Atactic distribution

Atactic. Substituents are distributed at random along the chain, for example, -DDLDLLLDLDLL-. 

Polystyrene produced by free-radical polymerisation techniques is part syndio-tactic and part atactic in structure and therefore amorphous. In 1955 Natta and his co-workers reported the preparation of substantially isotactic polystyrene using aluminium alkyl-titanium halide catalyst complexes. Similar systems were also patented by Ziegler at about the same time. The use of n-butyl-lithium as a catalyst has been described. Whereas at room temperature atactic polymers are produced, polymerisation at -30°C leads to isotactic polymer, with a narrow molecular weight distribution. 

When poly(propylene) was first made, it was found to exist in two possible forms. One was similar to poly(ethylene), but had greater rigidity and hardness the other was found to be amorphous and of little strength. The first of these is now known to be isotactic, that is with a regular stereochemistry at each alternating carbon atom. The other is now known to be atactic, that is with a random distribution of different stereochemical arrangements... 

Moreover it has been shown that PV0CC1 prepared by free-radical polymerization of vinyl chloroformate (V0CC1) is an atactic polymer having a Bernouillian statistical distribution as expected (J[9). In order to extend our studies on the chemical modification of PV0CC1, the stereoselective character of the nucleophilic substitution of the chloroformate units with phenol has been examined by the study of the 13c-NMR spectra of partly modified polymers in the region of the aliphatic methine carbon atoms. The results obtained in this field are presented here. 

The 29Si resonance is therefore a single narrow line. However for dialkylpolysilanes with two different alkyl groups on each silicon, (RR Si)n, each silicon atom is a chiral center and the resonance for a particular silicon will depend upon the relative stereochemistry of other nearby silicon atoms. For such polymers, a rather symmetrical cluster of peaks is observed (Figure 5). These results are consistent with atactic structures, having a statistical (Bernoullian) distribution of relative configurations.(32,33)... 

In atactic polymers, side groups are irregularly positioned on either side of the chain, as illustrated schematically in Fig. 1.8 c). A truly atactic polymer would comprise a random distribution of steric centers. In practice, atactic polymers typically show some preference for either meso or racemic placement The tendency towards stereoregularity is due to the fact that polymerization catalysts often contain steric centers, which tend to direct the incoming monomers and the growing chain into preferred configurations. 

Atactic polypropylene exhibits a greater vapor permeability relative to either syndiotactic or isotactic polypropylene of the same molecular weight distribution. Why is this so ... 

In contrast to the case of Cp2ZrX2/MAO giving atactic poly(alkene)s, Cp MCl2/MAO, M = Zr (139) and Hf (140), are the catalyst precursors of the syndiotactic polymerization of 1-butene and propylene [176]. Triad distribution indicated that this is chain-end controlled syndiospecific polymerization. The syndiospecificity is attributed to the increase of steric encumbrance around the metal center. Thus, Cp HfX2 is the most effective syndiospecific catalyst component in this system. 

Polymers in which the configuration of the X-substituted carbon atoms are randomly distributed are known as atactic polymers. 

Atactic. A characteristic of the spatial configuration of atoms or groups in a polymer chain. Atactic indicates a random distribution- of those atoms or groups, i.e., no symmetry to the spatial configuration. This characteristic is important, for example, in determining the properties of polypropylene. 

The tenn atactic was introduced in 1956 to indicate a polymer with no steric regularity (56). It was first used to indicate a behavior opposite to that of isotactic and syndiotactic polymers [in early articles the term anisotactic was employed (23)]. Its meaning is sometimes restricted to polymers having an equal number of randomly distributed substituents on both sides of the chain (21) deviations from this random disposition are interpreted in terms of microtac-ticity. 

The Suter-Flory model was successfully used to interpret the results of the epimerization reaction carried out on propylene oligomers (204) and on polypropylene itself (106, 205). In both cases a slight prevalence of the r dyad over the m (52/48) is observed. The epimerized polypropylene has a microstmcmre almost coincident with a Bernoulli distribution and represents the polymer sample closest to an ideal atactic polymer so far obtained. 

