Diisotactic polymer

Cyclic olefin monomers such as indene, benzofuran, and so on, can give rise to two diisotactic polymers eiythro and threo, illustrated in the Fischer projection in 26 and 27, and to two different disyndiotactic polymers 28 and 29 (Scheme 5), as each monomeric unit is clearly defined and quite distinct from its neighbors (58, 61). For polymers of this type the terminology eiythro and threo is used also. 

The stereocenters in all three stereoregular polymers are achirotopic. The polymers are achiral and do not possess optical activity. The diisotactic polymers contain mirror planes perpendicular to the polymer chain axis. The disyndiotactic polymer has a mirror glide plane of symmetry. The latter refers to superposition of the disyndiotactic structure with its mirror image after one performs a glide operation. A glide operation involves movement of one structure relative to the other by sliding one polymer chain axis parallel to the other chain axis. 

All four diisotactic polymers (cis and trans, erythro and threo) are chiral and possess optical activity. Each of the four disyndiotactic polymers possesses a mirror glide plane and is achiral. For symmetric 1,4-disubstituted 1,3-butadienes (R = R ), only the cis and transthreo-diisotactic structures are chiral. Each of the erythrodiisotactic and threodisyndiotactic polymers has a mirror glide plane. Each of the erythrodisyndiotactic polymers has a mirror glide plane. 

Later, Tieke reported the UV- and y-irradiation polymerization of butadiene derivatives crystallized in perovskite-type layer structures [21,22]. He reported the solid-state polymerization of butadienes containing aminomethyl groups as pendant substituents that form layered perovskite halide salts to yield erythro-diisotactic 1,4-trans polymers. Interestingly, Tieke and his coworker determined the crystal structure of the polymerized compounds of some derivatives by X-ray diffraction [23,24]. From comparative X-ray studies of monomeric and polymeric crystals, a contraction of the lattice constant parallel to the polymer chain direction by approximately 8% is evident. Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, resulting in diisotactic polymer chains. He also referred to the y-radiation polymerization of molecular crystals of the sorbic acid derivatives with a long alkyl chain as the N-substituent [25]. More recently, Schlitter and Beck reported the solid-state polymerization of lithium sorbate [26]. However, the details of topochemical polymerization of 1,3-diene monomers were not revealed until very recently. 

Given the high regioselectivity, the type of addition in the insertion step must always be the same. It was found that in the polymerization of cis- and trans-[l-2H ]-propene a yyn-type double-bond opening takes place during insertion, eventually resulting in em/iro-diisotactic and f/ireo-diisotactic polymers, respectively 332-334... 

The validity of the proposed mechanism is borne out by the results of the polymerisation of (E,E)-1 -(2H)-1,3-pentadiene with Nd(OCOR)3—AlEt2 Cl A1 (z-Bu)3 and Co(Acac)2 AIEt2Cl—H20 catalysts [20-22,195], The Nd-based catalyst gave a czj-1,4-eryt/zro-diisotactic polymer, while the Co-based catalyst yielded a cA-l,4-t/zra>-disyndiotactic polymer. The formation of such polymers is shown by the schemes in Figure 5.5 [7,41],... 

Natta, Farina, and Donati (47) have prepared diisotactic polymers from alkyl esters of sorbic acid and /5-styryl acrylic acids by anionic initiation with optically pure 2-methyl butyllithium. The polymers were optically active. 

In sterically random propagation, the probability of meso diad generation Pm in each addition is equal to 0.5, and the ratio of triads in the chain is i h s = = 1 2 1. In an atactic polymer, the diads m and r are statistically distributed in the chain. Natta [81] called the simplest stereoregular chains isotactic —mmmm— and syndiotactic —rrrr—. Monomers with both a and / carbons unsymmetrically substituted can form diisotactic polymers... 

The polymerization of cis oxides gives, in contrast to trans monomers, polymers of different stereochemistry, namely cis-2,3-butene oxide forms racemic (RR, SS) disyndiotactic polymers whereas cis-1,4-dichloro-2,3-butene oxide gives racemic mixtures of (RR, RR) and (SS, SS) diisotactic polymers. 

Explain why it is possible to synthesize two diisotactic polymer structures, but only one syndiotactic polymer structure. 

Analysis of the products obtained after hydrolysis under controlled conditions led to the conclusion that the trans-monomer gave a meso-diisotactic structure, while the cis-monomer produced racemic-diisotactic polymer. A more detailed discussion of the stereochemistry of this class of polymers has already been given in Part I of this review (Adv. Pol. Sci. 37). 

Sorensen and Campbell [110] indicate that, depending on the nature of the initiator that has been used, either an erythro-diisotactic or a threo-diisotactic polymer forms. The catalyst is formed from aluminum chloride and either (-I-)-/ -phenylalanine (to produce a polymer with ( + ) rotation) or phenylalanine (to produce a (-) rotating polymer) in toluene. Other initiators include aluminum chloride with (-)-menthoxytriethyltin, -germaniiun, or -silicon [111]. The following preparation is a brief summary of the detailed procedure given in Ref. [110]. 

When 1,2-disubstituted olefins are polymerized with Ziegler-Natta catalysts, the ditacticity of the products depends on the mode of addition. It also depends on the structure of the monomer, whether it is cis or trans. A threodiisotactic structure results from a syn addition of a trans monomer. A syn addition of a cis monomer results in the formation of an erythrodiisotactic polymer. For instance, cis and /ra/i -l- Z-propylenes give erythro and threo diisotactic polymers, respectively. To avoid 1,2-interactions in the fully eclipsed conformation, the carbon bond in the monomer units rotate after the addition of the monomer to the polymeric chain. 

Fig. 14. Diisotactic polymers from cis- and irans-l-chloro-2-butoxyethylene.

With the mode of presentation indicated, the new monomer gives, after insertion, a butenyl group superimposable on the preceding one hence a diisotactic polymer will be formed. 

Diisotactic polymers were prepared from cyclic vinyl ethers, such as 2,3-dihydropyran (XV) (212), and from benzofuran (XVI) a-naphthofuran (XVII) and 6-naphthofuran (XVIII). From the last three monomers predominantly isotactic, optically active polymers... 

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