Isotactic dyads

If we define p and Pj. as the probability of addition occurring in the meso and racemic modes, respectively, then p Pr since there are only two possibilities. The probability p is the analog of Pjj for copolymers hence, by analogy with Eq. (7.30), this equals the fraction of isotactic dyads among all dyads. In terms of the kinetic approach of the last section, p is equal to the rate of an iso addition divided by the combined rates of iso and syndio additions ... 

Most polymers formed by radical polymerization have an excess of syndiotactic over isotactic dyads [i.e, < 0.5]. / (m) typically lies in the range... 

The analysis shown in Figure 1 is however incomplete. The medium is a three-component system consisting of two polar components (monomer and THF) and one non-polar (toluene). Correlations in terms of two components only lead to incomplete characterization and, if extrapolated, invalid conclusions. In Figure 3 the meso (isotactic) dyad frequencies of polymers produced by t-butylmagnesium bromide at 225 are shown as a function of the mol fractions of the three components of the medium. It is apparent that monomer itself acts as a polar solvent component decreasing the isotactic content in comparison to toluene. It should be noted that the polymerizations carried out at =... 

Figure 3. Meso (isotactic) dyad frequency (m) of polyfmethyl methacrylate), prepared at 250 initiated by t-butylmagnesium bromide, as a function of solvent composition. The meso-frequency m is indicated for each point. The solvent composition, given in terms of mole fractions of THF, XThf, of toluene, Xtoiimie and of monomer, , ore indicated by the scale on each median, with each apex corresponding to the pure component indicated.

Bichromophoric model compounds can provide such standards, since these compounds can be highly purified and characterized by techniques inapplicable to polymers. For example, the racemic compound 2,4-diphenylpentane models the syndiotac-tic dyad of PS, while the meso 2,4-diphenylpentane models the isotactic dyad of PS. The 2,4-diarylpentanes are the best models of aryl vinyl polymers, since the populations of the rotational conformers of the model compounds are similar to those of the dyads of the polymer. 

The rate parameters for the model compounds of PS and P2 VN are given in Table 10. The values of M for isotactic, syndiotactic, and heterotactic triads of P2VN can be calculated as 0.035, 0.147, and 0.0565, respectively. For the same triads of PS, the values of M are 0.0097, 0.172, and 0.0184, respectively. If we assume that a typical atactic polymer has 50 % isotactic dyads, and if the dyads are independently distributed on the polymer, then there will be 25 % isotactic and 25 % syndiotactic triads. Thus, the value of M for a 50% isotactic P2VN sample in solution should be about 0.074 that for a 50% isotactic PS sample in solution should be 0.0545. 

The 100 MHz H-NMR spectra in benzene-d6 solution afforded additional information. Isotactic polymers prepared by zinc catalysis had a symmetrical quartet at 6.38, 6.48, 6.67, and 6.77 ppm, whereas amorphous polymers prepared by KOH catalysis had an additional quartet of equal intensity at 6.36,6.46,6.60, and 6.70 ppm. Therefore, the former absorption could be assigned to the isotactic dyad and the latter to the syndiotactic one. Amorphous polymers prepared by aluminum catalysis had an additional quartet assigned to a tail-to-tail dyad. 

Polymerization of propylene oxide-a-d was carried out by the EtZnNBu ZnEt catalyst in benzene solution in the presence of varying amounts of added water at 70° C, and was terminated after 7 days. The microstructure of the crude polymer was determined by the 1H-NMR method and the yields of amorphous and crystalline polymers were determined by a fractionation method (Fig. 16). When the amount of added water was increased up to 0.3 mole per mole of catalyst, the yield of crystalline polymer increased remarkably, whereas that of amorphous one remained nearly constant, and the isotactic dyad content (I) increased remarkably while syndiotactic one (S) remained almost constant. Thus, the striking parallel was observed between the yield of crystalline polymer and the isotactic dyad content, and between the yield of amorphous polymer and the syndiotactic dyad content. It is therefore concluded that water contributes more remarkably to the formation... 

The difference in the apparent activation energies for the formation of syndiotactic and isotactic dyads can be estimated by the following relationship ... 

