Tacticity polar solvents

Tacticity of products. Most solid catalysts produce isotactic products. This is probably because of the highly orienting effect of the solid surface, as noted in item (1). The preferred isotactic configuration produced at these surfaces is largely governed by steric and electrostatic interactions between the monomer and the ligands of the transition metal. Syndiotacticity is mostly produced by soluble catalysts. Syndiotactic polymerizations are carried out at low temperatures, and even the catalyst must be prepared at low temperatures otherwise specificity is lost. With polar monomers syndiotacticity is also promoted by polar reaction media. Apparently the polar solvent molecules compete with monomer for coordination sites, and thus indicate more loosely coordinated reactive species. 

A completely different tactic is the use of microwave energy absorption of this energy by a peptide results in decreased aggregation. Lithium salts facilitate the solution of peptides in these polar solvents, increase the rate of swelling of polar resins, and improve coupling yields. Sometimes the order of addition of two solvents to a peptide makes a difference. The inclusion of pyridine can also have a beneficial... 

Many radical polymerization systems were examined hy Challa at a/. A number of papers were devoted to polymerization of methyl methacrylate, MM, in the presence of poly(methyl methacrylate). The role of tacticity of template was stressed. Isotactic PMM forms stereocomplex with syndiotactic PMM. Polymerizations of MM in the presence of isotactic PMM were carried out and the product (template + daughter polymer) was analyzed by NMR. When MM is polymerized in the polar solvent in the presence... 

Warzelhan et al. (23,24) ascribed structure V to these associates) lead to distinctly different tacticities than the mono functionally growing species (cf. Table I). These tacticities are intermediate between those produced in polar solvents by. monofunctional initiators and those produced in non-polar solvents, where association of the growing ends with methoxide or with the monomer has been proposed. 

In polar solvents such as THF anionic polymerization of methyl methacrylate at low temperature is classically assumed to proceed via free anions and the products are syndiotactic. Indeed the structure of such polymers has been confirmed recently and tactic addition shown to conform to Bernoullian... 

Schrbder et al.81 studied the effect of solvent on the tacticity of poly(meth-acrylic acid). Unlike the methyl ester, the structure of poly(methacrylic acid) prepared at 60 °C was found to depend on the solvent, changing from 70% syndi-otactic in xylene to 91-92% syndiotactic in polar solvents such as tetrahydrofuran and hexamethylphosphoric triamide. 

Free-radical polymerizations were conducted under a wide range of conditions that included photochemical, thermochemical, and low-temperature alkylborane-oxygen initiation methods. Both bulk and solution methods were used with somewhat unexpected results. Temperature appeared to have relatively little effect on the tactic order (percent syndiotacticity) of VTFA polymers. Polymerization temperatures ranged from -80 to 150 C and gave syndiotacticities between 50 and 55% by triad analysis. Polar solvents affected tacticity more. For example, polymerization in 1,2-dichloro-ethane resulted in a syndiotacticity of 43%. Presumably, such effects were related to a disruption of the association of VTFA monomer with the growing chain end. 

Use of hydrocarbon solvents has an advantage in polymerizations of conjugated dienes, because they yield some steric control over monomer placement. This is true of both tacticity and geometric isomerism. As stated earlier, the insertions can be 1,2 3,4 or 1,4. Furthermore, the 1,4-placements can be cis or trans. Lithium and organolithium initiators in hydrocarbon solvents can yield polyisoprene, for instance, which is 90% cw-1,4 in structure. The same reaction in polar solvents, however, yields polymers that are mostly 1,2 and 3,4, or trans-lA in structure. There is still no mechanism that fully explains steric control in polymerization of dienes. 

There are some important structural aspects of the polymer which are necessary to take into account in the analysis of the polymer behavior in mixture solvents, such as its polarity, chemical structure, microtacticity, molecular weight. The analysis of these properties shows that they are determinant factors in preferential adsorption phenomena involved. It has been pointed out that the effect of tacticity, and particularly the molecular weight, is a complex problem. In the case of poly(2-vinylpyridine), when the polar solvent is preferentially adsorbed, preferential solvation is independent of molecular weight but when the non-polar solvent is adsorbed, there is a dependence on the molecular weight. ... 

Polymerization is propagated by ion pairs and free ions in polar solvents. There are relatively more pairs available at high initiator concentrations relative to monomer concentrations than there are at low initiator concentrations. But high initiator/monomer ratios also produce low molar masses. Thus, since ion pairs and free ions obviously behave differently with respect to stereocontrol, there is a variation in tacticity, even if only slight, with the molar mass of anionic polymerisates (Figure 18-6). 

Higher temperatures or polar solvents (toluene or ether) gave rise to faster rates and lower tacticities. The propagations were visualized as occurring on a surface because the catalysts are insoluble in hexane or toluene. 

Subsequent work by D. Braun and collaborators dealt with the effect of solvent polarity and countercation on the tacticity of poly(methyl methacrylate) (PMMA) initiated with various organo alkali metal compounds. By using proton NMR analysis they concluded that polar solvents and alkyl lithium initiators favored syndiotactic configurate on( ) while non-polar solvents favored isotactic piacement( )." ... 

In order to generate stereoregular (usually isotactic) polymers, the polymerization is conducted at low temperatures ia nonpolar solvents. A variety of soluble initiators can produce isotactic polymers, but there are some initiators, eg, SnCl, that produce atactic polymers under isotactic conditions (26). The nature of the pendant group can influence tacticity for example, large, bulky groups are somewhat sensitive to solvent polarity and can promote more crystallinity (14,27). 

Tautomerism on polymer should be quite sensitive to neighbouring group effects (composition and unit distribution, steric hindrance and tacticity) and to the microenvironment polarity in solution (copolymer-solvent interactions, critical concentration c of coil interpenetration). The determination of the tautomerism constant KT=(total conjugated forms)/(keto form) in dilute (csemi-dilute (c>c ) solution from H-NMR at 250 MHz and from UV spectroscopy has been reported elsewhere (39,43). The following spectrometric data related to keto-2-picolyl and keto-qui-naldyl structures are quite illustrative ... 

The tacticities of samples prepared in different solvents using different counterions show a distinct tendency concerning the polarity of the solvent and the solvatability of the counterion. 

The cationic polymerization of several para-substi-tuted a-methylstyrenes initiated by various Friedel-Crafts catalyst-cocatalyst combinations has been studied for the effects of catalyst type, monomer substituent and reaction solvent polarity on polymer structure and properties. By using solvent mixtures, the tacticity of the resulting polymers could be varied over a wide range, the syndiotactic form being favored in the more polar mixtures. 

Solvent Polarity Effects. To investigate the effect of solvent polarity on these systems, a series of polymerization reactions was performed using mixtures of 2 solvents with different dielectric constants (e) methylene chloride (e=14.8) and hexane (e-2.0). As seen in Table V, the solvent polarity had a great effect on the tacticity of the resultant polymers. Predominately syndiotactic polymers were formed in the more polar solutions as previously found ( ), and the level of isotacticity increased as the solutions became less polar. These results support the proposal that isotactic sequences come from backside attack on... 

Solvent polarity and counterion size largely influence the rate of polymerization and affect the tacticity of the polymers. At higher temperatures, in THF the termination reactions gain in importance as indicated by the deviation from first-order kinetics, broadening of MWD, and the generation of alkali methoxide. ... 

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