Polyisobutylene conformations

Some of the K values in Table 7 are not new, but are taken unchanged from the tabulations of Flory (5f) and Chinai (64). However, the a values have all been recalculated with the preferred theoretical number, 2.87 1021, for 0O (of course, with proper allowance for the effects of heterogeneity, as discussed in Section IIE). This revision has an interesting consequence in the case of polyisobutylene, for which Hoeve (124) has made a theoretical prediction of a using an internal rotation potential derived from the known helical conformation of the crystalline polymer. 

M. Vacatello and D. Y. Yoon, Macromolecules, 25, 2502 (1992). Conformational Statistics of Polyisobutylene by a Monte Carlo Study. 

For nonpolar linear polymers that form coils, Hershey and Zakin observed 40% less DR for polyisobutylene in benzene, a poor solvent, than in cyclohexane, a good solvent.Hunston and Zakin showed that polystyrene in the good solvents, toluene or benzene, gave more DR than the same samples in a poor, mixed solvent of toluene and / 6>-octane as shown in Figs. 1 and These results and those of White and Gordon showed that expanded coil conformations. 

To describe the effects of steric restrictions in another polymer, polyisobutylene, consider the Newman projections of the staggered conformations of two adjacent carbons in its repeat unit, as shown in Fig. 2.6. Here the chain substituent on the rear carbon shown is either between a methyl group and polymer chain or between two methyl groups on the front carbon. There is no significant energy difference between the conformers. Since no conformation is favored, polyisobutylene will tend to spiral into a helix (gauche conformers) as well to form into a zigzag (If),... 

Considering again two adjacent carbons in the main chain of the polymer, six conformations are now possible because of the presence of an asymmetrically substituted carbon atom, as shown in Fig. 2.6. Forms 1 and 6 can be neglected for steric reasons so four different conformations are still possible for the polymer. Atactic polypropylene (see Stereoisomerism) has two trans forms (2 and 5) in the fully extended state (IH) and so, unlike polyisobutylene, is incapable of crystallizing upon being stretched. Isotactic polypropylene, however, having all the methyl groups on one side, crystallizes easily. 

Geometric factors, such as the symmetry of the backbone and the presence of double bonds on the main chain, affect Tg. Polymers that have symmetrical structure have lower Tg than those with asymmetric structures. This is illustrated by two pairs of polymers polypropylene vs. polyisobutylene and poly(vinyl chloride) vs. poly(vinylidene chloride) in Table 4.4. Given our discussion above on chain stiffness, one would have expected that additional groups near the backbone for the symmetrical polymer would enhance steric hindrance and consequently raise Tg. This, however, is not the case. This discrepancy is due to conformational requirements. The additional groups can only be accommodated in a conformation with a loose structure. The increased free volume results in a lower Tg. 

In contrast to polyethylene, it is impossible sterically to accommodate the two methyl groups in polyisobutylene in the planar zigzag conformation. However, a hehcal conformation can accommodate the pendant groups. 

Another important result is the similarity of the temperature variation of the correlation time r, associated with conformational jumps, and observed for all the polymers considered except polyisobutylene, to the predictions of the Williams-Landel-Ferry equation for viscoelastic relaxation, which indicates that the segmental motions observed by NMR belong to the glass-transition phenomenon. Moreover, the frequency of these intramolecular motions is mainly controlled by the monomeric friction coefficient of the polymer matrix. 

A common use of the rotational isomeric state model is to learn how speciflc structural properties (at the local level) affect conformational properties at the level of a long chain. Polyisobutylene provides a good example. The bond angles in the backbone of pol5dsobutylene alternate between 110° (for CH2—C—CH2) and 124° (for C—CH2—C). How strongly, and in what direction, does this alternation in bond angles affect the mean square dimensions of the chains in a polyisobutylene... 

Figure 2.5 (a) Schematic diagram showing two adjacent carbons, C and C +i, in the main chain of polyisobutylene. (b) Newman projections of staggered conformations of adjacent carbons in the main chain. 

A characteristic property of amorphous polymers is the ability to sustain large strains. For cross-linked three-dimensional networks the strain is usually recoverable and the deformation process reversible. The tendency toward crystallization is greatly enhanced by deformation since chains between points of cross-linkages are distorted from their most probable conformations. A decrease in conformational entropy consequently ensues. Hence, if the deformation is maintained, less entropy is sacrificed in the transformation to the crystalline state. The decrease in the total entropy of fusion allows crystallization, and melting, to occur at a higher temperature than would normally be observed for the same polymer in the absence of any deformation. This enhanced tendency toward crystallization is exemplified by natural rubber and polyisobutylene. These two polymers crystallize very slowly in the absence of an external stress. However, they crystallize extremely rapidly upon stretching. 

Somewhat special is the position of polyisobutylene, which is due to the crowding of the two bulky methyl groups on the a carbon. Stereoregularity considerations are irrelevant in this case. Because of the crowding of the methyl groups, conformational states are separated by low barriers, and thus under normal conditions, at room temperature without stress, the polymer behaves like a rubber. When oriented by drawing, the chain shows a tendency to crystallize, with a chain conformation of an 83 helix, which is near to the tg conformation [41]. This behavior is a result of the regularity of the chain. 

While polyethylene adopts a planar zig-zag conformation in the crystalline state, polyisobutylene is a helix. This conformation allows the pendant methyl groups to have more freedom while maintaining a perfectly repeating stmcture. Most polymer chains are helical in the crystalline state. 

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