The treatment of irregular and atactic polymers, and, in general, real polymeia (with statistical distribution of molecular weight and defects) is much more complex. With regard to atactic vinyl polymers it is not possible to recognize the presence of mirror symmetry, not even in the infinite chain model. 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

Note As the definition above indicates, a regular polymer, the configurational base units of which contain one site of stereoisomerism only, is atactic if it has equal numbers of the possible types of configurational base units arranged in a random distribution. If the constitutional repeating unit contains more than one site of stereoisomerism, the polymer may be atactic with respect to only one type of site if there are equal numbers of the possible configurations of that site arranged in a random distribution. 

A polymer such as -[-CH=CH-CH(CH3)-CH2-hr which has two main-chain sites of stereoisomerism, may be atactic with respect to the double bond only, with respect to the chiral atom only or with respect to both centres of stereoisomerism. If there is a random distribution of equal numbers of units in which the double bond is cis and trans, the polymer is atactic with respect to the double bond, and if there is a random distribution of equal numbers of units containing the chiral atom in the two possible configurations, the polymer is atactic with respect to the chiral atom. The polymer is completely atactic when it contains, in a random distribution, equal numbers of the four possible configurational base units which have defined stereochemistry at both sites of stereoisomerism. 

In addition to isotactic, syndiotactic and atactic polymers (and other well-defined types of tactic polymers), there exists the whole range of possible arrangements between the completely ordered and the eompletely random distributions of configurational base units,... 

An isotactic polymer has only one species of configurational base unit in a single sequential arrangement and a syndiotactic polymer shows an alternation of configurational base units that are enantiomeric, whereas in an atactic polymer the molecules have equal numbers of the possible configurational base units in a random sequence distribution. This can be generalized as follows in zig-zag and horizontal Fischer projections ... 

The polymer chain end control model is supported by the observation that highly syndiotactic polypropene is obtained only at low temperatures (about —78°C). Syndiotacticity is significantly decreased by raising the temperature to —40°C [Boor, 1979]. The polymer is atactic when polymerization is carried out above 0°C. 13C NMR analysis of the stereoerrors and stereochemical sequence distributions (Table 8-3 and Sec. 8-16) also support the polymer chain end control model [Zambelli et al., 2001], Analysis of propene-ethylene copolymers of low ethylene content produced by vanadium initiators indicates that a syndiotactic block formed after an ethylene unit enters the polymer chain is just as likely to start with an S- placement as with an R-placement of the first propene unit in that block [Bovey et al., 1974 Zambelli et al., 1971, 1978, 1979]. Stereocontrol is not exerted by chiral sites as in isotactic placement, which favors only one type of placement (either S- or R-, depending on the chirality of the active site). Stereocontrol is exerted by the chain end. An ethylene terminal unit has no preference for either placement, since there are no differences in repulsive interactions. 

The rate parameters for the model compounds of PS and P2 VN are given in Table 10. The values of M for isotactic, syndiotactic, and heterotactic triads of P2VN can be calculated as 0.035, 0.147, and 0.0565, respectively. For the same triads of PS, the values of M are 0.0097, 0.172, and 0.0184, respectively. If we assume that a typical atactic polymer has 50 % isotactic dyads, and if the dyads are independently distributed on the polymer, then there will be 25 % isotactic and 25 % syndiotactic triads. Thus, the value of M for a 50% isotactic P2VN sample in solution should be about 0.074 that for a 50% isotactic PS sample in solution should be 0.0545. 

The configurational-conformational characteristics of PP are discussed by considering every polymer chain as constituted by the periodic repetition of a sequence of monomeric units in a given configuration. Calculations are presented for the special case in which mesa and racemic diads are distributed according to Bemoullian statistics. Numerical results show that the characteristic ratio of atactic PP reaches an asymptotic value of 5.34 when the size of the periodic sequence corresponds to six monomeric units. 

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