Polypropylene rotation isomerism

All isotactic vinyl polymers can be arranged in four ways in the usual crystals. Polypropylene has as stable conformation a 2 3/1 helix that consists of successive tg(—) or tg rotational isomeric states (left-handed or right-handed helices). In addition, it was possible for the helices to have the —CHj groups point up or down , relative to the crystallographic c-axis. In the ideal, monoclinic crystal form I (P2j /c) all four types of helices occur only at specific, symmetry-related positions The equilibrium melting temperature and entropy are 460.7 K and 15.1 J/(K mol), respectively. 

Based on relative intensity and, particularly, bandshape of skeletal bands, it has become possible to obtain the relative energy difference between various rotational isomeric states. In the first study of this type, Snyder and co-workers analyzed low frequency (0-600 cm ) Raman spectra of re-alkanes in the liquid state (35). The method was subsequently applied to analysis of the low frequency Raman spectrum of molten state isotactic polypropylene. The vibrational spectroscopic analysis was successfully used to differentiate the correct model governing the chain (80) and has been extended to analysis of higher frequency vibrations (0-1500 cm" ) of liquid re-alkanes (36). 

Conformational Analysis.— The major effort in this area has been directed towards empirical correlations of chemical shift and conformation. Tonelli and co-workers have examined the stereosequence dependence of C chemical shifts in polypropylene (PP) poly(vinyl chloride) (PVC), and polystyrene (PS) using the rotational isomeric-state scheme of polymer statistical mechanics. The structural variation of C shifts was shown to correlate well with the occurrence of three-bond gauche (y) interactions between carbon atoms together with y interactions between carbon and chlorine atoms in PVC and ring-current shifts in PS. It has been... 

Conformation and Chemical Shifts.—Chemical shifts have been correlated with conformation for H shifts in polystyrene and poly(vinyl chloride) and C shifts in model compounds of polypropylene. In all these papers, the chemical shift is related empirically to the occurrence of three- and four-bond steric interactions, similar to those used in the rotational isomeric state treatment of polymer statistical mechanics, " and the shift is expressed as a sum of compositional and conformational increments. The origin of the shielding contributions (magnetic anisotropy, electric field effects, etc.) is not stated, except for polystyrene in which the magnetic anistropy of the aromatic rings are incorporated... 

Flory reviewed in 1969 the development and applications of the rotational isomeric state scheme calculations, which allow, by matrix algebra, the statistical mechanical averaging over the rotational states of chain properties which may be expressed as a vector or tensor quantity associated with the chain bonds, and estimations of the probabilities of chosen conformational sequences. The methods were generalized and schemes for reducing the dimensions of certain generator matrices were presented in 1974, when comparisons were also made with an alternative Fourier expansion method, currently in use for atactic polypropylene. These techniques have greatly contributed to an understanding... 

Rotational Isomeric State (RIS) Calc ulations, with an Illustrative Application to Head-to-Head, Tail-to-Tail Polypropylene... 

Fig. 6 a) which, if repeated, is that of the 3 helical conformation which isotactic polypropylene, in common with most other isotactic polymers, is known to have in the crystalline state (Fig. 7). The calculations of Flory et al [75], using a three-state rotational isomeric state matrix treatment, showed that the mean... 

The configurational structure (stereoregularity) of 1-butene and of the higher polyolefins up to 1-nonene has been studied by NMR spectroscopy in solution [38, 39], interpreted with the aid of chemical shift calculations, consideration of the y effect and of the rotational isomeric state model of Flory. The evaluation of the results favors the bicatalytic sites model of polymerization [40] over simple Markovian statistics. In contrast to polypropylene, side-chain conformation also has to be considered. Comparison with alkane model compounds indicates that in meso-units of poly-1-butene, trans conformation of backbone is less favored than in isotactic polypropylene because of contiguous ethyl group interactions. Introduction of racemic units in both... 

In the case of isotactic polystyrene or of polyacrylates, other effects must be taken into account. In particular, together with the conformations TG and G T, for a piece of chain of the kind (40), also conformations near to TT planar are available. Without discussing the more complicated rotational isomeric model which result, it is interesting to explain why conformations near to the TT planar are possible for polystyrene and more unlikely for a polymer such as polypropylene. 

The chemical shift calculation (y-effect method) based on the y-effect of the chemical shift and the rotational isomeric state model (RIS model) has been developed as a reliable method for predicting chemical shift differences among pentad, hexad, and heptad sequences in various polyolefins [47-49, 14, 50, 51]. chemical shift assignments of tactic pentad and heptad sequences in polypropylene have been provided by this method [47-49]. Hayashi and co-workers [45,46] confirmed that the chemical shift due to the y-effect is also sensitive to different comonomer sequences in ethylene-propylene copolymers. Asakura and co-workers [52] have demonstrated that... 

The structures considered above have been concerned with the behaviour of the backbone of the polymer. On proceeding from polyethylene to the next member in the series of olefin polymers, polypropylene, [-CH2-CH(CH3)-] , an asymmetric centre has been introduced into the backbone, in this case the carbon bearing the methyl group. An asymmetric centre is one where it is possible to recognize two isomeric forms that are mirror images and not superimposable. These are often described as optical isomers and the terms d and I are introduced for dextro (right-) and laevo (left-) handed forms. For small molecules these isomers may be resolved optically since they will rotate the plane of polarization in opposite directions. 

It was also reported that elastomeric polypropylenes can be formed from the monomer with the aid of some metallocene catalysts [62-64]. Because rigid, chiral metallocene catalysts produce isotactic polypropylene, while the achiral ones produce the atactic form, Waymouth and Coates [62] prepared a bridged metallocene catalyst with indenyl ligands that rotate about the metal-ligand IxMid axis. The rotation causes the catalyst to isomerize between chivakic and nonchiralic geometries ... 

The most recent expansion of the metallocene catalyst families is represented by oscillating metallocenes such as bis(2-phenyl-indenyl)2ZrCl2, developed by Coates and Waymouth [28]. Rotation of the phenyl-substituted indenyl ligand accounts for formation of isoselective racemic and non-stereoselective meso-stereoisomers of the metallocene. Provided the isomerization rate is rapid in comparison to the chain propagation rate, segmented polypropylene is obtained, where crystalline isotactic and amorphous atactic segments alternate. The formation of such stereoblock polypropylenes is displayed in Fig. 13a and 13b. The ratio of amorphous and crystalline segments is controlled by polymerization temperature and monomer... 

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