Disyndiotactic structures

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. 

The polymer obtained from 9 by y-radiation was soluble in chloroform despite a high crystallinity. The alternating molecular stacking of 9 led to stereoregular polymer formation with a disyndiotactic structure. The racemo and meso structures of the resulting polymers were confirmed by NMR spectroscopy. A comparison of the NMR data of related polymers concludes that the chemical shifts for a series of the polymers are predominantly determined by the meso-racemo structure rather than the diisotactic-disyndiotactic one. 

Explain the capability of terminally symmetrically disubstituted 1,3-butadiene containing only deuterium atoms as substituents to yield stereoregular 1,4-polymers with a diisotactic or disyndiotactic structure. 

In the case of poly(l,2-cycloalkylene)s containing symmetrical rings such as poly(l,2-cyclobutylene)s, the erythro and threo structures are also referred to as meso (M) and racemo (R) structures for denoting the stereochemistry of the rings the relative stereochemistry between the rings is denoted by meso (m) and racemo (r) configurations corresponding to diisotactic and disyndiotactic structures respectively [21,22]. 

In particular, C2- and Cs-symmetrical zirconocenes, activated by methylalu-minoxane, exhibit very high activities in the polymerisation [18], Polymers of norbornene obtained with these catalysts are characterised by predominant erythro-disotactic and m7/ o-disyndiotactic structure respectively [15]. Active species formed in the rac.-Me2Si(Ind)2ZrCl2-[Al(Me)0]A. catalyst possess homotopic coordination sites for the incoming monomer, and hence pure... 

Note that stereoregular polymers of disubstituted epoxides with a threo-diisotactic and eryt/zro-disyndiotactic structure have not been synthesised. 

Early studies on the homopolymerization of E-l,3-pentadiene yielded polymers with a high cis-1,4-content and an isotactic structure, whereas E-2-methyl-l,3-pentadiene resulted in a polymer with a mixed czs-1,4/transit-structure [487-492]. Investigations on the polymerization of E-1,3-pentadiene with the system NdN/TIBA/DEAC partially support these findings as a poly(l,3-pentadiene) with a cis- 1,4-threo-disyndiotactic structure was obtained [492]. A somewhat lower cis- 1,4-content of 70% was obtained when the polymerization of E-l,3-pentadiene was catalyzed by (CF3COO)2NdCl/TEA [493,494]. When 2,3-Dimethyl-1,3-butadiene is polymerized with the catalyst NdN/TIBA/EtAlC the resulting poly(2,3-dimethyl-butadiene) predominantly contains cis-1,4-units [495,496]. 

A more complicated picture emerges when the polymerization of 1,2-disubstituted ethylenes (CHR=CHR ) is considered because now each carbon atom in the chain becomes a chiral center. The resulting ditactic structures are illustrated in Figure 6.1(d,ed). Two isotactic structures are obtained, the erythro, in which all the carbon atoms have the same configuration, and the threo, in which the configuration alternates. Only one disyndiotactic structure is possible. The differences arise from the stereochemistry of the starting material if the monomer is cis-substimted the threo form is obtained, whereas a trans monomer leads to the erythro structure. 

Explain why for vinyl monomers of type CE1R=CHR two diisotactic isomers are possible bnt only one disyndiotactic structure is possible. 

Acenaphthylene has been homopolymerized by radical emulsion polymerization to a product having predominantly a threo disyndiotactic structure (209). 

Very recently, these views have been definitively supported by the very elegant work of Porri (31), using cis-cis-1,4-dideute-rio-butadiene as moncmer, and correlating the different stereoregularities of the polymers obtained (trans-l,4-threo-diisotactic and cis-1,4-threo-disyndiotactic structures). 

