Isotactic polymers helical structures

The zinc complex of 1,1,1,5,5,5-hexafluoroacetylacetonate forms coordination polymers in reaction with either 2,5-bis(4-ethynylpyridyl)furan or l,2-bis(4-ethynylpyridyl)benzene. The X-ray crystal structures demonstrate an isotactic helical structure for the former and a syndio-tactic structure for the latter in the solid state. Low-temperature 1H and 19F NMR studies gave information on the solution structures of oligomers. Chiral polymers were prepared from L2Zn where L = 3-((trifluoromethyl)hydroxymethylene)-(+)-camphorate. Reaction with 2,5-bis(4-ethy-nylpyridyl)furan gave a linear zigzag structure and reaction with tris(4-pyridyl)methanol a homo-chiral helical polymer.479... 

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... 

The problem is further complicated for vinyl polymers with their problems of stereoisomerism. The first descriptions of the conformational state of isotactic polypropylene in solution go back 25 years (178, 179, 192, 193). Corradini, Allegra, and Ganis proposed a model, still essentially valid today, according to which macromolecules possess a local helical structure analogous to that observed in the crystalline state. The helix segments are rather short, only a few monomer units, after which an inversion of the helix sense occurs, with simultaneous alteration of its direction (Figure 15). As a whole this disordered con-... 

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. 

Anionic Catalysis Several bulky methacrylates afford highly isotactic, optically active polymers having a single-handed helical structure by asymmetric polymerization. The effective polymerization mechanism is mainly anionic but free-radical catalysis can also lead to helix-sense-selective polymerization. The anionic initiator systems can also be applied for the polymerization of bulky acrylates and acrylamides. The one-handed helical polymethacrylates show an excellent chiral recognition ability when used as a chiral stationary phase for high-performance liquid chromatography (HPLC) [97,98]. 

The intramolecular interaction energy was calculated for five isotactic polymers, namely, isotactic polypropylene, poly(U-methyl-l-pentene), poly(3-methyl-1-butene), polyacetaldehyde, and poly(methyl methacrylate) (23). The molecular structures of the first four polymers have already been determined by x-ray analyses as (3/1) (2k), (7/2) (18,25.,26), (U/l) (21), and (U/l) helices (28), respectively. Here (7/2) means seven monomeric units turn twice in the fiber identity period. For isotactic poly(methyl methacrylate) (29), a (5/l) helix was considered reasonable at the time of the energy calculation in 1970, before the discovering of... 

Isotactic poly(x-olcfin)s crystallise in a helical conformation, and, in the case of polypropylene, with three units per turn [4,5], Isotactic polypropylene has a melting point of 175°C and does not dissolve in boiling n-heptane [6,7], Note that, depending upon the configuration of the tertiary carbon atom of the polymer main chains, the poly(x-olefin) helices will be characterised by right-handedness or left-handedness. It should be mentioned that the helical structure of the poly(x-olcfin) chain per se is sufficient for the appearance of chirality of such a macromolecule [8], Figure 3.3 presents the helical conformation of chains of isotactic poly(a-olefin)s in the crystalline state (with three units per turn - the case of polypropylene) [5],... 

The cyclic intermediate complex was assumed to add monomer with difficulty up to 8—9 monomer units, at which length the polymer chain was thought to form a helical structure in solution and propagation then ensued at a faster rate to give isotactic polymer. The same pseudotermination reaction was also noted in the polymerization with Grig-nard reagent in toluene at low temperatures (20). 

Isotactic polypropylene is a rather stiff and tough solid material with a melting point of 164°C. Closely packed, CHs-studded helices (Figure 17), rigidly interwoven in crystalline domains (Figure 18), account for the mechanical and thermal resistance of isotactic polymers. Syndiotactic polypropylene has a related crystalline structure, but atactic polymers are amorphous and form oily or waxy materials depending on chain lengths. 

A helical structure for vinyl polymers with an excess helicity in solution was realized for isotactic poly(3-methyl-1-pentene) by Pino in I960.12 Although the chiral side groups affect the helical conformation in the polyolefin, the single-handed helix of poly-... 

The isotactic polyolefins prepared using a Ziegler— Natta catalyst form a helical conformation in the solid state (crystalline regions).11 38,42 This helical structure persists in solution, but because of fast conformational dynamics, only short segments of the helix exist among disordered conformations. When an isotactic polyolefin is prepared from an optically active monomer having a chiral side group, the polymer shows the characteristic chiroptical properties which can be ascribed to a helical conformation with an excess helicity.12,43-46 The chiroptical properties arise in this case predominantly from the helical conformation of the backbone. 

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... 

Some isotactic polymers such as polychloral and poly(triphenylmethyl methacrylate)289 are known to exist only in purely helical conformation. The helical structure of the polymers is rigid even in solution, owing to the bulkiness of the side-groups. This has been demonstrated by the measurement of high optical activity of the polymers prepared by asymmetric polymerizations the optical activity is based on a one-handed helical conformation of the polymer chain. 

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 ... 

X-ray and electron-diffraction studies of proteins have shown that these polymers in their crystalline state often have a spatial configuration resembling a helix. A similar helical structure was proposed in recent investigations of highly crystalline isotactic polymers prepared with stereospecific initiator systems. 

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