Main chain tacticity

Table 9. Erythro selectivity [e] in the initiation process and main chain tacticity of propylene polymerization. 

For reaction conditions, see the particular references. Fraction of cis double bonds in the polymer main chain Tacticity bias of the cis- or /ra 5-centred dyads Depends on the particular A-substituents of the A-heterocyclic carbene ligands(s), see Ref. [295] Data given for the initially yielded poly(a n-7-methylbornene) Syn-iosmer is polymerized slowly after complete consumption of anti isomer, thus forming a block copolymer Depends on monomer-initiator ratio. 

Additionally the H-NMR spectra can afford, at least in the case of PNVC, information on main chain tacticity. In fact the large splittings observed in methine and methylene signals have been attributed to conformational differences arising from different configurational enchainment of prochiral monomeric units Recently... 

From these results, and from the reported extrapolated values of 276 °C (syndiotactic) and 126 °C (isotactic) for the glass transition temperatures of PNVC it has been concluded that rotational eclipsing in isotactic NVC sequences is very much more favourable than in corresponding syndiotactic sequences. This contusion is strongly supported by examination of space filling molecular models. Such considerations must be borne in mind when considering the optical and chiroptical properties of carbazole containing polymers and it has to be expected that copolymers of NVC will have fluorescence emission properties that can be related to both main chain tacticity and distribution of monomeric units. Free radically polymerized mixtures of NVC and acrylic derivatives of (—)-menthol have fluorescence spectra... 

As for the stereochemistry, for the case of complete cyclization, besides the usual tacticity (possibly, isotactic or syndiotactic, referred to relative configurations of equivalent stereogenic carbons of subsequent monomeric units), the cis or trans configuration of the 1,3-cycloalkane rings which are present in the polymer main chain also has to be considered.70,74... 

C-l to C-4) increase, as expected, as a function of the distance from the anchoring main-chain, and increase with temperature, thus reflecting their ease of motion (see Table I). Unexpectedly, the n.O.e. values are less than their maximum, and the effect of field intensity is not marked. However, almost all Ti values, both those of main and side chains, increase on going from 22.6 to 67.9 MHz. Unfortunately, T2 values could not be measured in this study, because of line-broadening due to tacticity. 

Tactic macromolecule, essentially comprising only one species of configurational base unit, which has chiral or prochiral atoms in the main chain in a unique arrangement with respect to its adjacent constitutional units. 

Tactic polymer that contains two sites of defined stereoisomerism in the main chain of the configurational base unit. 

Tactic polymer in which the main-chain double bonds of the configurational base units are entirely in the Z configuration. 

The pioneering work of Natta and co-workers introduced the concept of tacticity, i.e. the orderliness of the succession of configurational repeating units in the main chain of a polymer. For example, in polypropene (polypropylene), possible steric arrangements are (shown in Fischer projections displayed horizontally) ... 

Linear macromolecules having a constitutional repeating unit such as -CH2-CHX- (X H) show two further stereoisomerisms, i.e., optical isomerism and tacticity. The stereoisomerism named tacticity has its origin in the different spatial arrangements of the substituents X. When we arrange the carbon atoms of the polymer main chain in a planar zigzag conformation in the paper plane. 

The changes were mainly caused by temporal main-chain scission followed by recombination. The trace of the tacticity change enabled the evaluation of the efficiency of temporal main-chain scission, G(TCS), giving G(TCS) = 18.6, which was much higher than the G value of permanent main-chain scission. The high value of G(TCS) is also very important to reveal the amount of the potential reactive intermediates, hence for the fate of energy deposited by radiation in the polymeric system. 

The RIS model, coupled with the Flory matrix method, is applied to the calculation of the unperturbed mean-square end-to-end distance in polylcyclohexene sulphone) as a function of several parameters. The calculations are performed for atactic, isotactic and syndiotactic chains the tacticity arises from the two possible ways, D and L, in which the rings can be attached to the main chain, assuming that the C—C bonds are all in the trans conformation, as indicated by dielectric measurements. 

Natta carried out the anionic polymerization of methyl sorbate, a 1,3-diene, with an optically active initiator and obtained an optically active homopolymer with main-chain chirality. The high molecular weight crystalline polymer produced with (P)-2-methylbutyllithium had a tritactic (di-iso-rra/w-tactic) structure. This was probably the first metal-catalyzed asymmetric polymerization 134). Polymerization of other dienes was attempted by using various asymmetric methods 135). 

It is convenient to classify these polymers on the basis of the tacticity of the main chain only thus, polymers obtained from one antipode, in which all the asymmetric carbon atoms of the lateral chains have the same absolute configuration, will be considered as homopolymers. On the contrary, polymers made up by macromolecules in the side chains of which asymmetric carbon atoms having R or S absolute configuration alternate more or less regularly, can be considered as copolymers of the two antipodes of the monomer having respectively R and S absolute structure. 

A cistactic polymer is a tactic polymer in which the main chain double bonds of the configurational base units are entirely in the cis arrangement. A transtactic polymer is a tactic polymer in which the main chain double bonds of the configurational base units are entirely in the trans arrangement [134]. Geometrical cis-trans isomerism also arises from different configurations of substituents on a cyclic structure. 

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