Isotactic polymers structural data

Three types of elastomeric polypropylene are known (1) a large molecular weight atactic polymer [125], (2) polymers with alternating isotactic and atactic blocks [108,109,126-129], and (3) polymers formed by the dual-site mechanism [110]. The structure of the obtained elastomeric polypropylene was elucidated by comparison of the 13C-NMR spectra with those of an atactic oily polymer and isotactic polypropylene prepared by the zirconium complex 4. On the basis of the NMR data, for all the samples the statistical lengths of the isotactic blocks between two neighboring epimerization stereodefects were calculated [41]. It was found that for the isotactic polymer 90%), the... 

This reduced partial structure factor analysis was also applied to the isotactic polymer. For surface concentrations less than 0.8 mgm a uniform layer model fitted the partial structure factor data for the spread polymer very well. Above this value, a Gaussian partial structure factor model fitted best however, even though the standard deviations of the Gaussians were reasonably large, the surface concentrations calculated from the parameters of the distribution were still less than the amount dispensed. This was again taken as evidence that some of the polymer penetrates the sub-phase and becomes lost insofar as the... 

TABLE 2.2 Structural Data of Isotactic (it) aud Syudiotactic (st) Polymers ... 

Further confirmation of the structure and tacticity of poly/5-methyl-l,4-hexadiene)was obtained from X-ray diffraction and u-NMR data of its hydrogenated polymer (Scheme 2). The hydrogenated polymer sample showed a highly crystalline pattern (Figure 7), with diffraction spots that were well defined. This pattern was identical to that of isotactic poly(5-methyl-l-hexene) as reported in the literature (26) (measured identity period, 6.2 A lit., 6.33 A). 

The program just described, for Rietveld analyses using generalized coordinates, has been used in the structural analysis of isotactic polypropylene recently undertaken both with x-ray and with neutron powder diffraction data. We believe this analysis (Immirzi, in preparation) to be the first Rietveld analysis of a polymer done from x-ray data. Rietveld analyses of polymers from neutron data have been done but, at least in the polyethylene case reported by Willis and co-workers (15), there was no use of generalized coordinates. 

The whole-pattern-fitting structure-refinement method can give correct, refined structural information for linear polymers. Use of generalized coordinates, in which accurately known structural information can be introduced as known parameters, makes the least-squares fitting problem tractable. Excellent fits of x-ray diffraction data for isotactic polypropylene permitted the selection of the correct space group and a preferred model. 

MAJOR APPLICATIONS The polymers referred to in this chapter include those families of homopolymers of propylene which are known to have elastomeric recovery properties at reasonable molecular weight and for which properties have been attributed to a crystallizable-noncrystallizable (e.g., isotactic-atactic) stereoblock structure, or to a major component with a stereoblock structure, whether or not the compositions are homogeneous by solvent fractionation tests. Copol)rmers and blends are not deliberately included in the data presented, but are described in some of the references. (See also some of the closely related elastomeric polymers presented in the entry on Polypropylene, atactic in this handbook.) The criterion of multiple crystallizable blocks per polymer chain may be met in significant fractions of low-tacticity, low-stereoregularity polymers of very high molecular weight. 

Poly(vinyl alcohol) is very high in head-to-tail structures, based on NMR data. It shows the presence of only a small amount of adjacent hydroxyl groups. The polymer prepared from amorphous poly(vinyl acetate) is crystalline, because the relatively small size of the hydroxyl groups permits the chains to line-up into crystalline domains. Synthesis of isotactic poly(vinyl alcohol) was reported from isotactic poly(vinyl ethers) like poly(benzyl vinyl ether), TOly(f-butyl vinyl ether), poly(trimethylsilyl vinyl ether), and some divinyl compounds. ... 

Some metallocenes with two interannular bridges have appeared in the literature, but with no polymerization data included in the reports of their synthesis and structure. Most of these examples possess Czv-symmetry. These types of precatalysts are expected to produce atactic polypropylene under enantiomorphic site control conditions (the most common scenario). Tacticity may vary if chain-end control is obeyed (rarer). Perhaps the authors did not publish polymerization data for propylene because of the expectation of obtaining atactic polymer, considered to be less indusdi-ally useful than its isotactic counterpart. (For more information on enantiomorphic site control and chain-end control, see Chapter 1.)... 

The inclusion of a variable temperature magic-angle spinning capability for solid state NMR spectroscopy makes feasible the investigation by relaxation parameters of structural and motional features of polymers above and below Tg and in temperature regions of secondary relaxations. Herein, we report variable temperature (50K to 323K) spectral data on semicrystalline poly(propylene) and glassy PMMA. Illustrative of the data are the Tj and results for isotactic... 

---