Isotactic diads

Table I shows some peculiarities. In contrast to what has been claimed in literature, the probabilities of forming an isotactic diad at an existing iso tactic diad (pj/j) do not always equal the probabilities of forming an isotactic diad at an existing syndiotactic diad (p8/i). The free radical polymerization of the six methacrylate monomers mentioned is thus not always Bemoullian, at least not at the temperature of — 78°C and in toluene as a solvent.

A positive value of (AHf/ — AHf/8) means that the formation of a heterotactic triad is easier with increasing temperature if isotactic diads are formed at syndiotactic ones and not vice-versa. More interesting are the differences of activation enthalpies and activation entropies for the other five ratios of rate constants. Because very few data for the temperature dependency of triad fractions have been reported in literature, the calculation of the corresponding activation enthalpies and en-... 

At existing syndiotactic diads, synoditactic diads are always formed more easily than isotactic diads (column II). At existing isotactic diads, syndiotactic diad formation is however preferred to isotactic diad formation (column III) if one considers the limits of error. 

Heterotactic triads are formed more easily by a formation of isotactic diads at existing syndiotactic diads in the case of methacrylic monomers than vice-versa (column VI). The probability for the formation of heterotactic triads is however independent of the mode of addition for vinyl polymers. 

A chiral Mt -O73-butenyl) species will give, depending on its structure and thus on the orientation of the incoming monomer, a new Mt ( -butenyl) species of the same chirality as the previous one (and hence an isotactic diad) or of the opposite chirality (and hence a syndiotactic diad) it is obvious that the tacticity may concern only 1,2-polymers of non-substituted or substituted butadiene and 1,4-polymers of terminally monosubstituted and symmetrically disubstituted butadiene. The mode of the formation of the butenyl group of the same or opposite chirality with respect to the preceding butenyl group is shown, for 1,3-butadiene insertion, in Figure 5.4 [7],... 

The crystalline fraction was found to be formed via a cleavage of the C1g-0(CH2—O) bond in the monomer molecule and proved many years ago to be an isotactic polymer (with regular head-to-tail linkages) (Figure 9.1) [42]. The structure of the amorphous fraction, on the other hand, varies depending on the kind of catalyst. Some amorphous polypropylene oxide)s prepared with catalysts such as diethylzinc-methanol [43] or aluminium isopropoxide-zinc chloride [44] consist of regular head-to-tail linked units, but they are atactic (the mole fraction of isotactic diads is less than 0.6) [43]. Some other amorphous polypropylene oxide)s obtained with catalysts derived from reactions in the diethylzinc-water [44,45], and triethylaluminium-water [46] systems, and with aluminium isopropoxide [44], have been found to be irregular, i.e. to contain head-to-head and tail-to-tail enchained monomer units. 

Some cadmium compounds, including simple salts, were revealed to be excellent catalysts for the enantiosymmetric polymerisation of propylene sulphide [156,157], For instance, the proportion of isotactic diads in the polypropylene sulphide) sample obtained in polymerisation with the cadmium (7 )-tartrate catalyst was more than 95%, higher than the 69% which was characteristic of a polymer sample prepared using the zinc (i )-tartrate catalyst [158]. The superior stereoselectivity of the cadmium (i )-tartrate catalyst is also borne out by the more effective separation into fractions having opposite optical rotations of the polypropylene sulphide) yielded by cadmium tartrate, compared with that yielded by zinc (i )-tartrate. Note the quite different behaviour of these two catalysts in terms of their stereoelectivity in the polymerisation of propylene sulphide only very slight optical activity was found for the polypropylene sulphide) sample prepared using cadmium tartrate, whereas that associated with the polymer sample obtained with zinc tartrate was found to have a much higher value [158]. 

NMR has also been used to study the degree of syndiotacticity in PVC as, for example, that prepared by the bulk polymerization process at temperatures ranging from -78 to 120 C. By the analytical techniques used, an atactic polymer would have a fraction of syndiotactic diads, 8 equal to a, the fraction of isotactic diads, or equal to 0.5. Polymer prepared at or near commercial temperatures had a syndiotactic diad fraction 8, equal to 0.51 to... 

