Steric purity

This highly sophisticated investigation has been superseded by C spectroscopy, which is mote sensitive to the stereochemical envirorunent and does not requite the preparation of deuterated polymers. C NMR analysis of steric purity and sequence distribution can, therefore, be directly carried out on commercially available polymers. Thus, any doubt regarding the previous conclusions about laboratory-produced polymers obtained under necessarily different... 

The development of simpler, more economic processes for the production of polyethylene and polypropylene did not signify the end of MgCl2 catalyst research, but rather constituted the first phase. New, ever more sophisticated requirements, both in terms of process and product quality, have been emerging, steadily increasing the performance range required for the catalyst control of the polymer molecular structure (MWD, branching, steric purity), of its morphological properties (shape and particle size distribution), and supply of copolymers with controlled structures. 

It should be noted that the tendency to form a nematic phase may be markedly diminished by departures from perfect helicity of the polypeptide chain. Experiments must therefore be conducted under conditions far removed from the helix-coil transition. Also, steric purity of the monomer units must be assured. Through their effect in disrupting perpetuation of the helix, as little as one percent of D-peptide units in an L-chain (or vice versa) may be expected to increase Vp considerably if the degree of polymerization is large, e.g., for a degree of polymerization DP > 1000, or X > 100. 

Isotactic polymers have only been formed in high steric purity by using well-formed, crystalline cocatalysts, so an explanation of... 

X-ray crystallography (Section 5.2.1) is an absolute method. With it, it is possible to determine the distances between the atoms in crystalline regions, and then the configuration from the position and intensity of the diffractions. The method does not depend on knowledge of model compounds. However, it is only applicable to substances that crystallize well and have a high steric purity. X-ray crystallography is used in configurational studies to calibrate relative methods. 

In general, therefore, polycondensation does not lead to products of complete steric purity. The degrees of polymerization are also usually low, since the conversions and yields are small. 

Of the polybutadiene rubbers only the cis- polybutadienes of high steric purity are capable of crystallization and in the case of unstrained samples at normal ambient pressures this only occurs below 3 C. For this reason the influence of crystallization on tensile strength and other properties is so minimal that it can be ignored in the case of polybutadiene rubbers. 

The strength of gum stocks of the trans-polypentenamer depends on steric purity. High trans- compounds do show a pronounced upward swing of the stress-strain curve and high values of tensile... 

Next, let us consider the relationship between the dimensions of an isotactic polymer in solution and its steric purity. Flory, Mark and Abe [75] have pointed out that in an isotactic chain, no steric conflicts occur in the conformational... 

There is strong evidence that the ring-opening polymerization of cycloolefins provides a convenient approach for the synthesis of poiyaikenamers with high trans or cis content [2, 3]. Except for polyhexen-amer, poiyaikenamers of predominantly trans structure have been obtained from cyclobutene to cyclododecene, and poiyaikenamers of predominantly cis structure from cyclobutene to cyclooctene. In addition, poiyaikenamers of high steric purity have been obtained from norbomene and its derivatives or from other bicyclic and polycyclic olefins. 

Polymerization of monomers having high steric purity... 

A very obvious way of producing stereoregular polymers using this synthetic approach is to start from monomers of high steric purity. Since the polymerization processes considered... 

A special type of stereoelectivity can be foreseen in a copolymerization of a racemic monomer with an optically active comonomer. Under the influence of the latter, one enantiomer of the racemic monomer can be preferentially incorporated in the polymer chains. Only a few systems of this type have been investigated. In the following sections we shall consider separately the synthesis of stereoregular polymers from monomers of high steric purity, and from racemic monomers. 

The C H NMR spectrum of this polymer shows fine structure for each carbon atom, arising from the possibility of syn and anti conformations of the side-chain formyl groups [2]. The steric purity of the main chain is therefore more easily investigated by examining the C H NMR spectrum of the hydrolysis product, polypropylenimine, 3. 

We may briefly consider the implication of these results on the mechanism of polymerization of 4-methyloxazoline. The steric purity of the optically active polymer is consistent with a single type of propagation step, namely (1) as postulated by Saegusa et al... 

E should be respectively above 100 or below 0.01 for S- and R-specific enzymes to ensure high steric purity. 

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