Configuration atactic

Polymers without configurational regularity are called atactic. Configurationally regular polymers can fonn crystalline stmctures, while atactic polymers are almost always amorjihous. Many polymers consist of linear molecules, however, nonlinear chain architectures are also important (figure C2.1.2). 

Any of the four monomer residues can be arranged in a polymer chain in either head-to-head, head-to-tail, or tail-to-tail configurations. Each of the two head-to-tail vinyl forms can exist as syndiotactic or isotactic stmctures because of the presence of an asymmetric carbon atom (marked with an asterisk) in the monomer unit. Of course, the random mix of syndiotactic and isotactic, ie, atactic stmctures also exists. Of these possible stmctures, only... 

For the 1,2- and 3,4-addition, a chiral carbon (marked by an asterisk) is formed which has an R or 3 configuration, but there is no net optical activity, because equal amounts of the R and S configurations are formed. The R and S configurations along the polymer chains lead to diastereomeric isomers called isotactic, syndiotactic, and atactic. In isotactic polyisoprene all monomer units have the same configuration as illustrated for isotactic... 

The configuration of a center in radical polymerization is established in the transition state for addition of the next monomer unit when it is converted to a tetrahedral sp1 center. If the stereochemistry of this center is established at random (Scheme 4.1 km = k,) then a pure atactic chain is formed and the probability of finding a meso dyad, P(m), is 0.5. 

PIB exists either as a low mw (about 12,000) viscous or atactic liq or as a cryst matl of about 1.00,000 mw. In this latter form the iso tactic (chain) type configuration predominates over the atactic and amorph forms (Ref 8). Since the isotactic form is the form designated by mil spec (Ref 2), its parameters are presented below ... 

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)... 

The term tactidty refers to the configuration of polymer chains when their constituent monomer residues contain a steric center. Figure 1.8 illustrates the three principal classes of tacticity as exemplified by polypropylene. In isotactic polypropylene, the methyl groups are all positioned on the same side of the chain, as shown in Fig. 1.8 a). In syndiotactic polypropylene, the methyl groups alternate from one side to the other, as shown in Fig. 1.8 b). Random placement of the methyl groups results in atactic polypropylene, which is shown in Fig. 1.8 c). We can readily observe the effects of tacticity on the properties of polypropylene isotactic polypropylene is hard and stiff at room temperature, syndiotactic polypropylene is soft and flexible, and atactic polypropylene is soft and rubbery. 

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. 

Monomer addition under radical propagation conditions leads to mainly an atactic configuration. As a consequence, radical polymerisations of asymmetric vinyl polymers usually lead to amorphous materials. However, if the substituent is small enough to enter into the crystal cell, atactic vinyl polymers can crystallise (an example is poly(vinyl fluoride)). 

Very active catalysts for the preparation of strictly alternating butadiene-propylene copolymers (BPR) consist of V0(0R)2C1/i-Bu Al (R = neopentyl). The CH3 side groups in BPR stiffen the polymer chain and were expected to promote the formation of strain-induced structures. The fact that we could not detect strain-induced crystallization is probably due to an atactic configuration of the propylene units. 

Figure 1.1 Schematic representation of configuration of isotactic, syndiotactic, and atactic vinyl polymers.

Figure 1.1 shows the typical representation proposed to distinguish easily between the different types of stereoisomerism present in vinyl polymers. Viewing the main chains as lying down on a plane, there are (i) isotactic sequence of configurations, (ii) syndiotactic sequence of configurations, and (iii) no order present, atactic sequence of configurations. 

Although the definitions of isotactic, syndiotactic, and atactic polymers according to International Union of Pure and Applied Chemistry (IUPAC) rules are well established in terms of succession of mesa (m) or racemic (r) dyads,12 the symbolism of (+) and (—) bonds allows the easy treatments of possible configurations in cases of any complexity.1 Moreover, the (+) or (—) character of the bonds in a polymer chain is strictly related to the accessibility of gauche+ or gauche conformations of the bonds and, therefore, to the formation of right-handed or left-handed helical conformations.1... 

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

A third configuration known as 7, 2 vinyl configuration, is also possible. In the case of 1, 2 vinyl configuration, the three types of optical isomerism, i.e., atactic, isotactic and syndiotactic configurations, are possible. 

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 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 versatility of Ziegler-Natta catalysis is shown in the polymerization of butadiene. Polybutadiene may have either a 1,2 or 1,4 configuration. The 1,4 polymer has a double bond as part of the main chain and this can be atactic, isotactic, or syndiotactic. Thus many different polybutadienes can be made and all of them have been made with the aid of Ziegler-Natta catalysts. 

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