Polymer reducing agents, preparation

A polymer-supported reducing agent, prepared by treating poly(2-vinyIpyridine) with BHj-MCjS, rapidly reduces aldehydes and ketones in high yield. ... 

The unique advantage of the nickel system is that it can produce either stmctures of i7j -I,4-polybutadiene, /n j -I,4-polybutadiene, or a mixture of both depending on the reducing agent and the co-catalyst used. For example, chloride catalyst yields i7j -I,4-polybutadiene, whereas bromide or iodide yields /n j -I,4-polybutadiene. The counterion also has an effect on the polymer microstmcture. A 50/50 cis- 4l/n j -I,4-polybutadiene has been prepared using a carboxyhc counterion (95—105). 

A second method of immunotoxin preparation by reductive amination involves the use a polysaccharide spacer. Soluble dextran may be oxidized with periodate to form a multifunctional crosslinking polymer. Reaction with antibodies and cytotoxic molecules in the presence of a reducing agent forms multivalent immunotoxin conjugates. The following sections discuss these options. 

Poly-p-xylylenes were prepared in excellent yield by electrolytic reduction of a,a -dihalo-/>-xylenes at controlled cathode potentials (28). Polymer and halides are formed at the cathode at the anode the halide is oxidized to halogen. It has been known that some of the a-a -dihalo- -xylene type of compounds have been polymerized by a variety of reducing agents, such as zinc, copper, phenyllithium, sodium and iron ... 

In the same study, redox polymers (223) were prepared that contained pendant viologens (Scheme 108). An active reducing agent was obtained by chemical reduction with dithionite or zinc, electrochemically, or by exposure to light. Utilization of the reduced poly(viologen) (224) as an electron transfer mediator was demonstrated by addition of a catalytic amount of the polymer to a mixture of zinc powder, ethyl benzoylformate (225) and water-acetonitrile (1 5). A quantitative yield of ethyl mandelate (226) was obtained after two days at room temperature (Scheme 109). Without the polymer, no reaction was observed after a month. 

Polymer-supported germanium hydrides have been investigated by Mochida and coworkers who report the preparation of poly(4-diethylhydrogermane)styrene and related polymers and examine their reactivity as free-radical reducing agents (equation 8)25. 

Dumartin and associates described the preparation of in situ polymer-supported organ-otin hydrides for use as clean reducing agents (equation 15)47, while Deleuze and coworkers reported the preparation of a novel, macroporous polymer-supported organotin hydride (37), for use in catalytic free-radical reductions (equation 16)48,49. 

Maxwell et al. 177, 178) studied the deactivation of reduced Cu2+Y catalysts for butadiene cyclodimerization in some detail. This work showed that the catalyst stability could be markedly improved by using NH3 as a reducing agent and choosing the activation conditions such that excess NH3 remains selectively chemisorbed on the zeolite acidic sites. Further, the Cu2+Y-derived catalyst was thermally stable to 850°C and was therefore able to withstand a regeneration procedure which involved a polymer burn-off at 550°C. By contrast, the catalysts prepared by direct exchange with monovalent copper, i.e., Cu+Y, formed CuO irreversibly when heated above 330°C. 

As a new approach to the preparation of water-soluble polymers in inverse miniemulsions, a redox initiation system consisting of ceric ions and carbohydrate-based surfactant Span 60 as a reducing agent has been successfully used for the... 

---