Polymerization in polar solvents

Angle-strain in the small-ring stannacycloalkanes confers on them an anomalously high reactivity. This is most obvious in the stannacy-clopentanes which, for example, undergo ionic polymerization in polar solvents such as methanol, react readily with acetic acid (128), and react additively with diarylsulfurdiimides (129). 

Equation 5-84 applies for the case where initiation is rapid relative to propagation. This condition is met for polymerizations in polar solvents. However, polymerizations in nonpolar solvent frequently proceed with an initiation rate that is of the same order of magnitude as or lower than propagation. More complex kinetic expressions analogous to those developed for radical and nonliving cationic polymerizations apply for such systems [Pepper, 1980 Szwarc et al., 1987],... 

Having established that a particular polymerization follows Bemoullian or first-order Markov or catalyst site control behavior tells us about the mechanism by which polymer stereochemistry is determined. The Bemoullian model describes those polymerizations in which the chain end determines stereochemistry, due to interactions between either the last two units in the chain or the last unit in the chain and the entering monomer. This corresponds to the generally accepted mechanism for polymerizations proceeding in a nonco-ordinated manner to give mostly atactic polymer—ionic polymerizations in polar solvents and free-radical polymerizations. Highly isoselective and syndioselective polymerizations follow the catalyst site control model as expected. Some syndioselective polymerizations follow Markov behavior, which is indicative of a more complex form of chain end control. 

Although there are no known reports on the use of (CH3)2NLi as catalyst for the polymerization of ethylene, there is ample evidence relating to the use of this species as a catalyst for styrene and 1,3-butadiene polymerization in polar solvents (17-19). It is not unreasonable to assume that TMEDA could activate (CH3)2NLi, (2) or both toward ethylene polymerization. 

The diene monomers give predominantly 1,4-polymers in hydrocarbon solvents if polymerized using lithium-based initiation. Isoprene, under these conditions, gives a predominantly cis-1,4 polymer but with butadiene the proportions of cis- and frans-1,4 are fairly evenly distributed. Once ain this phenomenon is characteristic of lithium compounds sodium- and potassium-based initiation gives mixed structures even in hydrocarbon solvents. Polymerization in polar solvents such as tetrahydrofuran leads to largely 3,4-polyisoprene or 1,2-butadiene with... 

The large dipole moment of ion pairs causes them to interact strongly with polar molecules with the result that small amounts of polar compounds profoundly affect the course of the polymerization. In polar solvents the following dynamic equilibrium will be set up involving ion pairs, solvent separated ion pairs and free ions ... 

Experimoits show that substantially larger conversions are obtained for styreiK polymerizations in polar solvents than in nonpolar media, an observation which has be i explained by subtle solvation effects that in turn influence the rate erf termination relative to that of propagation. 

The propagation rates for methyl methacrylate polymerization in polar solvents like tetrahydrofuran or dimethylformamide are lower than the rates of initiation.There is no evidence. 

Discuss steric control in homogeneous anionic polymerizations in polar solvents. 

Electron-transfer-initiated polymerizations in polar solvents proceed differently. An example of this is the polymerization of styrene with sodium naphthaline in tetrahydrofuran. The polymerization proceeds very rapidly, such that the concentration [P ] of growing chains is generally determined in... 

In addition, Percec and coworkers [253] reported that polymerizations in polar solvents in conjtmction with copper and appropriate ligands allow ultrafast syntheses of high-molecular-weight polymers at ambient temperature. The process is referred to as Single Electron Transfer-Living Radical Polymerization (SET-LRP). The mechanism proposed is based on disproportionation of cuprous ions to cupric ions and metallic copper. This is catalyzed by the polar solvents and the appropriate ligands. The proposed mechanism can be illustrated as follows ... 

The propagation rates for methyl methacrylate polymerization in polar solvents like tetrahydrofu-ran or dimethylformamide are lower than the rates of initiation [203]. There is no evidence, however, that more than one kind of ion pairs exist [204-206]. The ion pairs that form are apparently craitact-ion pairs [203]. Furthermore, based on the evidence, the counterions are more coordinated with the enolate oxygen atoms of the carbonyl groups than with the a-carbons. As a result, they exert less influence on the reactivity of the carbanions [203]. The amount of solvation by the solvents affects the reaction rates. In addition, intramolecular solvation from neighboring ester groups on the polymer chains also affects the rates. In solvents like dimethylformamide, tetrahydrofuran, or similar ones [203], the propagating chain ends-ion pairs are picmred as hybrid intermediates between two extreme structures. This depends upon the counterion, the solvent, and the temperature [203] ... 

The reaction procedures reported in the hterature were relatively similar and therefore only a few examples will be shown for each type of protocol. However, the work-up procedures for the reactions differed between research groups. Typical work-up procedures for polymerizations in polar solvents involved precipitation in aqueous solution (neutral, acidic or basic) while reactions carried out in nonpolar solvents were precipitated in aqueous methanol solutions. The crude polymer was washed with various solvents (acetone, methylvinylketone (MVK), hexanes, dichloromethane), typically through Soxhlet extraction, to remove lower molecular weight materials. The polymer was then extracted in chloroform or o-dichlorobenzene and repredpitated in methanol. Often, the polymer was washed with a metal scavenger, such as ethylenediaminetetracetic acid disodium salt, to remove residual metals, and this can be done before or after removal of low molecular weight materials. Filtration of the chloroform solution through celite or silica gel has also been used for this purpose. 

The various crystalline structures of PVF2 obtained by chemical and radiation-initiated polymerization are described by Gal perin et al. [536,537]. PVF2 samples of different molar masses could be prepared (0.3 to 10 x 10 g/mol, determined from viscosimetry in DMF). Radiation polymerization in the gaseous phase resulted in polymers with the highest molar masses. It was also shown that the conformation of PVF2 chains was independent of the method of initiation but was influenced by the polarity of the medium in which polymerization took place. Radiation polymerization in polar solvents promoted formation of the p phase, while nonpolar solvents (or gaseous polymerization) yielded the a phase [521,537]. 

Figure 9 shows the relationships between conversion and the number-average molecular weight (M ) of poly(pMOS) obtained with iodine in CCU solvent at 0 °C The M increases as the polymerization proceeds, and the lower the iodine concentration is, the higher the polymer molecular weight. No such increase in M was observed in polymerizations in polar solvents. 

Polar monomers such as 2-vinylpyridine and methyl methacrylate are normally polymerized in polar solvents such as tetrahydrofuran and at low temperature (-78 °C). In addition, additives such as LiCl are often added to help lower the rates of termination reactions to levels insignificant in the time frame of the reaction. Block copolymers made with nonpolar and polar monomers start with the nonpolar monomer because of its greater reactivity. These active centers are then typically capped with 1,1-diphenylethylene to lower then-reactivity before the addition of the polar monomer. This helps eliminate side reactions resulting from addition of the active center to electrophilic sites in the polar monomers. The two polar polymers, polystyrene-2-vinylpyridine (PS-P2VP) and polystyrene-poly methyl methacrylate (PS-PMMA) have been extensively studied in thin films. 

Alkali metal salts of fullerenes C ) (M ), were also tested as initiators for polymerization in polar solvents. Only the hexa-anion was able to initiate reactive monomers like methyl methacrylate, but not non-polar monomers hke styrene. In all cases, the initiation proceeds through electron transfer to the monomer so that no fullerene is incorporated [95]. The reduced fullerenes initiate polymerization like classical radical anions or dianions of aromatic or conjugated molecules and therefore this synthetic route is of limited interest. 

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