Stereoisomerism polypropylene

Important conclusions that can be drawn from considerations of polypropylene stereoisomerisms using various models are shown in Figure 3.5 [16]. 

A line of research that has aroused much interest in recent years is the study of head-to-head, tail-to-tail polymers (96-98). Their direct synthesis has little likelihood of being successffil as head-to-tail sequences usually predominate in vinyl polymerization. One possibility for their preparation is through the chemical modification of suitable preformed polymers. In the case of the head-to-head, tail-to-tail polypropylene, different stereoisomeric forms have been isolated, depending on the method of preparation. In the general scheme, the precursor is an unsaturated polymer obtained by polymerization of the disubsti-tuted butadiene (2,3-dimethylbutadiene or 2,4-hexadiene) then, by chemical or catalytic reduction, this polymer is converted into the desired polypropylene, whose stmcture can then be examined by NMR spectra. Head-to-head, tail-to-... 

The problem is further complicated for vinyl polymers with their problems of stereoisomerism. The first descriptions of the conformational state of isotactic polypropylene in solution go back 25 years (178, 179, 192, 193). Corradini, Allegra, and Ganis proposed a model, still essentially valid today, according to which macromolecules possess a local helical structure analogous to that observed in the crystalline state. The helix segments are rather short, only a few monomer units, after which an inversion of the helix sense occurs, with simultaneous alteration of its direction (Figure 15). As a whole this disordered con-... 

Fig. 4.17 illustrates the potential of carbon-13 NMR to detect the presence of isotactic (a), syndiotactic (b), and atatactic (c) vinyl polymers with polypropylene as sample [521], The spectrum of atactic polypropylene (Fig. 4.17(c)) displays the signals of all possible stereosequences including iso- and syndiotactic ones. Using the empirical increment systems for alkane carbon shift prediction [85, 201, 202] and including y effects of Zy = — 5 ppm specifically obtained by analysis of stereoisomeric polypropylene partial sequences between 3,5-dimethylheptane and 3,5,7,9,11,13,15-heptamethylheptadecane as a heptad model, the methyl carbon-13 shifts of all 36 possible heptads can be calculated... 

Nuclear magnetic resonance has been extensively used to investigate stereoisomerism in polymers such as polypropylene, polyacrylates, vinyl polymers. 

Considering again two adjacent carbons in the main chain of the polymer, six conformations are now possible because of the presence of an asymmetrically substituted carbon atom, as shown in Fig. 2.7. Forms 1 and 6 can be neglected for steric reasons so four different conformations are still possible for the polymer. Atactic polypropylene (see Stereoisomerism) has two trans forms (2 and 5) in the fully extended state (III)... 

Additional structural information can also be derived by Py-GC. Information on stereoisomerism, crystallinity, and sequence distribution data is sometimes able to be obtained. Isotactic and syndiotactic polypropylene may be identified by the ratio of oligomeric components. High- and low-density polyethylene can also be determined by the proportion of branch chain alkane pyrolysis products using a novel pyrolysis hydrogenation technique. 

Figure 1.10 Stereoisomerism of polypropylene. Regular organisation in isotactic polypropylene can lead to highly crystalline materials, whereas the irregular organisation in atactic polypropylene cannot.

A polymer in which at least one chain atom in a mer can exhibit stereoisomerism (e.g., -CH2C HX-), but has no preference for one particular configuration, e.g., atactic vinyl polymers (PVC or PS), atactic polypropylene, and PP. 

Stereorepeating unit n. A configurational repeating unit having defined configuration at all sites of stereoisomerism in the main chain of a polymer molecule (lUPAC). In stereoregular polypropylene, the two simplest possible stereorepeating units are ... 

NMR s ability to characterize the tacticity, or stereoisomerism, of polymers, such as polypropylene, polystyrene, or poly(vinyl chloride), is also significant. The tacticity of these, or of any other stereoirregular polymer, is analyzed as sequences of stereo pairs, or dyads. Each pair is designated as either meso (m) or racemic (r), depending on the stereostructure. In isotactic... 

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