Conformation, isotactic polymers

Crystallinity of polypropylene is usually determined by x-ray diffraction (21). Isotactic polymer consists of heHcal molecules, with three monomer units pet chain unit, resulting in a spacing between units of identical conformation of 0.65 nm (Fig. 2a). These molecules interact with others, or different... 

A polymorphic behavior involving packing of chains having completely different conformations has been found also for isotactic polymers. For instance, isotactic polystyrene, under suitable experimental conditions, can produce crystalline gels in which the chains assume a nearly fully extended conformation [11,12], very close to a truns-planar, rather than the classical conformation of three-fold helix [13]. The two possible conformations proposed for the two crystalline forms of i-PS are shown in Fig. 2. 

The conformation of the chains of isotactic polymers in the crystalline state is generally helical and corresponds to a succession of nearly trans and gauche torsion angles, the exact values depending on the bulkiness of the side groups. Molecular mechanics calculations have been extensively used for the prediction of the chain conformation of polymers in the crystal.29... 

Calculations of the conformational energy are performed according to the equivalence principle and, as a consequence, a succession of backbone torsion angles. .. 0i020i 20i 2- is generally assumed for isotactic polymers. 

Figure 2.10 Maps of conformational energy of various isotactic polymers as function of backbone torsion angles 0i and 02 (a) Isotactic polystyrene, (b) polypropylene, (c) poly(l-butene), and (d) poly(4-methyl-l-pentene). Succession of torsion angles. .. 0i020i02 [s(M/N) symmetry] has been assumed. Isoenergetic curves are reported every 10 (a,c,d) or 5 (b) kJ/mol of monomeric units with respect to absolute minimum of each map assumed as zero.

Data concerning the chain conformations of isotactic polymers are reported in Table 2.1. In all the observed cases the torsion angles do not deviate more than 20° from the staggered (60° and 180°) values and the number of monomeric units per turn MIN ranges between 3 and 4. Chains of 3-substituted polyolefins, like poly(3-methyl-l-butene), assume a 4/1 helical conformation (T G )4,45,46 while 4-substituted polyolefins, like poly(4-methyl-1-pentene), have less distorted helices with 7/2 symmetry (T G )3.5-39 When the substituent on the side group is far from the chain atoms, as in poly(5-methyl-1-hexene), the polymer crystallizes again with a threefold helical conformation (Table 2.1). Models of the chain conformations found for the polymorphic forms of various isotactic polymers are reported in Figure 2.11. 

For the isotactic polymer, highly extended chains with chain axis of 7.5-7.6 A can be obtained with a helix repeating after two structural units [s(2/l) line repetition group] when 01 02, and 03 are in the range 180° 8, with 8 being 25-30° and 8i + 82 + 83 0.125 The experimental values of the torsion angles found in the crystal structure of i-STCO123 are indeed 0i = —161.5°, 02 = 155.3°, and 03 = -171.4°. Similar values have been found by conformational... 

In the crystal structures of many other isotactic polymers, with chains in threefold or fourfold helical conformations, disorder in the up/down positioning of the chains is present. Typical examples are isotactic polystyrene,34,179 isotactic poly(l-butene),35 and isotactic poly(4-methyl-l-pentene).39,40,153,247... 

Mesomorphic forms characterized by conformationally ordered polymer chains packed in lattices with different kinds of lateral disorder have been described for various isotactic and syndiotactic polymers. For instance, for iPP,706 sPP,201 sPS,202 syndiotactic poly(p-methylstyrene) (sPPMS),203 and syndiotactic poly(m -methylstyrene),204 mesomorphic forms have been found. In all of these cases the X-ray fiber diffraction patterns show diffraction confined in well-defined layer lines, indicating order in the conformation of the chains, but broad reflections and diffuse haloes on the equator and on the other layer lines, indicating the presence of disorder in the arrangement of the chain axes as well as the absence of long-range lateral correlations between the chains. 

Figure U. 3/l, 7/2, and 4/1 helical conformations of isotactic polymers in the crystalline state. From ref. (166) Copyright Soc. Chim. llal.

The ORD and CD curves of optically active polymers containing chromo-phoric groups show that the chromophores can be asymmetrically perturbed by the chirality of the substituents and of the main chain conformation. This is the case with poly( ec-butyl vinyl ketone) (377), which presents a Cotton effect at 292 nm, its intensity being greater in the prevalently isotactic polymer than in the atactic polymer. 

It is important to realize that polymer configuration and conformation are related. Thus, there is a great tendency for isotactic polymers (configuration) to form helical structures (conformation) in an effort to minimize steric constrains brought about because of the isotactic geometry. 

In spite of the similarity of the structure of the monomer units the two corresponding isotactic polymers crystallize in two different chain conformations tiie helix of poly-3-methyl-l-butene contains four monomer units per turn (4/1) with a chain repeat of 6.85 A the helix of poly-4-methyl-l-pentene contains 3.5 units per turn (7/2) and has a repeat of 13.85 A. The copolymers tend to crystallize. Their chain conformation and cross sectional area in the crystal lattice are analogous to those of the homopolymer corresponding to the predominant comonomer. For 4-methyl-l-pentene contents higher than 50% some evidence exists that the system simultaneously contains both chain conformations. 

Such a model is in agreement with all the experimental findings till now ascertained in the field of optically active vinyl-polymers in fact it explains, in the case of polymers having asymmetric carbon atoms in a or j position with respect to the principal chain, the relationships between absolute structure of monomers and sign of the rotatory power of polymers, and the high rotatory power observed in isotactic polymers. The rapid and reversible variation of the optical rotation with temperature (105) is probably connected with the existence of a conformational equilibrium that is rapidly attained at each temperature. 

Aldehyde Polymers Asymmetric anionic polymerization can lead trichloroacetalde-hyde (chloral) to a one-handed helical, isotactic polymer having a 4/1-helical conformation with... 

Olefin Polymers Isotactic polymers of propylene and 1-butene obtained by optically active metallocene catalyst (145) have been reported to show large specific rotation in suspension ([a]D-123°, -250° for polypropylene [a]D+130° for polybutene), which was lost when the polymers were completely dissolved or heated [176,177]. The optical activity was ascribed to a helical conformation of the polymer chain with preferential screw sense. 

These considerations, thus, lay the groundwork for tests among several semi-empirical approaches to the estimation of optical rotation of bond systems regarded as helices. Should it be necessary to use Eq. (lb) rather than (la), then a sweeping reassessment of the use of the helical conductor model will be required. However that test turns out, a test between that model and the simple conformational dissymmetry model becomes possible on the basis of the material shown in Table 1. At this point it should be said that our calculations on twistane 16> support the helical conductor model but that the results obtained by Pino and his co-workers 17 18> on the chiroptical properties of isotactic polymers prepared from chiral a-olefins support the conformational dissymmetry model. [We are not able, at present anyhow, to account for their results with the helical conductor model]. 

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