Diisotactic structures

By the presence of cyclic monomer units obtained, for example, from cyclic olefins (benzofuran, indene, etc.) (58, 254). The erythro- and threo-diisotactic stractures (26, 27 or in a different representation 77, 78) are chiral. If B is equal to A (cyclobutene or analogous monomers) only the threo-diisotactic structure 27 is chiral. 

CHR—CO—), polyoxy acids (—O—CHR—CO—), poly-l-alkylbuta-dienes (—CH=CH—CHR—CH2—). To the same class iKlong the polymers with two asymmetric atoms for every monomer unit, such as polysoibates (—CH=CH—CHA—CHB—, 32 and 33) where both eiythio- and thieo-diisotactic forms are chiral, or polyhexadiene (—CH=CH—CHA—CHA—) and poly-2,3-epoxybutane (—O— CHA-CHA-), 39, where only the thieo-diisotactic structures are chiral, and the polymers of some bicyclic monomers such as those shown in 41 and 42. Other examples are the polymers obtained by hydrogen transfer fk m substituted benzalacetone (79, Scheme 17) (266, 267). 

The polymerization of 1,2-disubstituted ethylenes, RCH=CHR, such as 2-pentene (R = — CH3, R = —C2H5), presents a different situation. Polymerization yields a polymer structure II in which there are two different stereocenters in each repeating unit. Several possibilities of ditacticity exist that involve different combinations of tacticity for the two stereocenters. Various stereoregular structures can be defined as shown in Fig. 8-2. Diisotactic structures occur when placement at each of the two stereocenters is isotactic. 

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. 

If the polymerisation of cis- l- FQ-propene occurred with monomer enchainment via another route (namely via the addition of the trans type), poly(l-deuteropropylene) of t/zrco-diisotactic structure [scheme (48)] would be formed, but this is not the case ... 

Similarly, in the polymerisation of cis- l, 2,3,3,3-pcnta-1 -(2H)-propcnc run with the homogeneous (IndCH2)2ZrCl2—[Al(Me)0]x catalyst, the respective poly(-pentadeuteropropylene) of eryt/zro-diisotactic structure is formed, which confirms the insertion to be of the cis type [22],... 

On the other hand, the polymerisation of trans-1 -(2 H)-propene with heterogeneous Ziegler-Natta catalysts leads to the formation of the respective polymer of threo-diisotactic structure [scheme (49)], which proves the m-insertion of the monomer [26,275] ... 

It should be added that alternating ethylene/2-butene copolymers can exhibit stereoregularity namely the ethylene/cA-2-butene copolymer, which possesses an erythro-diisotactic structure and is a crystalline polymer. It may be interesting to note that from the formal point of view the alternating eryt/zro-diisotactic ethylene/cA-2-butene copolymer, i.e. erythro-diisotactic poly[ethylene- //-(c/.v-2-butene)], can be treated as isotactic head-to-head and tail-to-tail polypropylene. Isomeric trans-2-bu. ene gives atactic amorphous copolymers with ethylene [2,82]. 

Helix-sense-selective polymerization has also been attempted for several bulky monomers including an acrylonitrile derivative105 and a-substituted acrylates.106107 Triphenylmethyl crotonate (TrC, 35) affords optically active, helical polymers by the polymerization using DDB—FILi and PMP—FILi complexes.108109 The polymers possess a nearly completely threo-diisotactic structure. Although the polymers indicate relatively small specific rotation ([(x]d +5.6° and +7.4° for the samples with DP =15 and... 

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). 

It was difficult until recently to assess the relative merits of these mechanisms owing to the lack of information on the opening of the double bond. Such information was reported recently by Natta et al. (1962c). In these studies, raws-l-chloro-2-butoxy-ethylene and cis-l-chloro-2-butoxyethylene were polymerized in toluene at —75°C using ethylaluminum dichloride as catalyst. The trans monomer afforded a polymer having a fereo-diisotactic structure (VII), whereas the cis monomer gave an erythro-diisotactic structure (VIII). 

Fig. 16. Schematic drawing s of the two diisotactic structures of polybenzo-furan (only one of the enantiomers is shown for each structure) (a) erythro-diisotactic, (b) tfereo-diisotactic.

Cyclopolymerization. As discussed earlier, nonconjugated dienes can be polymerized with metallocene-based catalysts to afford cyclopol5miers. In contrast to linear polyolefins which have only two microstructures of maximum order (isotactic and syndiotactic), cyclic polymers have four microstructures due to the possibility of configurational isomerism (cis vs trans) in the main chain (Fig. 16). Of these the frares-diisotactic structure contains no mirror planes of S5unmetry and is chiral by virtue of its main-chain stereochemistry (481). Two criteria for chirality of this microstructure are the presence of trans rings and isotacticity (the same... 

Hexadiene was found to polymerize in DCA and apoCA canals on heating over 100°C for 10 to 20 days after Y-ray irradiation. Particullary trans, trans-2,4-hexadiene polymerized in an inclusion state via radical mechanism for the first time. The polymers from the monomer prefer erythro structure to threo structure in a DCA canal, while they do slightly threo to erythro in an apoCA canal. It is considered that the polymerization proceeded preferentially in the canals through trans opening to yield erythro diisotactic structure in a DCA canal [ll]. 

In poly cis 2,3 dimethyl thiirane the peak located at 45.8 ppm (CDCU solvent, reference to IMS) was clearly assigned to the me-thine chain carbon of-diisotactic structure as it was directly increasing with the optical activity of the polymer. Three other peaks corresponding to chain carbons were found, showing that other structures than the simple disyndiotactic one (II) are also present. 

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