Syndiotactic diads

Three kinds of polymer segments are formed in the polymerization of dienes 1-4 cis-, 1-4 trans-, and 1-2 segments (or 3-4 in polymerization of isoprene or other monosubstituted dienes). The latter may form isotactic or syndiotactic diads when the proportion of the 1-2 form is sufficiently high, e.g. a syndiotactic, highly 1-2 polybutadiene was described recently by Ashitaka et al. 123), although the so far examined 1-2 polybutadienes produced by homogeneous anionic polymerization were found to be atactic (unpubl. results of Bywater, Worsfold). 

It has been suggested however that isotacticity derives from polymerization occurring on colloidal particles formed by thermal decomposition of the catalysts. As stated previously, in the presence of the monomer even the allyl compounds are stable at 65°C and none of the thermal decomposition products (black to yellow solids) could be detected. As a check on these results a polymerization of propylene was carried out with Zr (benzyl) 4 in toluene at 0°C in a sealed tube. The reaction was very slow and analytical quantities of polymer could be obtained only after 312 hr. NMR analysis showed peaks assignable to isotactic sequences, and these were much stronger than the peaks assignable to syndiotactic diads. It was concluded... 

On the (110) face there are many possibilities. In fact, although an isolated TiCl3 unit adsorbed on the (110) face can be thought to produce a poorly stereoregular polymer with a prevalence of syndiotactic diads,103 C - and... 

TP he free radical polymerization of vinyl and acryl monomers normally does not lead to a true atactic polymer (I). A true atactic polymer is defined in this context as a polymer consisting of 50 isotactic and syndiotactic diads each, 25 iso- and syndiotactic triads each, and 50 heteroactic triads, etc. Furthermore all diads, triads, tetrads, etc., must be distributed at random. 

Most free radical polymerized polymers exhibit a preponderance of syndiotactic diads. The syndiotacticity normally increases with decreasing temperature. This phenomenon has lead some authors to conclude that the stereocontrol is governed by the bulkiness of the substituent. Implicit in this argument are the assumptions that only repulsive forces... 

Intuitively, the idea of an influence of the size of the substituent on the stereocontrol in free radical polymerization is very appealing. However, it is not supported by quantitative data. Table I shows the probabilities p of the formation of isotactic and syndiotactic diads at existing iso- and syndiotactic diads for the free radical polymerization of the methacryl type monomers CH2=CH(CH3)COOR in toluene at —78°C. These probabilities have been calculated from published mole fractions of iso- and syndiotactic diads (Xi, X8) and iso-, syndio-, and heterotactic triads (Xu, X88 and Xu) via... 

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.

From the ratio of iso- and syndiotactic diad fractions, one thus does not get the ratio of the sum of rate constants leading to iso- and syndiotactic diads (Equation 11). Instead one gets only the ratio of rate constants of processes leading to the formation of heterotactic triads. In... 

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],... 

Excimer formation and its potential applications for polymers continues to be widely studied. Poly(N-vinylcarbazole) continues to be investigated with the monomer exhibiting yet again a new emission below 400 nm assigned to a second excimer state similar to that seen in the homopolymer. Poly(3,6-di-tert-butyl-9-vinylcarbazole) has been found to give unique sandwich excimer formation due to syndiotactic diads while the effect of hydrostatic pressure on intramolecular excimer formation for meso-2,4-di-N-carbazolylpentane dissolved in poly(propylene oxide) has indicated that the compressibility of free volume cannot be considered to be constant. A new scheme has been proposed to... 

Figure 6.24. Illustration of the two limiting cases of perfect stereoregularity, for a vinyl-type chain backbone. One diad (pair of -CXR- groups) is shown. X and R denote atoms or groups attached to backbone carbon atoms, with X R. The thick lines represent bonds coming out of the plane of the paper, while the dashed lines represent bonds going below the plane of the paper. X s and the R s point above and below this plane, respectively, for the isotactic chain. They alternate regularly in direction (handedness) for the syndiotactic chain. The limit of total lack of stereoregularity (an atactic chain) occurs when the isotactic and syndiotactic diads are equal in number on a polymer chain and are distributed completely randomly over its length.

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... 

