Isotactic vinyl

In addition to existing as helices in crystals, there is evidence that certain vinyl polymers also show some degree of regular alternation between trans and gauche conformations in solution. In solution, the chain is free from the sort of environmental constraints that operate in a crystal, so the length of the helical sequence in a dissolved isotactic vinyl polymer may be relatively short. 

The special stability of the helix with three repeat units per turn for isotactic vinyl polymers in which the substituent is not too large was discussed in connection with Fig. 1.10a. Rodriguez has suggested a simple paper model which is also helpful in illustrating this structure. The following steps summarize this method additional details are available in this readily accessible reference. 

As is well known, the most simple head-to-tail stereoregular vinyl polymers were called isotactic (22-24) and syndiotactic (25) by Natta. The first compounds to be recognized as such were polypropylene and 1,2-polybutadiene, respectively (26). Ideal isotactic vinyl polymers (4, 5, Scheme 1) have all the substituents on the same side of the chain while in syndiotactic polymers (6, 7) the substituents regularly alternate between the two sides of the chain (27). 

In the crystal state most stereoregular polymers have helical conformations. Group s(M/N) 1 comprises all the isotactic vinyl polymers [polypropylene, polybutene, polystyrene, etc., M/N = 3/1 poly-o-methylstyrene, etc., 4/1 ... 

The relationship for an isotactic vinyl chain characterized by two chain bonds... 

When it is necessary to specify the internal stereochemistry of the group, a prefix is required. In vinyl polymers there are meso (m) and racemic (r) diads and mm, mr, rr triads. The latter may be called isotactic, heterotactic and syndiotactic triads, respectively. Stereoregular vinyl polymers can be defined in terms of the regular sequences of diads thus an isotactic vinyl polymer consists entirely of m diads, i.e., it corresponds to the following succession of relative configuration -mmmmmm-, whereas a syndiotactic vinyl polymer consists entirely of r diads, corresponding to the sequence -rrrrrrr-. Similarly, a vinyl polymer consisting entirely of mr (= rm) triads is called a heterotactic polymer. 

The chain conformation of isotactic vinyl polymers in the crystalline state is ... 

FIGURE 2.13 Helical conformation of isotactic vinyl polymers. (From Gaylord, N., Linear and Stero-regular Addition Polymers, Wiley, New York, 1959. With permission.)... 

The second requirement is perhaps best illustrated by the cases of isomorphism in isotactic vinyl copolymers (7, 2, 4, 5, 6) only if the helical conformations of the two crystalline homopolymers are not too different, a regular helical conformation is also possible for the copolymer chains. 

Statistical substitution between chains having the same helix handedness, but different orientations of the side groups ("up" and "down chains) was proved or at least suggested as probable for a variety of isotactic vinyl polymers. In particular we will mention here the cases of isotactic polypropylene (53), isotactic poly-w-butene-1 (54), isotactic polyaldehydes (55) and isotactic-1,2-poly-4-methylpentadiene (56). 

Isotactic vinyl polymers -(CH2-CHX)- can, because of sterical hindrance, in most cases not lie in zig-zag shape the chain then takes, as the most regular conformation, a helical shape. An example is given in Figure 4.13, again as a projection on the plane of drawing, for PP, which as a helix crystallizes in a monoclinical lattice. 

Since isomerically pure polymers were not available, three different kinds of BR, each relatively high in one of the three kinds of base units were used as standards [35]. The band near 1308 cm 1 was identified [38,39] with the cis isomer and used for analyses [43]. The 1308 cm 1 band is weak and relatively broad, with the appearance of an unresolved doublet (1306,1311 cm 1). The cis band at 730 cm 1 is more frequently used in spite of some difficulties. Relatively pure, crystalline stereoregular polymers have been prepared and structures were determined by X-ray diffraction for cis [44], trans [45] and syndiotactic vinyl [46] and isotactic vinyl [47]. Infrared spectra [48-50] have been published for the four stereoregular polybutadienes, with detailed analyses of the spectra and band assignments for cis [51], trans [51] and syndiotactic vinyl [51] polymers. For the spectrum of isotactic vinyl BR, bands at 1232, 1225, 1109, 943, 876, 807 and 695 cm"1... 

Helical conformations were also proposed for the isotactic copolymer derived from (/T)-3,7-d imethyl-1 -octene and styrene.48,49 The copolymer showed intense CD bands based on the styrene units incorporated into the polymer chain. The CD intensity was much larger than that of a model compound of an adduct of the chiral olefin and styrene. The helical structure of polyolefins has also been supported by force field calculations.50 The relationship of these considerations to isotactic vinyl polymers and more recent studies have recently been reviewed.41... 

Isotactic vinyl polymers often possess a helical conformation in the solid state however, without bulky substituents present (vide infra) in solution at room temperature, helix—helix reversal takes place fast and no optical activity is observed. Ortiz and Kahn reported a borderline case in which a non-bonded interaction between the monomers leads to the formation of isotactic 39 (Chart 7) by anionic polymerzation at —78 °C. Optically active polymers can be isolated, but in solution the proposed one-handed helicity is lost in less than 1 h.148 An intriguing class of polymers formed by polycondensation of diboronic acid and chiral tetraalcohols has been studied by Mikami and Shinkai and is exemplified by polymer 40 (Chart 7). In this D-mannitol-based polymer, the noncovalent intramolecular interaction between the amines and the boron atoms imposed a sp3-hybridization on boron, which, according to calculations, results in a helical conformation of the macromolecule.149... 

