Syndiotactic content

Millan (98) studied the effect of tacticity on the ionic dehydrochlorination and chlorination of PVC. For the dehydrochlorination reaction, both the reaction rate and the polyence sequence distribution depend markedly on the syndiotactic content. Chlorination appeared to be easier through heterotactic parts than through syndiotactic sequences as shown by C-NMR. 

The chain tacticity of PMMA synthesized by GTP catalyzed by nucleophiles at different temperatures was analyzed by Webster and coworkers The syndiotactic content increases from 50% at 60 °C up to 80% at —90°C in THF, using tris(dimethylamino)sulfonium bifluoride [(Me2N)3S+ HF2 ] as catalyst . In contrast to the anionic polymerization of MMA, the stereoselectivity of GTP is less sensitive to solvent. It must be noted that PMMA is less syndiotactic when the GTP is catalyzed by nucleophiles rather than by Lewis acids . GTP was extended to the living polymerization of many acrylates and methacrylates, such as nBuMA, glycidyl-MA, 2-ethylhexyl-MA, Me3SiOCH2CH2-MA, sorbyl-MA, allyl-MA, lauryl-MA), acrylates (EA, BuA), acrylonitrile, methacrylonitrile and Al,A-dimethylacrylamide . 

In a few instances, poly(methylmethacrylate) has been prepared exceeding the syndiotactic content attainable through a free ion pair or free radical mechanism at the same temperature [20]. A possible mechanism for homogeneous syndiotactic propagation has been proposed. However, none of these highly syndiotactic systems has been reproducible [10], and it appears to be no real need for such a mechanism. Coordination-directed stereospecific polymerization of methyl methacrylate seems to be limited to isotactic propagation. 

A particularly interesting result in Table V is seen for the solutions containing about 66% methylene chloride. At this point a sharp break occurred in several properties (1) the syndiotactic content decreased and the isotactic content increased, (2) the yield dropped drastically, (3) the molecular weight decreased, and (4) the molecular weight distribution broadened. These changes all point to a change in the reaction mechanism, probably from one of solvated ion pairs to one of predominantly contact ion pairs at that solvent concentration. Nevertheless, the tacticities of all the polymers formed in this series were found to fit on a Bovey plot, which indicates that they were formed by a Bernoullian process and no penultimate effect was present. 

For a simple unoriented mixture of two tactically pure samples of a vinyl polymer, measurement of the ratio of the IR absorbances or Raman intensities of two modes, one specific for isotactic and one for syndiotactic material, would be a measure of the ratio of the isotactic and syndiotactic contents. Even in this ideal case the method would require calibration with known mixtures of the two types of polymer, because it is not possible to predict how the absolute values of the absorbances or Raman intensities depend on concentration, but only that they are proportional to the concentration. 

Group-transfer polymerizations yield very narrow molecular weight distribution polymers. When mixtures of monomers are used, random copolymers form. The polymerization reaction is very tolerant of other functional groups in the monomer. Thus, for instance, p-vinylbenzyl methacrylate is converted to poly(p-vinylbenzyl methacrylate) without the polymerization of the vinyl group [397]. In addition, it is possible to form polymers with high syndiotactic content. 

MMA pol3nnerized in THF at -78 C are often much lower (e.g., 56%). The reasons for these discrepancies may be attributed to the manner in which the monomer is added to the reaction flask. The literature values reflect the addition of MMA in large quantities this causes a warming of the reaction solution due to the exothermicity of the reaction. Thus, the higher syndiotactic content probably reflects a better temperature control caused by the slow vapor distillation of MMA into the reaction vessel. 

Homopolymers Prepared With Bifunctional Initiators. Warzelhan et al. showed that the syndiotactic content of PMMA was significantly lowered (from 58 to 20%) in going from a monofunctional to a bifunctional sodium initiator in THF at low temperatures. 

From Table 3, TrMA and DMA show no major differences in tacticities or GPC results using monofunctional or bifunctional lithium initiators. Interest inly, with MMA, the syndiotactic content was somewhat higher, and the pol3nner had a bimodal distribution when the bifunctional lithium initiator was used. Whether these results indicate intramolecular associations of ion pairs is not entirely clear. In any event, subsequent reactions with bifunctional initiators were carried out with DMA and TrMA. 

AB Stereoblock PMMA Synthesized From MMA and TrMA. In order to determine how tacticity influences sterocomplex formation, DMA was replaced by MMA in the syntheses of PMMA A-B stereoblock polymers. Homopolymer analysis showed that syndiotactic content was 10% lower for PMMA synthesized from MMA than that prepared from DMA. The AB stereoblock pol)7mers of PMMA obtained from MMA and TrMA were synthesized in the same manner as those obtained from TrMA (Scheme 3), the exception being that MMA was substituted for DMA. 

Barrall and co-workers [13] attempted to determine the syndiotactic content of some experimental PP. NMR spectra or samples dissolved in o-dichlorobenzene were obtained at 170 °C at 100. Spectra of syndiotactic and isotactic samples gave the NMR parameters in Table 6.1. 

The rate of propagation and consequent polyene sequence length are also enhanced by syndiotactic arrangements, since polymers with high syndiotactic content produce abnormally long polyene sequences [1484]. 

The stereochemical sequences of the prevailingly syndiotactic polymers follow a perturbed first order Markov model (a strictly first order Markov model should be expected from the propagation step [22]) (20). The secondary (or Markownikow)regiospecificity of the step [22] turns primary across ethylene units with contemporary decrease of the syndiotactic content. Syndiotactic control is therefore lost whenever the last unit is achiral. 

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