Stereochemical regularity

First, the short chain length PHAs, poly(HASCL), are composed of monomeric units containing up to 5 carbon atoms. The most well-known representatives are poly(3-hydroxybutyrate) (PHB), and its copolymers with hydroxyvalerate. Of all the PHAs, PHB is by far the most commonly encountered in nature [18]. It is the simplest PHA with respect to chemical structure, having a methylene (-CH3) group as the pendent R-unit in Fig. 1. Owing to its enzymatic synthesis, PHB has an exceptional stereochemical regularity. The chains are linear and the chiral centers all are in the R-stereochemical conformation, which implies that this polymer is completely isotactic. 

The study of the hydrogenation of D-fructose is of both practical importance and theoretical significance since it allows one to investigate stereochemical regularities (equation 27). 

Some refinement procedures include, at certain stages, criteria for stereochemical regularity in addition to the agreement between observed and calculated structure factor magnitudes. 

Owing to the mechanism of the topochemical reaction the polyconjugated polymer chain is of exceptional purity and stereochemical regularity. Polydiacetylene crystals are thus ideally suited to study the inherent optical and electrical properties of polyconjugated chains. These unique features have attracted considerable attention and in recent years the topochemical polymerization of diacetylenes has developed to... 

While both isotactic and syndiotactic polypropenes are partially crystalline materials with relatively high melting points (up to 160—170 °C for i-PP, and 150 °C for 5-PP), atactic polypropene (a-PP) is a fully amorphous polymer, since it lacks long-range stereochemical regularity. 

Although in the poisoner examples above, the use of stereoselective is preferred because of its denotation in simple organic reactions, we must also consider another term, stereoregular. Its acceptance in polymer chemistry is indicated by its use in reviews by Gaylord and Mark (1959) and by Bawn and Ledwith (1962). This term is generally used for any reaction which leads to stereochemical regularity in macromolecules, whether stereospecific or stereoselective as defined by Eliel. 

Of the three possible isomeric forms, poly(L-lactic acid) and pofy(D-lactic acid) are both semicrystalline in nature, and pofy(mesolactic acid) or poly(D,L-lactic acid) is amorphous. Racemic PLA - synthesized from petrochemicals - is atactic, i.e. it exhibits no stereochemical regularity of strueture, is highfy amorphous and has a low glass transition temperature. Amorphous grades of PLA ate transparenL... 

The norbornadiene dimer exo-trans-exo-pentacyc ote-tradeca-5,11-diene has been employed as a crosslinking agent for the controlled synthesis of these star-shaped copolymers in the catalytic presence of W(CHr-Bu)(NAr)(0/-Bu)2 or Mo(CHr-Bu)(NAr)Or-Bu)2 [66], Star polymers in which the arms are block copolymers of different polarities can be prepared by this method, as well as star polymers in which the composition of the arms is different, as are the solubility characteristics and stereochemical regularity. Heterostars thus obtained, in which the number of the different arms will be approximately equal, may exhibit interesting intramolecular phase separations. 

---