Figure 3.9 Helical structures adopted by various isotactic vinyl polymers (Natta and Corradini26). The side group R-of the polymers that form the helical structures shown is -CH3, -C2H5, -CH=CH2, -CHr-CH2-CH-(CH3)2, -o-ch3-o-ch2 -CH-(CH3)2, and -C6H5 for (a), -CH2-CH-(CH3)-C2H5 and -CH2-CH-(CH3)2 for (b), and -CH-(CH3)2 and -C2H5 for (c).

Hence, this reduced Tm/Tg correlation system is far from ideal. Besides, each of the three groups of polymers contain a number of strongly deviating systems. The isotactic vinyl polymers (group A) which form helical structures for instance deviate strongly. The (Tm(exp.)-Tm(calc.)] values of a number of this type of polymers are ... 

All isotactic vinyl polymers can be arranged in four ways in the usual crystals. Polypropylene has as stable conformation a 2 3/1 helix that consists of successive tg(—) or tg rotational isomeric states (left-handed or right-handed helices). In addition, it was possible for the helices to have the —CHj groups point up or down , relative to the crystallographic c-axis. In the ideal, monoclinic crystal form I (P2j /c) all four types of helices occur only at specific, symmetry-related positions The equilibrium melting temperature and entropy are 460.7 K and 15.1 J/(K mol), respectively. 

Rg. 4.7 (a) The threefold helix for an isotactic vinyl polymer. The X group of the structural unit -fCHX—CH2T- is shown shaded. The carbon backbone forms a right-handed helix and the right-handed 3i and the left-handed 32 helices are indicated by the ribbons, (b) The 13e helix of PTFE, showing the twisted backbone, and a spacefilling model viewed from the side and viewed along the chain axis, ((a) reproduced from The Vibrational Spectroscopy of Polymers by D. I. Bower and W. F. Maddams. 

A certain isotactic vinyl polymer crystallises with the chains in a simple 3i helical conformation, with only one chain passing through each unit cell. At approximately what angle will the second layer line be observed in an X-ray fibre photograph obtained using radiation of wavelength 0.194 nm ... 

Figure 5.34 indicates the repeating-unit isomorphism of atactic poly( vinyl alcohol). The -OH group on every second chain-atom occurs randomly along the chain, in contrast to isotactic vinyl polymers. Two positions are available for the -OH, i.e., in the actual crystal half of these positions remain filled by the smaller -H. Only because the size difference between -H and -OH is sufficiently small, is such a compromise possible. 

Fig. 2.9 Illustration of the helical conformation of isotactic vinyl polymers...

Figure 9 (a) Isotactic vinyl chain, (b) Syndiotactic chain, (c) Vinyl polymer chain with isotactic blocks, (d) Atactic vinyl chain. 

However, an isotactic chain can be chiral as a whole if the directions forward and backward along the chain are not equivalent. This is trae, for example, for an isotactic vinyl polymer whose two ends are modified with different substituents. Another example is given by a vinyl chain with two isotactic blocks of different lengths (Figure 9(c)). Such polymers may exhibit a weak optical activity (due to end-effects). 

The case in which this site (e.g. a double bond or an asymmetric carbon atom) is repeated along the polymer main chain with the same configuration is much more frequent both in natural (natural rubber, natural peptides, poly-p-hydroxybutyrate) and synthetic polymers (isotactic vinyl polymers) than the case in which stereoregularity arises from ordered successions of different configurations. Indeed, up to now, only the ordered alternation of the two possible configurations of asymmetric carbon atoms (syndiotactic vinyl polymers and alternating copolymers of D- and L-amino acids) has been recognized. 

Fig. 13.2. Helical conformation of an isotactic vinyl polymer c stalline chain (from F. Rodriguez, Principles of Polymer Systems, 2nd Ed., McGraw Hill Inti., Auckland, 1982).

Most isotactic vinyl polymers assume helical conformations in the crystalline state [4], but ovnng to the substantial achirality of the macromolecules both screw senses are found in the lattice ceils in equal amounts. This is even more true in a melt or in solution, where left-handed and right-handed helical sections alternate even within the same macromolecular chain. Certainly, an appreciable optical rotation would be observed in the crystalline state provided that crystallization occurred under a chiral field inducing a single screw sense helidty in all chains. Such an optical rotation would promptly be lost on melting or dissolution, as an immediate equilibration between the two opposite helical senses would occur. 

Figure 1. Spin Systems in Isotactic Vinyl Polymers...

All definitions in polymers refer to ideal situations, so an "ideal isotactic vinyl polymer is a polymer caracterized by a succession of all m diads an "ideal" syndiotactic vinyl polymer is a polymer which is caracterized by a succession of all v diads. According to these conventions, the isotactic polymer of propylene can be indicated ... 

Note that a structure such as the one above would be the first one, according to our knowledge, in which the helical repetition refers, in an isotactic vinyl polymer, to pairs of monomeric units. 

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