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yield the polyenes like that whose spectrum is shown in Fig. 7.5. The
specific absorbance at the indicated wavelengths was measured for 1%
solutions of the products after HC1 elimination:
t A. Winston and P. Wichacheewa, Macromolecules 6:200 (1973). tlbid.
fl Specific absorbance
X = 312 nm X = 367 nm X = 41 2 nm
0.829 74 13 -
0.734 71 19 -
0.551 154 77 20
0.490 151 78 42
As noted in Sec. 7.7, these different wavelengths correspond to
absorbance by sequences of different lengths. Compare the appropriate
absorbance ratios with the theoretical sequence length ratios calculated
above and comment briefly on the results.
7. Use the values determined in Example 7.6 for the vinylidene chloride
(M^-isobutylene (M2) systemf to calculate Fi for various values of fi
according to the terminal mechanism. Prepare a plot of the results. On
the same graph, plot the following experimentally measured values of fi
fi Fi fi Fi
0.548 0.83 0.225 0.66
0.471 0.79 0.206 0.64
0.391 0.74 0.159 0.61
0.318 0.71 0.126 0.58
0.288 0.70 0.083 0.5 2
Comment on the quality of the fit.
8. Some additional dyad fractions from the research cited in the last
problem^ are reported at intermediate feedstock concentrations (ML =
vinylidene chloride; M2 = isobutylene):
fi Mole fraction of dyads
11 12 22
0.418 0.55 0.43 0.03
0.353 0.48 0.49 0.04
0.317 0.44 0.52 0.04
0.247 0.38 0.58 0.04
0.213 0.34 0.62 0.04
0.198 0.32 0.64 0.05
Still assuming terminal control, evaluate rL and r2 from these data.
Criticize or defend the following proposition: The copolymer composition
equation does not provide a very sensitive test for the terminal control
t J. B. Kinsinger, T. Fischer, and C. W. Wilson, Polym. Lett. 5:285
Polymers with Microstructure
Dyad fractions are more sensitive, but must be examined over a wide
range of compositions to provide a valid test.
9. Fox and Schneckof carried out the free-radical polymerization of
methyl methacrylate between -40 and 250 C. By analysis of the a-methyl
peaks in the NMR spectra of the products, they determined the following
a, the probability of an isotactic placement in the products
prepared at the different temperatures:
T(°C) a T(°C) a
250 0.36 0 0.20
150 0.33 - 20 0.18
100 0.27 -40 0.14
Evaluate Ej* - Es* by means of an Arrhenius plot of these data using
a/( 1 - a) as a measure of kj/ks. Briefly justify this last relationship.
10. As apparent from structure [XVIII], a hetero triad occurs at each
between iso and syndio triads. The total number of hetero triads,
therefore, equals the total number of sequences of all other types:
= 2Nni + 2Nns..
Use this relationship and Eq. (7.71) to derive the expression Ph =
vi + "s) = 2/nj + ns. Criticize or defend the following
proposition: The sequence DL_ is already two-thirds of the way to
becoming a hetero triad, while the sequence DD_ is two-thirds of the way
toward an iso triad. This means that the fraction of heterotactic triads
is larger when the average length of syndio sequences is greater than the
average length of iso sequences.
11. Randall$ used 13C-NMR to study the methylene spectrum of
polystyrene. In 1,2,4-trichlorobenzene at 120°C, nine resonances were
observed. These were assumed to arise from a combination of tetrads and
hexads. Using m and r notation, extend Table 7.8 to include all 20
possible hexads. Criticize or defend the following proposition: Assuming
that none of the resonances are obscured by overlap, there is only one
way that nine methylene resonances can be produced, namely, by one of the
tetrads to be split into hexads while the remaining tetrads remain
12. In the research described in the preceding problem, Randall:}: was
able to assign the five peaks associated with tetrads in the 13C-NMR
spectrum on the basis of their relative intensities, assuming zero-order
Markov (or Bernoulli) statistics with pm = 0.575. The five tetrad
intensities and their chemical shifts from TMS are as follows:
tT. G. Fox and H. W. Schnecko, Polymer 3:575 (1962).
tj. C. Randall,/. Polym. Sci. Polym. Phys. Ed. 13:889 (1975).
13 Ñ (PPm) Relative area under peak
The remaining 21% of the peak area is distributed among the