Comparison to the model spectra strongly suggests that in the CF2 resonance region of the polymer spectrum, the peak at 104.2 represents CFa groups which are between monomer units in isotactic dyads, — dd or ll —, whereas the peak at 106.0 represents CFa groups which are between syndiotactic dyads, dl or Id. As before, the first type of CFa group may be designated as meso and the second as racemic. If the chain were random or atactic , these peaks would be of equal... 

Signals are split into two closely located singlets and comparison of the C-NMR spectrum of the racemic polymer, obtained from the racemic monomer, with that of the optically active polymer (prepared from (+)-( R, 5S)-6,8-dioxabicyclo[3,2,l]oc-tane indicate, that the lower field, smaller peak of each signal pair comes from the D-L (syndiotactic) dyad. The hi er field signal can thus be ascribed to the dyad structures of D-D and L-L consecutive units (isotactic dyads). 

Referential formation of the isotactic dyads (L-L and D-D units), i.e. the higher rates of homopropagation than of crosspropagation has been explained by Okada in terms of stereoselection due to the steric hindrance when, e.g. the D-monomer approaches the growing center composed of L-unit ... 

Comparisons of the intensity of signals led to the conclusion that the polymer is richer in isotactic dyads (Table 8.6). 

The experimental values for.atactic PMA given in Table 8 are estimated to correspond to 49-56% isotactic dyads, as estimated from the work of Matsuzaki et al.94)... 

An explanation for the preference for syndiotacticity during MMA polymerization was proposed by I suruta et al. I hey considered that the propagating radical should exist in one of two conformations and showed, with models, that attack on the less hindered side of the preferred conformation (where steric interactions between the substituent groups are minimized) would lead to formation of a syndiotactic dyad while similar attack on the less stable conformation would lead to an isotactic dyad,... 

Much literature precedent supports the assignment of tacticity in methyl acrylate polymers using NMR techniques [40,41]. In the H-NMR spectrum, the shift of the methylene protons is sensitive to dyad stereochemistry. For example, in an isotactic (meso) dyad 28, the methylene protons are chemically non-equivalent and appear as two separate sets of signals, whereas in a syndiotactic (racemic) dyad 29, the methylene protons are equivalent. The H-NMR spectrum of 27 showed multiplets at 1.89 and 1.5 ppm due to the two diastereotopic methylene protons of the isotactic dyad. The rest of the spectrum is consistent with the structure of the n=4 tetrad 27. A racemic dyad structure would have been expeeted to give resonances of intermediate shift to that of the two resonances observed for the telomer 27. This evidence strongly implies that 27 has the allisotactic configuration shown in Scheme 8-12. 

Czv-Symmetric Catalysts. Syndiotactic polymers have been formed using metallocene catalysts where the polymer chain end controls the syndiospecificity of olefin insertion. Resconi has shown that Cp 2MCl2 (M = Zr. Hf) derived catalysts produce predominantly syndiotactic poly(l-butene) with an approximate 2 kcal/mol preference for syndiotactic versus isotactic dyad formation." At —20 °C. Cp 2HfCl2/MAO produces poly(l-butene) with 77% rr triads. Pellecchia had reported that the diimine-ligated nickel complex 30 forms moderately syndiotactic polypropylene at —78 °C when activated with MAO ([rr] = 0.80)." " Olefin insertion was shown to proceed by a 1.2-addition mechanism." in contrast to the related iron-based systems which insert propylene with 2.1-regiochemistry. ... 

Catalyst Monomer Isotactic dyads (%) Syndiotactic dyads (%) Reference... 

Sample (% isotactic dyads) (gmi ) Heat of fusion (J g Degradation time (h) Weight loss (%)... 

Tacticity Commercial grade materials generally have 50-70% 30% atactic, and <10% isotactic dyads syndiotactic. (3)... 

A simultaneous coordination of the two aldehydes prior to addition may explain the observed sequence of isotactic dyads. 

Enzymatic degradation of racemic PHB films with different stereoregularities (crystallinity from 33 to 8% for an isotactic dyad fraction [/] varying from 0.88 to 0.54) (Abe and Doi 1996) was carried out at 37°C at pH 7.4 in the presence of PHB depolymerase (1.0 pg) from A faecalis. In the absence of depolymerase, no hydrolysis was observed for over 48 h for aU films. The racemic PHB film with an isotactic... 

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