Heterotactic triads

Use zero-order Markov statistics to evaluate the probability of isotactic, syndio-tactic, and heterotactic triads for the series of p values spaced at intervals of... 

Millan and coworkers (99-101) also studied the effect of tacticity on the nucleophilic substitution reactions of PVC. Sodium thiophenate and phenol were used for these reactions. The central chlorine in isotactic triads and, to a lesser extent, in heterotactic triads was found to be most reactive. It was concluded that initiation of degradation may occur by normal structures, and polyene build-up may be favored by syndiotic sequence. This... 

Such configurational as well as conformational effects have been also reported by MILLAN et al. in the case of nucleophilic substitution of poly(vinyl chloride) with sodium thiophenate (14) and with sodium isooctylthioglycolate or isooctylthiosalicylate (15). The authors have shown that these reactions proceed selectively on the isotactic TT diads which can only exist either in the GTTG isotactic or in the TTTG heterotactic triads, the former ones being much more reactive than the latter ones. 

In a recent study on poly(a-methyl-a-ethyl-piopiolactone) and poly(a-methyl-a-propyl-propiolactone) (112) Spassky et al. extended the earlier examination carried out on oxiranes and thiiranes (113) to the tetrad and pentad level. Because of the chirality of these polymers, Spassky avoids the use of the terms m and r as being inapplicable (21). To indicate the dyads RR (or SS) and RS (or SR) 64 he used the symbols ij and Sj where the indices stand for dyad. In their turn the triads RRR (or SSS) are indicated with i, RSR (or SRS) with s, RRS (or SRR) with hi (heterotactic triad beginning with an dyad) and RSS (or SRR) with (heterotactic triad beginning with a dyad), 65 (Scheme 14) (114). 

The probability of forming a particular triad is the product of the probabilities of forming the two dyads making up the triad. The coefficient of 2 for the heterotactic triad is required since the heterotactic triad is produced in two ways thus, mr is produced as mr and rm. Tetrad probabilities are given by... 

Meso (isotactic) and racemic (syndiotactic) triads result from Reactions 8-91 and 8-94, respectively. Reactions 8-92 and 8-93 yield the heterotactic triad. 

The absorptions of the a-CH3 group reflect the respective triade substructure (see Fig. 2.7) One observes different values of 6 for the isotactic, syndiotactic, and heterotactic triade, respectively, but all signals are singlets due to the lack of a proton at the carbon atom next to the CH3 group (see Fig. 2.8). Via quantitative evaluation of the intensity of the respective absorptions information is available about the homogeneity of the chain configuration. 

If energy migration were ignored, the fluorescence kinetics of aryl vinyl polymers in solution could be expressed by the same set of rate constants as utilized for the meso- and d/-2,4-diarylpentanes. However, each chromophore in the polymer (ignoring chain ends) has two neighbors which may participate in excimer formation. A chromophore centered in a heterotactic triad, for example, would generate excimers on the syndiotactic side and on the isotactic side at the rates k M, and ki w r.r. 

The general case of nonzero k o can be approximately treated by averaging M over the triad population. The probability M for the isotactic, syndiotactic, and heterotactic triads is taken equal to that for a polymer which is purely isotactic, purely syndiotactic, and alternately iso- and syndiotactic, respectively. It can be shown that M is given exactly for a purely isotactic polymer by Eq. (3), provided that kiio = k iD, me o a°d that kJjM = 2. A purely syndiotactic polymer can be treated in a similar way by utilizing rates from the dl isomer. A strictly alternating syndiotactic-isotactic polymer has the following exact expression for M ... 

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. 

Stereocontrol of free radical polymerization is influenced by monomer constitution, solventy and temperature. Most polymerizations seem to follow at least a Markov first-order one-way mechanism. Ratios of the four possible rate constants ki/iy ki/8, k8/i, and k8/8 can be calculated from the experimentally accessible concentrations of configurational triads and diads. With increasing temperature, more heterotactic triads are formed at a syndiotactic radical whereas the monomer addition at an isotactic radical favors isotactic and not heterotactic triads. Compensation effects exist for the differences of activation enthalpies and activation entropies for each of the six possible combinations of modes of addition. The compensation temperature is independent of the mode of addition whereas the compensation enthalpies are not. 

