Sequence isotactic

The fraction of isotactic sequences increases as increases, as required by the definition of these quantities. 

The number of isotactic sequences containing nj iso repeat units is Nnj. 

Metallocene isotactic polypropylenes (MET.PP) are accessible with different melting points under the commercial range of melt flow rate (Fig. 11). The variation of melting points in these polymers is linked with the presence of different lengths of isotactic sequences. Mechanical properties of polypropylene... 

The products of the thermal transformation of PAN were used for the examination of the effect of stereoregularity of the initial polymer on the CTC properties. At equal transformation degrees, the concentration of complete charge transfer states in a bromine-complexed PCS, obtained on the basis of PAN-c (t e., a polymer with elevated content of isotactic sequences), exceeds by two orders of magnitude this parameter in the polymer obtained on the basis of PAN-r. 

The nucleophilic substitution on poly(vinyl chloroformate) with phenol under phase transfer catalysis conditions has been studied. The 13c-NMR spectra of partly modified polymers have been examined in detail in the region of the tertiary carbon atoms of the main chain. The results have shown that the substitution reaction proceeds without degradation of the polymer and selectively with the chloroformate functions belonging to the different triads, isotactic sequences being the most reactive ones. 

Nuclear magnetic resonance has proved to be a valuable tool in determination of configurational sequences in poly(MMA) (14). In Figure 3 is shown the NMR of poly(MMA) synthesized with an anionic polymerization catalyst known to produce predominantly isotactic sequences. In these polymers, the NMR spectrum of the methylene units In the polymer backbone gives an unequivocal determination of tacticity. The methylene signal, occurring about 1.8... 

It has been suggested however that isotacticity derives from polymerization occurring on colloidal particles formed by thermal decomposition of the catalysts. As stated previously, in the presence of the monomer even the allyl compounds are stable at 65°C and none of the thermal decomposition products (black to yellow solids) could be detected. As a check on these results a polymerization of propylene was carried out with Zr (benzyl) 4 in toluene at 0°C in a sealed tube. The reaction was very slow and analytical quantities of polymer could be obtained only after 312 hr. NMR analysis showed peaks assignable to isotactic sequences, and these were much stronger than the peaks assignable to syndiotactic diads. It was concluded... 

By studying mixtures of L- and D-LA of varying composition, Spassky et al. have demonstrated that (256) yields PLA containing long isotactic sequences, with a ratio of homo cross propagation of 2.8.796 Hence, an 80/20 L D mixture when polymerized to 70% conversion displayed an optical purity of 87%. Even at the relatively low optical purity of 65/35 L D, isotactic block lengths of >12 repeat units were reported. Achiral (259) converts rac-LA into highly isotactic PLA a Tm of 192 °C indicates that the chains of P(L-LA) and P(D-LA) form a stereocomplex.792 797-799... 

By the time the concentration of monomer is low, the back-skip of the polymer chain to the less-hindered site is faster than the formation of the high-energy alkene coordinated intermediate (IV). For this reason, at low propene concentrations and elevated temperatures isotactic sequences are formed. The probability of monomer coordination at the aspecific site (IV) is enhanced when the propene concentration increases. The consequence is that single stereoerrors [mrrm] are introduced in the isotactic polymer chain. 13C-NMR was able to prove the mechanism because a... 

Tacticities of the polypropylenes above 60% [mmmm] highlights the tendency of the backward oriented substitution (4a) to produce plastomeric materials. During experiments in liquid propylene, no similarity is observed between the two isomeric catalysts. Catalyst 4b/borate does not follow the same trend in the catalytic performance as previously mentioned, but leads to polypropylenes with sufficient amount of isotactic sequences and relatively high molecular weight for the design of plastomeric polypropylenes. 

Figure 1.1 shows the typical representation proposed to distinguish easily between the different types of stereoisomerism present in vinyl polymers. Viewing the main chains as lying down on a plane, there are (i) isotactic sequence of configurations, (ii) syndiotactic sequence of configurations, and (iii) no order present, atactic sequence of configurations. 

A polymer is defined as isotactic when the bonds issuing from the successive stereoisomeric centers (conventionally indicated by a vertical line) always have the same disposition when observed in the same direction -F —F —f- —1- —h — or— -l—1+—13—13— + itis defined as syn-diotactic when the disposition of the signs is inverted at each stage -t- --I++I----- -t-+ —. An isotactic sequence is, therefore, represented by... 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

The advent of high-resolution 13 C NMR allows the determination of tetrad, pentad, and even higher sequence distributions in many polymers [Bovey, 1972 Bovey and Mirau, 1996 Farina, 1987]. The tetrad distribution consists of the isotactic sequence mmm, the syndiotactic sequence rrr, and the heterotactic sequences mmr, rmr, mrm, rrm. The sum of the tetrad fractions is unity, and the following relationships exist ... 

