Monomers unit configuration

A "0" represents one monomer unit configuration, "1" is its opposite configuration. 

Identical stereosequences are initiated by intermediates that are oppositely handed in the configurations of both site and chain-end unit. R and S monomer unit configurations in the polymer are indistinguishable. The probabilities for pentads in the total polymer are therefore equal to those of the pentads initiated by either an S or an R active site with S and R chain-ends. For the sake of brevity, only the events occurring at S configuration sites are considered in the following discussion on stereosequence initiation. 

Figure C2.1.1. (a) Constitutional isomerism of poly (propylene). The upper chain has a regular constitution. The lower one contains a constitutional defect, (b) Configurational isomerism of poly(propylene). Depending on tire relative configurations of tire asymmetric carbons of two successive monomer units, tire corresponding dyad is eitlier meso or racemo.

Poly(vinyl fluoride) [24981-14-4] (PVF) is a semicrystaltiae polymer with a planar, zig-zag configuration (50). The degree of crystallinity can vary significantly from 20—60% (51) and is thought to be primarily a function of defect stmctures. Wide-line nmr and x-ray diffraction studies show the unit cell to contain two monomer units and have the dimensions of a = 0.857 nm, b = 0.495 nm, and c = 0.252 nm (52). Similarity to the phase I crystal form of poly (vinytidene fluoride) suggests an orthorhombic crystal (53). 

Any of the four monomer residues can be arranged in a polymer chain in either head-to-head, head-to-tail, or tail-to-tail configurations. Each of the two head-to-tail vinyl forms can exist as syndiotactic or isotactic stmctures because of the presence of an asymmetric carbon atom (marked with an asterisk) in the monomer unit. Of course, the random mix of syndiotactic and isotactic, ie, atactic stmctures also exists. Of these possible stmctures, only... 

Distribution of the monomer units in the polymer is dictated by the reactivity ratios of the two monomers. In emulsion polymerization, which is the only commercially significant process, reactivity ratios have been reported (4). IfMj = butadiene andM2 = acrylonitrile, then = 0.28, and r2 =0.02 at 5°C. At 50°C, Tj = 0.42 and = 0.04. As would be expected for a combination where = near zero, this monomer pair has a strong tendency toward alternation. The degree of alternation of the two monomers increases as the composition of the polymer approaches the 50/50 molar ratio that alternation dictates (5,6). Another complicating factor in defining chemical stmcture is the fact that butadiene can enter the polymer chains in the cis (1), trans (2), or vinyl(l,2) (3) configuration ... 

For the 1,2- and 3,4-addition, a chiral carbon (marked by an asterisk) is formed which has an R or 3 configuration, but there is no net optical activity, because equal amounts of the R and S configurations are formed. The R and S configurations along the polymer chains lead to diastereomeric isomers called isotactic, syndiotactic, and atactic. In isotactic polyisoprene all monomer units have the same configuration as illustrated for isotactic... 

The configuration of a center in radical polymerization is established in the transition state for addition of the next monomer unit when it is converted to a tetrahedral sp1 center. If the stereochemistry of this center is established at random (Scheme 4.1 km = k,) then a pure atactic chain is formed and the probability of finding a meso dyad, P(m), is 0.5. 

The application of NMR spectroscopy to tacticity determination of synthetic polymers was pioneered by Bovey and Tiers.9 NMR spectroscopy is the most used method and often the only technique available for directly assessing tacticity of polymer chains. "2 7 8 0JI The chemical shift of a given nucleus in or attached to the chain may be sensitive to the configuration of centers three or more monomer units removed. Other forms of spectroscopy (e.g. TR spectroscopy l2 lJ) are useful with some polymers and various physical properties (e.g. the Kerr effect14) may also be correlated with tacticity. 

This physically unrealistic assumption may be abolished by dividing by x, the number of permutations of the monomer units for a given configuration as originally defined. Hence... 

In the simpler cases, the discrimination between the two faces of the prochi-ral monomer may be dictated by the configuration of the asymmetric tertiary C atom of the last inserted monomer unit (chain-end. stereocontrol) or by the chirality of the catalytic site (chiral site stereocontrol). The distribution of steric defects along the polymer chain may be indicative of which kind... 

On the basis of the microstructure of the prevailing isotactic polymer chains, it is well established that the steric control of the heterogeneous Ziegler-Natta catalysts is due to the chirality of the catalytic site and not to the configuration of the last inserted monomer unit.28,29,95... 

The first report of ROMP activity by a well-characterized Mo or W species was polymerization of norbornene initiated by W(CH-t-Bu)(NAr)(0-f-Bu)2 [122]. In the studies that followed, functionality tolerance, the synthesis of block copolymers, and ring-opening of other monomers were explored [30, 123]. Two important issues in ROMP concern the cis or trans nature of the double bond formed in the polymer and the polymer s tacticity. Tacticity is a consequence of the presence of two asymmetric carbons with opposite configuration in each monomer unit. The four ROMP polymers (using polynorbornene as an example) that have a regular structure are shown in Scheme 3. 

Prior to a discussion of the theory of rubber elasticity, it is important to review how isolated polymer chains behave as this will provide a picture of the size and shape of a polymer. Clearly a polymer chain in a vacuum will collapse into a dense unit, but when in a solution the molecule will take on a conformation which is a function of the interaction with the surrounding molecules and the balance between the entropically driven tendency to maximise the spatial configuration and the connectivity of the monomer units. This is the case whether the chain is surrounded by small molecules (solvent) or other macromolecules that may or may not act like a solvent. 

The butadiene polymers represent another cornerstone of macromolecular stereochemistry. Butadiene gives rise to four different types of stereoregular polymers two with 1,2 linkage and two with 1,4. The first two, isotactic (62) and syndiotactic (25), conform to the definitions given for vinyl polymers, while the latter have, for eveiy monomer unit, a disubstituted double bond that can exist in the two different, cis and trans, configurations (these terms are defined with reference to the polymer chain). If the monomer units all have the same cis or trans configuration the polymers are called cis- or trans-tactic (30 and 31). The first examples of these stereoisomers were cited in the patent literature as early as 1955-1956 (63). Structural and mechanistic studies in the field have been made by Natta, Porri, Corradini, and associates (65-68). 

If, during ring opening polymerization a tertiary carbon is involved in the reaction, a configurational inversion may occur in which case a change in the stereochemistry of the monomer unit with respect to the monomer will be observed. Thus, the cf5-2,3-epoxybutane gives rise to a polymer with threo monomer units, 39, whereas the trans isomer is transformed into erythro or meso units, 40 (Scheme 8) (81, 82). 

This phenomenon of chirality degradation is carried to the extreme in the polymerization of (-F)-rrinactive polymer One of the two equivalent asymmetric atoms inverts its configuration during polymerization giving rise to a monomer unit with eiythro or meso strac-ture. The isotactic polymer, 40, so formed is clearly achiral (280). 

The Arcus approach can be extended to the analysis of the polymerization of 1,4-disubstituted butadienes. The main differences concern the presence of a further element of stereoisomerism, the cis or trans configuration of the double bond in the chain, and the fact that the chemical monomer unit (—CHA—CH=CH—CHB—) does not coincide with the stereochemically significant chain segment (corresponding to =CH—CHB—CHA—CH=). 

There are four types of possible stereoregular structures for each frans-1,4 polymer of 1,4-disubstituted butadiene (Fig. 13). The stereochemistry of polymers is represented by two kinds of relationship as follows, one of which is the relative configuration between the two repeating monomer units. When all the... 

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