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... 

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... 

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-... 

In all of these systems, there is a tendency to form more isotactic triads than heterotactic triads (coming from an s to an i addition) with increasing temperature. With the exception of vinyl formate, all differences (AHzf/i — Aff ) are positive however. A formation of heterotactic triads (from an i to an s addition) is thus favored over the formation of syndiotactic triads with increasing temperature. Because the syndio-... 

A similar result has been recently found for the free radical polymerization of methyl methacrylate in 14 solvents (32). All differences (Aff. — AHf/8) were found to be positive, but only three of the 14 differences (AH /g — AH. ). Again, isotactic triad formation is favored over heterotactic triad formation and heterotactic triad formation over syndiotactic with increasing temperature as long as the individual modes of addition are considered and not the net result. Except for meth-acrylic acid in alcohols (cf. Lando et al. (28)) no model is known which shows why a certain solvent acts differently from another one with respect to stereocontrol in free radical polymerization. 

With the exception of methacrylic acid in various alcohols, a homotriad (iso- or syndiotactic triad) formation is always preferred to the formation of heterotactic triads (columns IV and V). 

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. 

For a triad of silicon atoms, there are four possibilities M,M (isotactic), R,R (syndiotactic), and M,R and R,M (heterotactic) the last two are equivalent since the polymer chain may be read in either direction. If there is no stereochemical preference, as in an atactic polymer, the amounts of isotactic, syndiotactic, and heterotactic triads would be 1 1 2, the heterotactic arrangement being twice as probable. 

The stereochemistry of GTP of MMA polymerization was measured for Lewis acid as well as for bifluoride catalysis. Lewis acid catalysts gave a ratio of syndiotactic heterotactic triads of 2 1 while bifluoride catalysis gave ratios near 1 1 [9, 41]. The amount of isotactic triads was about 5%. The effect of temperature on triad and diad composition provided data to calculate the difference in activation enthalpy (AAH ) and activation entropy (AAS ) for... 

Fig. 14. The probabilities Pi, and of formation of isotactic, syndiotactic, and heterotactic triads, respectively, as a function, of o, the probability of isotactic placement of monomer units during chain propagation. Experimental points at the left are for methyl methacrylate polymers prepared with free radical initiators . those at the right for polymers prepared with anionic initiators ( O) t peaks ...

Although several techniques have been used to characterize stereosequence distribution, we suggest that the percent crystallinity and temperature of melting measurements are more generally applicable than any other technique presently available. Bovey and co-workers (7) showed how NMR measurements can be used to determine the triad distribution in polymers such as polymethyl methacrylate in which there is sufficient difference between the NMR spectra corresponding to syndiotactic, isotactic, and heterotactic triads to allow quantitative measurements to be made. This type of measurement unfortunately is restricted to few systems and would lead to a unique description of the stereostructure of the chain only when a model involving one or two probability parameters is applicable (See Appendix I). 

Although the anionic polymerization of MMA in THF leads to highly syndiotactic PMMA (Table 1), that one of tBuMA results in an equimolar mixture of syndiotactic and heterotactic triads (Table 1). For both monomers an increase in temperature results in higher heterotacticity at the expense of the syndiotacticity . The chain tacticity has a direct influence on the macroscopic properties, as illustrated in Table 2 for both the glass transition (Tg) and the melting (T a) temperatures of PMMA . 

Figure 2.16 Isotactic, syndiotactic, and heterotactic triad configurations for poly(methyl methacrylate).

Since P and Pr are synonymous with isotactic and syndiotactic dyad fractions m and r, respectively, probabilities for isotactic, syndiotactic, and heterotactic triad sequences will be given by... 

The coefficient of 2 for the heterotactic triad arises from the fact that the heterotactic triad can be produced in two ways (as mr and rni). 

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