The combination of cis-trans isomerism with iso-syndio and erythro-threo dispositions gives complex stractures as exemplified by the 1,4 polymers of 1-or 4-monosubstituted butadienes, such as 1,3-pentadiene (72, 73), and 2,4-pentadienoic acid (74, 75) and of 1,4-disubstituted butadienes, for example, sorbic acid (76). This last example is described in 32-35 (Scheme 6, rotated Fischer projection). Due to the presence of three elements of stereoisomerism for each monomer unit (two tertiary carbons and the double bond) these polymers have been classed as tritactic. Ignoring optical antipodes, eight stereoregular 1,4 structures are possible, four cis-tactic and four trans-tactic. In each series (cis, trans) we have two diisotactic and two disyndiotactic polymers characterized by the terms erythro and threo in accordance with the preceding explanation. It should be noted that here the erythro-threo relationship refers to adjacent substituents that belong to two successive monomer units. 

Turning to propylene, cis addition was found also for syndiotactic polymers (4(X), 401). This result deserves additional comment. It is known that only one disyndiotactic polymer is obtained from a CHA=CHB olefin (see Sect. II-B) but this is no longer true when one considers the syndiotactic copolymers between two differently labeled monomers. The syndiotactic copolyriKr between perdeuteropropylene and propylene-l-d, can have either of the two structures 99 Ot 199. Hew 9ve motvomet mil deti ixv% from Ihe second mononvei (present in small quantity) can be clearly identified as to its stereochemistry. 

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. 

Stereoregular polymers that can be afforded by 2,4-hexadiene and other symmetric terminally disubstituted butadienes (of the CHR CH CH CHR type) exhibit still more complex stereoisomerism, since each monomeric unit in these polymers possesses three sites of isomerism. The formation of these polymers involves 1,2- and 1,4-polymerisation. The 1,2-polymers derived from the CHR=CH—CH=CHR monomers exhibit the same type of stereoisomerism as polymers with a 3,4 structure obtained from monomers of the CH2 CH CH=CHR type. However, owing to the presence of the R substituent at the double bond in the side group of the polymer derived from a monomer of the CHR=CH—CH=CHR type, two types of eryt/zro-diisotactic, t/zraz-diisotactic and disyndiotactic polymer are foreseeable, each type with either cis or trans configuration of the double bond, as in the 1,2-polymer derived from a monomer of the CH2 CH CH CHR type. Thus, six stereo-isomeric forms of 1,2-polymer are possible for the CHR CH CH CHR monomer. The 1,4 monomeric units in the polymers formed by the polymerisation of CHR CH CH CHR monomers contain one double bond (in either cis or trans configuration) and two tertiary carbon atoms and therefore can exist as two sets of enantiomers, erythro and threo ... 

Several possibilities exist for ditacticity in macromolecules formed by polymerizing 1,2-disubstituted ethylenes of the type RHC=CHR, structures (XVIII)-(XX). It can be seen that each unit contains two different asymmetric carbon atoms in the chain. The original definitions of tacticity have been extended to include these modifications and Newman s (1956) definitions of erythro and threo structures. Structures (XVIII)-(XX) illustrate ifereo-diisotactic, erythro-diisotactic, and disyndiotactic poisoners, respectively. 

FIGURE 19.1 Maximum order structures of the cyclopolymers produced by the cyclopolymerization of 1,5-HD (a), 1,6-heptadiene (b), and 1,7-OD (c) with metallocene catalysts. These structures are also referred to as cis-diisotactic (me o-diisotactic), cis-disyndiotactic (m 56>-disyndiotactic), trans-diisotactic (racemo-diisotactic), and trans-disyndiotactic (rac mo-disyndiotactic). 

The terms isotactic and syndiotactic refer as above to the structure of each diasteric atom relative to comparable atoms in the other repeating units. The prefix erythro indicates that when a pair of adjacent diasteric atoms is rotated into an eclipsed conformation, at least two similar substituents can be superimposed. Threo denotes the nonsuperimposable isomer. In a disyndiotactic polymer each diasteric center is erythro to one of its neighbors and threo to the other, so there is only one isomer. 

According to Vandenberg (2 )the ring-opening of cis compounds involves a process with inversion of configuration of the attacked carbon and depending on the type of enchainment one can obtain di-isotactic (I) or disyndiotactic(II) structures. 

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. 

Nuclear magnetic resonance studies showed the methyl groups resonance was composed of four peaks of approximate equal intensity from 1.3 to 1 ppm, supporting diisotactic 31A and disyndiotactic 31B structures... 

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