POLYMERIC REDUCING

Additional empirical observations concerning this reaction were that use of BF3 as its etherate or addition of small amounts of water (both apparently common practices in certain polymerizations) reduce the overall yield of cyclic products because of chain termination. In a typical reaction mixture, 12-crown-4, 15-crown-5, and 18-crown-6 were formed in 15%, 5% and 4% yields respectively. Dioxane constituted 40% of the product mixture and the remainder was less than 3% each of identified components below the cyclododecamer. ... 

In this paper, the pseudo-kinetic rate constant method in which the kinetic treatment of a multicomponent polymerization reduces to that of a hcmopolymerization is extensively applied for the statistical copolymerization of vinyl/divinyl monomers and applications to the pre- and post-gelation periods are illustrated. 

Applying the pseudo-kinetic rate constants, the explicit formulation of the kinetics of a multicomponent polymerization reduces to that of a homopolymerization. 

B. General Oxidation Procedure for Alcohols. A sufficient quantity of a 5% solution of dipyridine chromium (VI) oxide (Note 1) in anhydrous dichloromethane (Note 7) is prepared to provide a sixfold molar ratio of complex to alcohol. This excess is usually required for complete oxidation to the aldehyde. The freshly prepared, pure complex dissolves completely in dichloromethane at 25° at 5% concentration to give a deep red solution, but solutions usually contain small amounts of brown, insoluble material when prepared from crude complex (Note 8). The alcohol, either pure or as a solution in anhydrous methylene chloride, is added to the red solution in one portion with stirring at room temperature or lower. The oxidation of unhindered primary (and secondary) alcohols proceeds to completion within 5 minutes to 15 minutes at 25° with deposition of brownish-black, polymeric, reduced chromium-pyridine products (Note 9). When deposition of reduced chromium compounds is complete (monitoring the reaction by gas chromatography or thin-layer chromatography analysis is helpful), the supernatant liquid is decanted from the (usually tarry) precipitate and the precipitate is rinsed thoroughly with dichloromethane (Note 10). 

The monomer concentration in this adsorbed layer is kept constant by diffusion from the monomer droplets. The growing surface of the polymer adsorbs more and more monomer, and at fairly high conversions the pure monomer phase disappears because all residual monomer is adsorbed. Further polymerization reduces the surface concentration of the monomer according to a first-order law. 

At the high temperatures of the high-pressure process, T is 140 - 210°C, the rate of polymerization reduces with increasing temperature because of the deactivation of the... 

For preparing gels choose an acrylamide concentration such that the protein will migrate as a sharp, tight band with an Rj between 0.2 and 0.8. Casting a gel in advance allows complete polymerization, reduces the amount of oxidants and free radicals, and minimizes the possibility of blocking the N terminus or other amino groups of the protein. 

When the concentration of the terminating agent is low, the rate of polymerization is proportional to the square root of the radiation dose whereas Rp increases linearly with / , at high concentrations of terminating agent. Thus, the rate of polymerization reduces to Eq. (66) under superdry conditions. 

Sodium borohydride reduces disulfides to thiols, which can then be used to reduce nitro groups. Based on the redox properties of 1,2-dithiolane, lipoamide (17) was used for the selective reduction of mono-substituted nitrobenzenes to the corresponding anilines. Lipoamide (17) can also be immobilized on hydrophilic polymers such as polyvinylamine, polyethyleneimine and chitosan. These polymeric reducing catalysts can be recycled and are easy to separate from the reaction mixture. The system has been used to reduce nitroarenes to anilines. ... 

Timasheff s preferential interaction mechanism also explains the influence of solutes on the degree of assembly of multimeric proteins. Preferentially excluded solutes tend to induce polymerization and stabilize oligomers since the formation of contact sites between constituent monomers serves to reduce the surface area of the protein exposed to the solvent. Polymerization reduces the thermodynamically unfavorable effect of preferential solute exclusion. Conversely, preferential binding of solute induces depolymerization because there is greater solute binding to monomers than to polymers. 

Poly(4-vinylpyridine) boran has b n used as a reducing agent for carbonyl compounds This polymeric reducing agent was prepared by the reaction of poly(4-vinylpyridinium hydrochloride) with sodium borohydride. 

R R Conditions of polymerization Reduced viscosity (04 g/100ml CHCk) V.P.O. M.W. 

If the chiral amino alcohol is incorporated into polystyrene, a chiral polymeric reducing agent is obtained. Acetophenone O-methyloxime was reduced by such a polymeric borane complex to give optically active 1-phenylethylamine with 99% ee35 36 With borane-tetrahydrofuran and 1 mol% of the chiral amino alcohol (lS,3S,5S)-(a,a-diphenyl)hydroxymethyl-2-azabicy-clo[3.3.0]octane (7), (7 )-1-phenylethylamine can be synthesized from acetophenone O-methyloxime in 64% yield with 17% ee37. 

TDB polymerization reduces the number of dead polymer molecules. Therefore, it causes an increase in both number- and weight-average molecular weights. A practical corollary is that an isothermal batch polymerization yielding a number-average molecular weight that does not change with conversion would mle out the importance of TDB polymerization. 

An unusual synthetic application of a soluble macromolecular reducing reagent described by Smith is the use of aqueous solutions of hydrazonium polyacrylate or hydrazonium poly[2-(acryl-amido)-2-methylpropanesulfonate] to prepare stable colloidal dispersions of red, amorphous selenium. (34). In these reactions, the solutions were prepared sufficiently dilute so that each macromolecular hydrazonium polyacrylate could react individually. When a solution of H2Se02 was added to this polymeric reducing agent, the selenious acid molecules in the... 

When the side chain is modified to include a rigid rod-like biphenyl moiety [ 130] [(CH2)3-0-0-0-(CH2CH20)9CH3] and the degree of polymerization reduced to 55 (P55C3BiE9), subtle differences are seen in the phase behavior. H, (34-75%, -2 -> 68 °C) and La (60-95%, 20- 100 °C) phases are still seen but with no Vj phase between them. Clouding is observed up to 58% polymer between 66 and 82 °C. Above this a biphasic Li-i-La region is seen to 80%. ... 

A resin injection gun was developed to interface with the injection gate valves, with an anti-dripping system. Molds are opened automatically when the sensors measure a preset degree of polymerization, reducing cycle times. Five hydraulic hoists on the five parallel molding fines... 

FIGURE 3.17 General principles of controlled radical polymerization reducing the concentration of active radicals. 

Figure 4 The 17-P-sterol acrylate ester template 6, used by Bystom et al. (Ref. 9) to prepare regio- and stereo-selective polymeric reducing agents for the diketone 7.

Electroosmotic Pulsed or Polymeric, Reduce fouling, low power... 

Results indicate the following improvement over isothermal batch polymerization reduced initiator consumption, reduced batch time, good control of average molecular weight, and reduced product polydispersity. The noted... 

Shrinkage during polymerization reduces die forces and should always be arranged to be greater than the thermal expansion caused by the temperature rise. 

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