Vinyl polymers, stereoisomerism

Fig. 2. Stereoisomerism in vinyl polymers (a) isotactic (b) syndiotactic and (c) atactic.

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. 

Tertiary carbon atoms along the chain have been defined as asymmetric (22-25, 34-37), pseudoasymmetric (6, 10, 38-40), stereoisomeric centers (30, 31), and diasteric centers (41). The first two terms put the accent on chirality and are linked to the use of models of finite and infinite length, respectively the last two consider only phenomena of stereoisomerism. Note the relationship between these last definitions and Mislow s and Siegel s recent discussion (42), where the two concepts—stereoisomerism (or stereogenicity) and chirality—are clearly distinguished. The tertiary carbon atoms of vinyl polymers are always stereogenic whether they are chinotopic or achirotopic (42) depends on stmctural features and also on the type of model chosen (43). 

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... 

Stereoisomerism occurs in vinyl polymers when one of the carbon atoms of the monomer double bond carries two different substituents. It is formally similar to the optical isomerism of organic chemistry in which the presence of an asymmetric carbon atom produces two isomers which are not superimposable. Thus glyceraldehyde exists as two stereoisomers with configurations shown in 4-13. (The dotted lines denote bonds below and the wedge signifies bonds above the plane of the page.) Similarly, polymerization of a monomer with structure... 

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

Optical Activity in Polymers Stereoisomerism in polymers is formally similar to the optical isomerism of organic chemistry. In a vinyl polymer with the general structure shown in (XIH) every other carbon atom in the chain, labeled C, is a site of steric isomerism, because it has four different substituents, namely, X, Y, and two sections of the main chain that differ in length (Rudin, 1982). 

Linear macromolecules having a constitutional repeating unit such as -CH2-CHX- (X 7 H) show two further stereoisomerisms, i.e., optical isomerism and tacticity. The stereoisomerism named tacticity has its origin in the different spatial arrangements of the substituents X. When we arrange the carbon atoms of the polymer main chain in a planar zigzag conformation in the paper plane, X is either above or below that plane ( Natta projection ). If the substituents X of the vinyl polymer are either all above or all below that plane (case I), the polymers are... 

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. 

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-... 

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