Polymers absolute stereoregularity

The most common states of a pure substance are solid, liquid, or gas (vapor), state property See state function. state symbol A symbol (abbreviation) denoting the state of a species. Examples s (solid) I (liquid) g (gas) aq (aqueous solution), statistical entropy The entropy calculated from statistical thermodynamics S = k In W. statistical thermodynamics The interpretation of the laws of thermodynamics in terms of the behavior of large numbers of atoms and molecules, steady-state approximation The assumption that the net rate of formation of reaction intermediates is 0. Stefan-Boltzmann law The total intensity of radiation emitted by a heated black body is proportional to the fourth power of the absolute temperature, stereoisomers Isomers in which atoms have the same partners arranged differently in space, stereoregular polymer A polymer in which each unit or pair of repeating units has the same relative orientation, steric factor (P) An empirical factor that takes into account the steric requirement of a reaction, steric requirement A constraint on an elementary reaction in which the successful collision of two molecules depends on their relative orientation. 

Topochemical [24-2] photoreactions of diolehn crystals has been reviewed. The reactions clearly depart from typical solution chemistry crystal-lattice control offers a unique synthetic route into photodegradable polymers, highly strained [24-2] paracyclophanes, stereoregular polymers, and absolute asymmetric synthesis. However, achieving the desired type of crystal... 

Stereoregularity. Representative stereoregularities of the two adjacent constitutional units in the homopolymer and co-polymers of terminal epoxides and symmetrical m-epoxides are listed in Figure 1. A diad is described as meso m) if the absolute configurations of the two adjacent constitutional units are the same, and racemo (r) if they are the... 

In this case a considerable optical activity should be expected for completely stereoregular isotactic cis (V) or trans (VI) polymer, since all the tertiary carbon atoms of the main chains bound to a > CH2, -CH3 and -CH= group, have the same absolute configuration. 

In the cases in which remarkable differences exist between the molar rotation of stereoregular polymers and models, the absolute value of the optical activity depends on stereoregularity, being lower in the samples of lower stereoregularity. In these cases the absolute value of the rotatory power decreases by increasing temperature and the temperature coefficients of the molar rotation are at least ten times higher in the polymers than in the model compounds (Table 24). 

Stereoregularities of the resulting polymers depend on the sizes of the host channels. Moreover, the space effect in chirality was observed in asymmetric inclusion polymerization of trans- or cis-2-methy 1-1,3-pentadiene by using a pair of hosts, deoxycholic acid and apocholic acid. We obtained optically active polymers with predominant absolute configurations (R). Optical yields varied with the polymerization conditions and the hosts. A maximum optical yield of the trans monomer was 36% m the channel of apocholic acid. 

Modem readers can hardly imagine the absolute novelty of those findings and the embarrassing inadequacy of vast sectors of the chemical community to assess or even understand them. As a matter of fact, until 1954 it was largely believed that stereoregular polymers can only have natural origin (as can be read in the motivation of the Nobel Prize to Natta [25]). That the world was just not ready is demonstrated... 

Mathematical Quantification of the Absolute ( abs)/ Relative ( rel)/ nd Total ( ) Stereoregularity of a Polymer in the Hemiisotactic Continuum... 

From the examples discussed in this chapter it results clearly that chiroptical techniques supply a powerfull method for investigating the molecular conformation of macromolecules in solution. The evident limitation of the method is that optically active polymers only display chiroptical properties. While this occurs in the most naturally occurring polymers, it is not true for the most common stereoregular synthetic polymers. In these last the asymmetric carbon atomsliave either one or the other absolute configuration with the same probability and inter-and intra-molecular compensations cancel any optical rotation. 

There is no doubt that these principles are applicable too to other systems which undergo topochemically controlled polymerizations, such as the diacetylenes [33] and, to a lesser extent perhaps, to mixed crystals (to give copolymers) and to inclusion complexes, in urea for example. Trans, fraws-pentadiene in urea has been reported to give a stereoregular polymer [42]. Further, it is known that crystals of the urea channel complexes have chiral structures. Thus we expect that this polymerization in a single crystal would give rise to an asymmetric polypentadiene, and therefore would provide a further example of absolute synthesis. 

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