Unfortunately, this kind of discrimination is unfeasible in the polymethyl methacrylate system. However, the tactidty of the polymer provides a useful way of differentiating between these two alternatives. The polymethyl methacrylate formed in THF by monofunctional initiators is, to a high degree, syndiotactic. The tacticity should have remained unaltered had the association-dissociation mechanism been operative, whereas the polymer formed in the early stages of a reaction initiated by bifunctional initiators shows a larger proportion of isotactic diads. This demonstrates that the slow addition involves a different center, i.e. it takes place on the associated end-groups. A further discussion of tacticity is postponed here and will be continued later. 

A variety of stereoregulating mechanisms have been invoked to explain the stereochemistry of anionic polymerization of alkyl methacrylates. As discussed by Pino and Suter [189], although syndiotactic diads are thermodynamically slightly more favored over isotactic diads, the free energy differences are so small that the formation of stereoregular... 

It must be remembered here that the mole fractions of isotactic diads in chiral monomeric units, as, for example, with a-amino esters, is given by the sum of... 

Since the two hydrogen atoms of a methylene group in an isotactic diad are not nmr-spectroscopically equivalent, they have also been called (and also called heterosteric or diastereotopic). In analogy to this, the methylene group of a syndiotactic diad has been called racemic (and also called homosteric or enantiotopic). For these reasons the composition fractions of isotactic and syndiotactic diads are often given in the literature as (m) or (r) instead of asXi or The names racemic and meso are not equivalent... 

Isotactic diads of both dd and ll units are formed in the copolymerization of d and l monomers. Thus, for a first order Markov mechanism, the following is obtained from equations (15-33) and (15-35) with respect to the monomeric units ... 

The instantaneous isotactic diad mole fraction is given by... 

The mole fraction of isotactic diads in final polymer can be calculated as follows. The rate of formation for the number of isotactic diads is equal to... 

From the change in the number of isotactic diads with conversion, i.e., by combining Equations (15-61), (15-62), and (15-59), we obtain... 

With the definition of the mole fraction of isotactic diads, we obtain with Equation (15-59) and the corresponding expression for the syndiotactic diads ... 

Comparison of Equations (15-65) and (15-60) shows that, the instantaneous isotactic diad mole fraction is only equal to that of the final polymer when the following holds ... 

The influence of the medium is stronger. Polymers with a relatively high syndiotactic triad fraction are produced in highly polar solvents (see Table 18-5). The isotactic diad fraction increases with increasing apolarity of the solvent. The heterotactic triad fraction remains more or less constant. Even here, the increase in isotacticity obviously parallels the increase in ion pairs. This conclusion can be readily understood. The linkage in chain propagation by free ions is determined by the most sterically favorable transition state but... 

If the configuration of the central carbon atoms of the molecule I in Figure 3-1 is observed from the position of carbon atom C-1 in direction of carbon atoms C-2, C-3, etc., then the nearest substituents H CH2 -> Cl follow in a clockwise direction. With C-4 of molecule I in Figure 3-2, the sequence of substituents H CH2 -> CH3 is also in a clockwise direction. The sequences thus follow in the same direction. Such a diad is called an iso tactic (it) diad. Consequently molecules IV (Figure 3-1) and I and IV (Figure 3-2) also contain only isotactic diads. 

Each configurational triad consists of a pair of diads. The pair of diads can be isotactic-isotactic or syndiotactic-syndiotactic or the triad can consist of a syndiotactic and an isotactic diad. The sum of the mole fractions of these three kinds of triads must also be equal to unity. 

The macromolecular substance is better characterized when the experimentally quantified J-ad is largest. A block copolymer consisting of a block of isotactic diads and a block of syndiotactic diads... 

Polymers are described as atactic in the literature when they do not completely or predominantly consist of only one type of J-ad. Strictly speaking, one must here distinguish between conventionally atactic and ideally atactic polymers. An ideally atactic polymer is described as one where, in the formation of each diad, a syndiotactic linkage and an isotactic linkage are equally probable. Thus, an isotactic diad can add on either an isotactic diad or a syndiotactic diad. Thus, Xj = Xg = 0.5 x = x s = 0.25 Xjs = 0.5 etc. Heterotactic triads can be formed in two ways (i is and... 

Since the two hydrogen atoms of a methylene group in an isotactic diad arc not NMR-chemically equivalent, they have been called meso ... 

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