectra of predominantly (a) syndiotactic observation of such sequences permits rather searching
and (b) isotactic films of polymethyl methacrylate. tests of polymerization mechanisms.
Another example is provided by polypropylene, partic-
ularly instructive as it is one of the few vinyl polymers
that can be prepared in both
so if strongly complexing ether solvents such as dioxane
or glycol dimethyl ether are employed rather than hydro-
CH 3
carbon solvents as in polymer (b) of Fig. 19.
[ CH CH 2 [
Vibrational spectra also reveal stereochemical differ- n

ences. In Fig. 21 infrared spectra of films of predominantly
syndiotactic (a) and isotactic (b) methyl methacrylate isotactic and syndiotactic forms with coordination cata-
polymers are shown. In addition to other smaller differ- lysts. The proton spectra are relatively complex because
ences, there is a conspicuous band at 1060 cm −1 in the of vicinal coupling between α and β protons and α and
syndiotactic polymer spectrum (arrow), which is absent methyl protons as well as geminal methylene proton cou-
in that of the isotactic polymer. Such observations can pling in isotactic sequences. In Fig. 22, 220-MHz proton
serve as quick measures of chain stereochemistry, but in spectra of isotactic (a) and syndiotactic (b) polypropylene
general infrared is not as discriminating nor as quantitative are shown. The β protons of the syndiotactic polymer ap-
as NMR. pear as a triplet at 1.03 ppm corresponding to a single
It is evident that in the spectra of Fig. 19 there is fine chemical shift and J-coupling to two neighboring α pro-
structure in both the methylene and methyl regions that we tons. In the isotactic polymer they appear as widely spaced
have not discussed. In spectrum (a) this corresponds prin- multiplets at 1.27 and 0.87 ppm, corresponding to syn and
cipally to residual resonances of the stereoirregular por- anti positions in the trans–trans conformation:
tions of the chains; in (b) such residual resonances are less
H H
conspicuous. These arise from sensitivity to longer stereo- (ANTI) (SYN)
chemical sequences than dyad and triad. In Table III planar
H CH 3 H
zigzag projections of such sequences, together with their CH 3
frequency of occurrence as a function of P m , assuming
Analysis of these spectra yields the following values for
Bernoullian propagation are shown. The tetrads—and all
the vicinal main-chain couplings (in both polymers the
“even-ads”—refertoobservationsof β-methyleneprotons
vicinal CH 3 H α coupling is 5 Hz and germinal methylene
(or β carbons), while the “odd-ads” refer to substituents on
proton coupling is −13.5 Hz):
the α-carbons (or α-carbons themselves). Resonances for
Isotactic
tetrad sequences or higher even-ads should appear as fine
structure in the dyad spectra, while pentad sequences or H syn :6.0Hz
J H α
higher odd-ads should appear as fine structures on the triad
resonances. The assignments to longer sequences as indi- J H α H anti :7.0Hz
cated on the spectra are based on Bernoullian probabilities
Syndiotactic
in spectrum (a); those in spectrum (b) are primarily based
on (a). It may be noted that r-centered tetrads (e.g., mrr) J CH 2 CH 2 :4.8, 8.3Hz