The possible origin of the low stereoselectivity for the chain-end-controlled catalytic systems based on metallocenes including two cyclopentadienyl rings has been discussed by Corradini, Guerra, and co-workers.2 The two possible diastereomeric preinsertion intermediates for si and re coordinations of the monomer for the case of a si chain—that is, a growing chain in which the last monomeric unit has been obtained by addition of a si-coordinated propene—are shown in Figure 24, parts A and B, respectively. These models would lead to a si—si and re—si propene enchainments, that is, to an isotactic and syndiotactic diad, respectively. 

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... 

Atactic polymers are also regular polymers. They contain, by definition, the possible configurational monomeric units in equal proportions, but with an ideally random distribution from molecule to molecule. Such distributions are caused by symmetric Bernoulli mechanisms during polymerization (see Section 15). They are distinguished by having equal numbers of iso- and syndiotactic diads (A i = ATJ, iso-, hetero-, and syndiotactic triads (Na = A is =... 

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... 

On the other hand, stereocomplexes and solvatophobic bonds are specific to macromolecular multimerization. Certain polymers with mutually complementary stereostructures form what are called stereocomplexes whose stoichiometry is dependent on the stereosequence length. Examples of such stereocomplexing are provided by the pairs poly(7-benzyl-D-glutamate) with poly(7-benzyl-L-glutamate) and isotactic with syndiotactic poly(methyl methacrylates) (PMMA). The specific enthalpy of stereocomplex formation with it- and st-PMMA is, for example, a linear function of the syndiotactic diad mass fraction, with a maximum at Wst = 0.58 (Figure 6-11). If, however. 

Materials. Atactic PPBA was prepared by radical polymerization in bulk or in benzene solution at 60-75°C, using BZ2O2 as initiator. It contained 55% syndiotactic diads. Isotactic PPBA was prepared by polymerizing the monomer in toluene at -78°C with Bu Li as initiator. The procedure was very similar to that reported in the literature for the preparation of other isotactic... 

Monomers with bulky side groups do not form syndiotactic-rich polymers readily. Several attempts to prepare syndiotactic PPBA were made. The best results were achieved by polymerizing the monomer in toluene solution at -78° under U.V. irradiation, when a sample was obtained with 65% syndiotactic diads. 

As we have already discussed, the extent of syndiotacticity in the polymer here described as syndiotactic was actually quite low (( 65%) Table I), and in fact only slightly higher than the polymer obtained by radical polymerization and here described as atactic (55% syndiotactic diads). From Figure 2 it can be seen... 

In statistical considerations, it is immaterial what the nature of the unit is. Two constitutionally different monomers may, for example, be units (i.e., acrylonitrile and styrene), or they may be two optical antipodes (i.e., D- and L-alanine), or they may be two configurational diads (e.g., iso- and syndiotactic diads), etc. 

The mole fraction of syndiotactic diads is obtained from the condition... 

The mole fractions of syndiotactic diads obtained by an asymmetric Bernoulli polymerization mechanism carried out on optical antipodes can never exceed a value of 2/ 3 (see also Figure 15-3). In addition, the heterotactic triad fraction must always be twice as large as the syndiotactic triad fraction. 

Figure 15-3, Relationship between triad fractions and the syndiotactic diad fraction. The lines give the theoretical relationship for an enantiomorphic catalyst with placements independent of the previously occurring placement the circles give the experimental results for the polymerization of methyl vinyl ether with Ab(S04)3/H2S04 in toluene. (From data of T. Higashimura, Y. Ohsumi, K. Kuroda, and S. Okamura.).

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

Thus, according to mechanism, the ratios of mole fractions of different kinds of diads, triads, etc., lead to very different rate constant combinations (Table 15-7). For example, the mole fraction ratio of iso- and syndiotactic diads gives the ratio of rate constants for iso- and syndiotactic linking in the case of Bernoulli mechanisms, but gives the ratios of the rate constants for the cross-steps and not a mean of the rate constants in the case of first-order Markov mechanisms. 

Table 20-6. Syndiotactic Diad Fractions and Transition Probabilities p for the Formation of Iso- and Syndiotactic Diads in the Free Radical Polymerization of Ethylene Derivatives, CHj CRR ...

The tetrahedral state of the carbon atoms first must be destroyed in the isomerization of iso- and syndiotactic polymers. For this, however, bonds (either main-chain bonds or bonds from the main chain to substituents) must be destroyed. The most probable state occurs on reformation of destroyed bonds, that is, polymers with the ratio of iso- to syndiotactic diads corresponding to the conformational equilibrium are formed. 

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