Homopolymers, optically active

A series of optically active poly alkyl(phenyl)silane derivative copolymers with different chiral molar composition 44 and 45, are shown in Chart 4.7, along with homopolymers poly(methyl(phenyl)silane) (42) and poly(n-hexyl(m-tolyl)silane) (43). 

It is considered that, if ideal, optically active poly(alkyl(aryl)silane) homopolymer and copolymer systems could be obtained which had stiffer main-chain structures with longer persistence lengths, it should be possible to clarify the relationship between the gabs value and the chiral molar composition. The magnitude of the chirality of the polyisocyanates allowed precise correlations with the cooperativity models.18q In the theory of the cooperative helical order in polyisocyanates, the polymers are characterized by the chiral order parameter M, which is the fraction of the main chain twisting in one helical sense minus the fraction of the main chain twisting in the opposing sense. This order parameter is equal to the optical activity normalized by the value for an entirely one-handed helical polymer. The theory predicts... 

Moller and co-workers co-polymerized dichlorodi- -pentylsilane with either dichloro-bis-(d )-2-methylbutylsilane or dichloro-(d )-2-methylbutyl- -pentylsilane in various ratios and found a linear dependence of optical activity on mole fraction of chiral co-monomer.313 On the other hand, studies by Fujiki on co-polymers 109 formed by the copolymerization of achiral (racemic) dichlorohexyl-2-methylbutylsilane and chiral dichlorohexyl-(d )-2-methylbutylsi-lane or dichlorohexyl-(l )-2-methylbutylsilane have shown that a preferential helical screw sense can be induced by even as little as 0.6 mol% of chiral co-monomer, and that at 5 mol%, the helicity, as gauged by the gabs value, is essentially the same as that of the chiral homopolymer, as shown in Figure 40. This indicates a positive non-... 

A somewhat different situation arises in the copolymerization of a racemic monomer with an optically active monomer of similar structure in the presence of a conventional stereospecific (or stereoselective) catalyst (299, 321). Examples concern the copolymerization of racemic 3,7-dimethyl-1-octene with (5)-3-methyl-l-pentene and of racemic jec-butyl vinyl ether with various optically active vinyl ethers. In all cases there was preferential copolymerization of one of the two enantiomers of the racemic monomer with the second monomer and simultaneous formation of an optically active homopolymer containing predominantly the noncopolymerized antipode, according to Scheme 20. The two products are easily separated, due to their different solubilities. 

However, there will be a possibility of obtaining an optically active polymer when the polymerization proceeds by forming a tight helix of polymer chian and the helix is sufficiently stable in solution (26). This concept was first demonstrated with poly(tert-butyl isocyanide) [ = - ( ) ] (27). In this case, the polymer was resolved into two fractions of positive and negative optical rotations. This is not a vinyl polymer. The results of the copolymerization of (j3)-MBMA and TrMA prompted us to investigate the possibility of forming an optically active homopolymer of TrMA. 

Natta carried out the anionic polymerization of methyl sorbate, a 1,3-diene, with an optically active initiator and obtained an optically active homopolymer with main-chain chirality. The high molecular weight crystalline polymer produced with (P)-2-methylbutyllithium had a tritactic (di-iso-rra/w-tactic) structure. This was probably the first metal-catalyzed asymmetric polymerization 134). Polymerization of other dienes was attempted by using various asymmetric methods 135). 

According to the above experiments an optically active homopolymer might be obtained by this method, for instance by polymerizing by free... 

The research in the field of optically active addition homopolymers, which have been considered in this review, was remarkably stimulated by the discoveries of Natta and co-workers on the stereospecific polymerization, and has advanced considerably in the last ten years. 

Cyclopolymerization of bifunctional monomers is an effective method of chirality induction. Optically active vinyl homopolymers and copolymers have been synthesized by using optically active distyrenic monomers (41) based on a readily removable chiral template moiety. Free-radical copolymerization of 41a with styrene and removal of the chiral template moiety from the obtained copolymer led to polystyrene, which showed optical activity ([Oc]365 -0.5-3.5°) (Scheme 11.6) [84], The optical activity was explained in terms of chiral (S,S)-diad units generated in the polymer chain through cyclopolymerization of 41a [85], Several different bifunctional monomers have been synthesized and used for this type of copolymerization [86-90]. 

Acetylene Polymers Homopolymers of optically active acetylenes, including (/ )-153 synthesized by [RhCl(norbomadiene)]2 catalyst, show intense CD bands in the UV-visible region, probably based on a predominant helical sense of the main chain [204]. Excess single-handed helicity of the main chain can be induced for polymers of achiral acetylenes (154 and 155) by adding chiral molecules. The chiral induction is based on acid-base interaction or complex formation between the polymer and the additives [205-2081. 

Optically Active Homopolymers with a Preferential Screw Sense. 131... 

Figure 13 shows several optically active vinyl polymers and copolymers with azobenzene or stilbene residues in the side groups. All these polymers were prepared by the conventional radical polymerization, so that the main chains should be atactic or syndiotactic and may not be able to take a helical conformation even in the presence of chiral pendants. The homopolymers (28-31) [83-85]... 

With metallocene catalysts, not only homopolymers such as polyethylene or polypropylene can be synthesized but also many kinds of copolymers and elastomers, copolymers of cyclic olefins, polyolefin covered metal powders and inorganic fillers, oligomeric optically active hydrocarbons [20-25]. In addition, metallocene complexes represent a new class of catalysts for the cyclopolymerization of 1,5- and 1,6-dienes [26]. The enantio-selective cyclopolymerization of 1,5-hexadiene yields an optically active polymer whose chirality derives from its main chain stereochemistry. 

Angiolini L, Caretti D, Carlini C, SalateUi E. 1995. Optically active polymers bearing side chain photochromic moieties synthesis and chiroptical properties of methacrylic and acrylic homopolymers with pendant l lactic acid or l alanine residues connected to trans 4 aminoazobenzene. Macromol Chem Phys 196(9) 2737 2750. 

Over the past several decades, polylactide - i.e. poly(lactic acid) (PLA) - and its copolymers have attracted significant attention in environmental, biomedical, and pharmaceutical applications as well as alternatives to petro-based polymers [1-18], Plant-derived carbohydrates such as glucose, which is derived from corn, are most frequently used as raw materials of PLA. Among their applications as alternatives to petro-based polymers, packaging applications are the primary ones. Poly(lactic acid)s can be synthesized either by direct polycondensation of lactic acid (lUPAC name 2-hydroxypropanoic acid) or by ring-opening polymerization (ROP) of lactide (LA) (lUPAC name 3,6-dimethyl-l,4-dioxane-2,5-dione). Lactic acid is optically active and has two enantiomeric forms, that is, L- and D- (S- and R-). Lactide is a cyclic dimer of lactic acid that has three possible stereoisomers (i) L-lactide (LLA), which is composed of two L-lactic acids, (ii) D-lactide (DLA), which is composed of two D-lactic acids, and (iii) meso-lactide (MLA), which is composed of an L-lactic acid and a D-lactic acid. Due to the two enantiomeric forms of lactic acids, their homopolymers are stereoisomeric and their crystallizability, physical properties, and processability depend on their tacticity, optical purity, and molecular weight the latter two are dominant factors. 

The monomers of L- and D-lactic acids, LLA, and DLA, and their homopolymers, PLLA and PDLA, are optically active. The specific optical rotation ([aj sse) values of PLLA and PDLA... 

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