# Polymer Chemistry. The Basic Concepts - Himenz P.C.

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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.

§ Ibid.

Problems

499

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

and FL:

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

mechanism.

t J. B. Kinsinger, T. Fischer, and C. W. Wilson, Polym. Lett. 5:285

(1967). tlbid.

500

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

values of

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

95 0.27

60 0.24

30 0.22

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

interface

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 =

VhKvh +

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

unsplit.

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).

Problems

501

13 Ñ (PPm) Relative area under peak

45.38 0.10

44.94 0.28

44.25 0.13

43.77 0.19

42.84 0.09

The remaining 21% of the peak area is distributed among the

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