Dyad tacticity

Probabilities Pm and P, are synonymous with the dyad tactic fractions (m) and (r) defined in Sec. 8-2b. Triad probabilities, synonymous with the triad fractions, follow as... 

Possible Dyad Tacticity and Double Bond Configuration Associated with ROMP of Norbornene... 

The proportions of these various sequences provide important information about the mechanism of the propagation reaction and the nature of the propagating species. The cis content (o. ), the proportions of cc, ct and tt pairs (2), and the proportions of HH, HT and TT dyads (3) are readily determined from the n.m.r. spectra of the polymers, but the ring dyad tacticity is not always so easily found. For example, there is no fine structure in the C n.m.r. spectrum of that can be attributed to m/r splitting. This problem was originally tackled through the use of enantiomers of... 

Table V. Dyad tacticities for poljnners of a series of norbornene derivatives...

The Tg of pdymethacrylates increases as the bulkiness of side chain increases and its flexibility decreases. Tg also correlates with the tacticity of poly(oe-substituted acrylate) and is lower for isotactic pdymer than for syndiotactic one. Good correlation was obtdned between the dyad tacticity and Tg. Therefore, the Tg value of 100 % isotactic or 100 % Syndiotactic polymer could be determined by the extrapolation of observed values even if it has not been prepared. The Tg for purely syndiotactic pdy(methyl methacrylate) thus obtained is... 

The probabilities represent the dyad tactic fractions m and r. The triad tacticity represent isotactic, syndiotactic, and heterotactic (or atactic) arrangements. They are designated as mm, rr, and nor respectively. One way to understand these definitions is by examining a representation of a section of a polymeric chain which has a total of 9 repeating units but only 8 dyads and 7 triads, as shown below ... 

It should also be noted, however, that in a 5-substituted norbomene derivative the whole molecule is chiral. Normally, samples of such molecules are prepared as racemic mixtures, but if an optically resolved sample is prepared and polymerised using ROMP, then the placement of m/r dyads (tacticity) is necessarily related to the head/tail placement of repeat units (see Figure 10). 

Alternatively, a solvated ion pair or a free ion in which the counterion is too far away to interfere with the approaching monomer, would as in a free radical propagation reaction, prefer a frontside, fs, attack for the conformation shown, so that the syndiotatic or r dyad would again be formed. Hence, the four possibilities for direction of approach of the monomer relative to the placement of the counterion (bs or fs) in combination with endgroup conformations (anti or syn) are listed below with the dyad tacticities which would be formed from each combination ... 

Figure 1 presents the dyad tacticity determined from 300 MH NMR (5) versus the Tg measured by DSC for the poly(alkyl a chforoacrylates) studied. All values are corrected to "infinite" molecular weight as follows. The molecular weight dependence of Tg for a linear polymer is given by the Flory-Fox relationship... 

The structure of Bj corresponds to a monomer unit of vinyl alcohol, while the structure B2 is two monomer units. By comparison of observed chemical shifts with those of the model compounds, the signals at 61.30 and 61.24ppm are assigned to the ultimate carbon atom in the end-group in model Bj and two peaks also coming from essentially dyad tacticity. The content of the PVA was about 0.027 B2 end-groups per 100 repeat units. When VAc was polymerized in bulk or methanol solution at the range of conversion 40-90%, the concentrations of short branches in the PVA s were almost same. The correlation between the concentration and the conversion was not found. 

The mechanism of B polymerization is summarized in Scheme 4,9. 1,2-, and cis- and trews-1,4-butadiene units may be discriminated by IR, Raman, or H or nC MMR speclroseopy.1 92 94 PB comprises predominantly 1,4-rra//.v-units. A typical composition formed by radical polymerization is 57.3 23.7 19.0 for trans-1,4- c7a -1,4- 1,2-. While the ratio of 1,2- to 1,4-units shows only a small temperature dependence, the effect on the cis-trans ratio appears substantial. Sato et al9J have determined dyad sequences by solution, 3C NMR and found that the distribution of isomeric structures and tacticity is adequately described by Bernoullian statistics. Kawahara et al.94 determined the microslructure (ratio // measurements directly on PB latexes and obtained similar data to that obtained by solution I3C NMR. They94 also characterized crosslinked PB. 

