Isotactic macromolecule

Note In an isotactic macromolecule, the configurational repeating unit is identical with the configurational base unit. 

These materials, however, as a rule exhibit rather broad chemical composition distribution. Block copolymers may contain important amounts of parent homopolymer(s) [232,244,269], In any case, it is to be kept in mind that practically all calibration materials contain the end groups that differ in the chemical composition, size, and in the enthalpic interactivity from the mers forming the main chain. In some cases, also the entire physical architecture of the apparently identical calibration materials and analyzed polymers may differ substantially. The typical example is the difference in stereoregularity of poly(methyl and ethyl methacrylate)s while the size of the isotactic macromolecules in solution is similar to their syndiotactic pendants of the same molar mass, their enthalpic interactivity and retention in LC CC may differ remarkably [258,259]. 

In spite of the fact that asymmetric carbon atoms1 are formed in the synthesis of vinyl polymers, head-to-tail linear vinyl polymers showing remarkable optical activity in solution cannot be prepared by the usual polymerization processes unless asymmetry centers are already present in the monomers (6, 13). In fact, by considering the principal types of vinyl polymers prepared up to now, the ideal isotactic macromolecules, having such terminal groups as to render both terminal segments of the macromolecule chemically identical, possess a symmetry plane and therefore the polymers cannot show optical activity they might be considered as a peculiar type of meso compounds. 

In other words, isotactic macromolecules deriving from non-asymmetric monomers, spiraled in one screw sense only, might in principle exist in Hie solid state however on dissolution, it should be expected at least a partial despiralization, as well as the formation of sections spiraled in both screw senses, having equivalent average length, and hence the disappearance of optical activity, which in principle, might exist in the solid state. 

According to the above model, the left- and the right-handed helix sections in an ideal isotactic macromolecule are supposed to change continuously their length with time, the percent of spiraled principal chain in each screw sense being constant at a given temperature (116). 

Birshtein and Luisi (17) made attempts to prove the theoretical validity of the above model by applying the theory of VOLKENSTEIN (137, 138, 139), Birshtein and Ptitsyn (16) on the conformation of isotactic macromolecules, to optically active poly-a-olefins. From their results it is possible to conclude that the presence of asymmetric carbon atoms in the lateral chains might cause the prevalence of main chain sections spiraled in one of the two possible screw senses, mainly for entropic reasons. 

Isotactic macromolecule - A tactic macromolecule, essentially comprising only one species of repeating unit which has chiral or pro chiral atoms in the main chain in a unique arrangement with respect to its adjacent constitutional units. [8]... 

Much of the work on the conformation of polymers concerns itself with reactions of the lower olefins. By the use of certain catalytic systems, crystalline syndiotactic polypropylene may be formed exclusively, rather than as a co-product in the formation of the isotactic macromolecule. To accomplish this, rather specific reaction conditions are required. For example, a catalytic system of approximately five molar equivalents of aluminum dialkylmonochloride to one of vanadium triacetylaceonate at less than 0°C initiates the stereospecific polymerization of propylene to a syndiotactic resin. Other catalytic systems have also been described [108]. As discussed above, these polymers are not expected to be optically active. 

Wiyatno, W. Pople, J. Gast, A. P Waymouth, R. M. Dynamic response of stereoblock elastomeric polypropylene studied by rheooptics and X-ray scattering. 1. Influence of isotacticity. Macromolecules 2002, 35, 8488-8498. 

Isotactic polymers are composed of isotactic macromolecules. In isotactic macromolecules, all the substituents are located on the same side of the macromolecular backbone. An isotactic macromolecule consists of 100% meso chads. PP synthesized by a Ziegler-Natta catalysis is an isotactic polymer (Figure 20.1). Isotactic polymers are usually semicrystalline and often form a helix configuration. 

Also in this case the inequality of the coupling constants between the two methylenic protons and the methynic ones [i] indicates that isotactic macromolecules in solution are not perfectly spiralized. Hence, it seems neither necessary nor sufBcient to suppose the presence of steric irregularities to explain the end to end distance observed by viscosimetric measurements on isotactic poly-a-olefins [ ]. 

Isotactic macromolecules derived from achiral monomers have no preference for right- or left-handed screw senses, and the two are perfectly balanc at inter-and intramolecular level. However, distribution of left- and right-handed helical secondary structures affects markedly the free energy of the system, alternation of the two senses in the same chain being favored for entropic reasons [4,5]. If this last situation takes place, conformational optical activity cannot be obtained... 

An isotactic macromolecule is formed through a series of successive asymmetric additions of the same sign on the monomer double bond ... 

---