The pentad distribution consists of the isotactic sequence mmmm, the syndiotactic sequence rrrr, and the heterotactic sequences rmmr, mmrm, mmrr, rmrm, rmrr, mrrm,... 

More recent work indicates that the conformational equilibrium is not between rac and meso conformers, but between the two enantiomeric anti conformers of the rac mixture [Busico et al., 2002], The prime support for this interpretation is the nature of the stereoerrors in the isotactic sequences of the polymer. The stereoerrors are XXXV instead of XXXVI. The isotactic sequences consist of all-/ isotactic blocks alternating with all-5 isotactic blocks. 

The proton NMR spectrum of a highly isotactic sample of PP is examined. The polymer is shown to contain 2% racemic diads occurring randomly at junctions of isotactic sequences of opposite configurations. The mean-square end-to-end distance of this polymer is measured under 9 conditions. Comparison of the value obtained with theoretical predicitons of Flory, Mark, and Abe [J. Am. Chem. Soc. 1966, 88, 639] permit an approximate measurement of the strength of the steric interactions within the PP chain. 

Polyacrylonitrile, as another example, displays well separated nitrile and methine carbon signals for isotactic, syndiotactic and atactic sequences [523]. 13C NMR analysis of polyacrylonitrile samples obtained by different methods of polymerization clearly indicates the isotactic sequence to predominate in the material obtained by y irradiation of the acrylonitrile-urea channel adduct [523],... 

Spin-lattice relaxation times of carbon-13 in different polypropylene stereosequences differ slightly while nuclear Overhauser enhancements are almost identical (1.8-2.0) [533] isotactic sequences display larger Tx values than the syndiotactic stereoisomers. Other vinyl polymers behave correspondingly [534]. Carbon-13 spin-lattice relaxation times further indicate that dynamic properties in solution depend on configurational sequences longer than pentads. The ratio 7J(CH) 7J(CH2) varies between 1.6 to 1.9 thus, relaxation can be influenced by anisotropic motions of chain segments or by unusual distributions of correlation times [181],... 

A more detailed picture of the influence of the ionicity of the catalyst have been given in other studies. Coleman and Fox (18) showed that the production of isotactic sequences decreased as the alkylmetal catalysts became more anionic. Phenyl magnesium bromide had a high probability for isotactic sequences. Butyl lithium and octylpotassium were progressively less able to give steric control (Table 2). 

At higher temperatures in Fig. 25,sharp resonance lines become visible down-field to the CH2 and CH resonances as pointed out above. Since their chemical shifts are very close to those of the CH2 and CH resonances of the isotactic sequence of this polymer in solution, these resonance lines are assigned to the amorphous component. These assignments are also evidenced by examining the spin-lattice and spin-spin relaxation phenomena. 

As an independent approach Fourier transform infrared studies of isotactic polystyrene gels favours isotactic sequences in an ordered conformation different from the Natta type three-fold helix (38). 

Thus, for both chiral and prochiral a-olefins, the isotactic sequence of the stereogenic tertiary carbon atom of the backbone is due to the enantioselectivity of the chiral active sites to the prochiral carbon atom of the monomer. The stereoselectivity (namely the selection, among the enantiomers, of a racemic... 

The R sites accept (/ )-propylene oxide in preference to the (5)-enantiomer, resulting in the formation of RRR isotactic sequences of polymer chains. On the other hand, the S sites accept (.Sj-propylcnc oxide in preference to the (R)-enantiomer, resulting in the formation of SSS isotactic sequences of polymer chains. 

When D, L-alanine A-carboxylic acid anhydride was subjected to polymerisation with triethylaluminium as the catalyst, the polymer formed, polyalanine, was found [168,174] to consist of two fractions a water-insoluble fraction which was rich in isotactic sequences and a water-soluble atactic fraction. This indicated that some stereoselective polymerisation had occurred [75]. 

D,L-copolymerization of enantiomerically imbalanced mixtures of 6,8-dioxybicy-clo[3.2. l]octane (1) has revealed that an isotactic sequence along the polymer chain is preferentially formed by the enantiomer selection at the chiral growing chain end. [21] If this is generally the case, asymmetric copolymerization should occur when a racemic bicyclic acetal is allowed to copolymerize with an optically... 

However, the syndiotactic segments of extended chains (613 cm"1) are obviously situated in a different local environment. These more rigid segments form into microcrystalline phases which orient as units to much higher degrees than the amorphous isotactic sequences. The C-Cl peak at 613 cm"1 arises mainly from seqeunces of four or more trans conformations in syndiotactic repeat units. With syndiotactic sequences of four or more repeat units, crystallinity can form (17,18). This ease of crystallization has been ascribed to the strong dipole-dipole interaction between the C-Cl bonds. 

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