The process is favored by low monomer concentrations as occurs at high conversions and in starved feed polymerizations.307 Theoretical calculations suggest that the incidence of backbiting should be strongly dependent on the tacticity of the penultimate dyad. 08 Double backbiting in VC or VAc polymerization will lead to 2-chloroethyl or 2-acetoxy ethyl branches respectively (as for E in Scheme 4.41 ).302... 

There are a few exceptions to this general rule. One of the few examples of an effect on polymer stereochemistry was provided by Dais et al.m who found that polymerization of 31 above the cmc initiated by y-irradiation at 25 °C yields polymer composed entirely of syndiolaclic dyads P(m) =0. When the double bond was distant from the polar head group in 32, the tacticity observed was similar to that observed in solution polymerization / ( )-0,18. Polymerization of 31 at higher temperatures (50 °C) initiated by AIBN also showed no sign of tacticity control. The stcrcospccific polymerization of 31 was attributed to organization of the methacrylate moiety on the surface of the micelle. 

The presence of hydroxyl end groups in PPG can be assigned from 13C NMR data (Figure 24), with resonances (g) observed between 65-68 ppm in the spectrum shown in Figure 24. There are two different resonances here (g) due tacticity effects in the polymer (meso/racemic (m/r) isomerisation of the terminal dyad). Estimations of the amount of these ends in the polymer, along with the average molecular weight of the PPG, can be made from the 13C NMR data. 

Stereoselective ROMP has been reported with monomer (213). Initiator (211) affords highly stereoregular polymer with >98% trans C=C bonds in the polymer backbone.534 However, when (210) is used, >98% cis-poly-(213) is obtained.535 A similar situation occurs for the diester monomer (214). Furthermore, a rapidly equilibrating mixture of (210) and (211) can be used to allow intermediate cis/trans contents to be manipulated by the stoichiometry of the initiator mixture. 13C NMR536 and dielectric analyses537 suggested that trans-poly-(213) is highly syndio-tactic (92% r dyad content). The ROMP of other fluorinated olefins has been recently reviewed.538... 

In figure 1 we present the experimental and calculated mK values of the copolymer poly(styrene-co-p-bromostyrene). From this study (3) we were able to show unequivocally that the tacticity of this polystyrene sample is pr — 0.55, where pr is the probability of racemic dyad replication. 

The cyclization rate kc is higher for B B than for B B dyads and a drastic influence of tacticity is observed, the meso dyads being by far more reactive than the racemic ones,as expected (8). Some characteristic Kc values measured at 20°C in NaOH 0.1 N for different systems are collected in table 5. 

Table I Tacticities of samples obtained in DME and THF, using mono- and bifunctional initiators and Na+ and Cs+ as the counterions. I,H,S isotactic (ami), heterotactic (mr/rm) and syn-diotactic (rr) triads m,r meso and racemic dyads p 2mr/H persistence ratio ( p 1 for Bernoullian statistics).

While the study of diarylpentanes is helpful in understanding the conformational behavior of aryl vinyl polymers, a simple weighting of the properties of the model compounds by the tacticity of the polymer does not yield the properties of the polymer. For example, the presence of dl dyads surrounding a meso dyad will suppress the tt conformer in the meso dyad 14fl). Thus, in order to obtain the fraction of tt meso conformers within an atactic P2VN sample, it is necessary to resort to a Monte Carlo calculation utilizing an extended product of statistical weight matrices, 26). 

Barriers to rotation about Mt=C have been measured by observation of NMR coalescence temperatures123,330,331. In some cases these are sufficiently high that epimerization by rotation about Mt=C is unlikely to be important, but in other cases such a process may be as fast as or faster than the propagation step. More detailed considerations show that when both cis and trans double bonds are formed in accordance with the enantiomorphic sites model then the cis junctions will always be associated with r dyads, and trans junctions with m dyads27,332. This model thus correctly predicts the observed tacticities in the first, third and fifth groups of results listed in Table 7. Cases of intermediate tacticity can also be interpreted in terms of this model if it is modified to include partial epimerization of P/ and Pr between propagation steps. 

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