Polymers elimination reactions

Synthesis of Conjugated Polymers via Polymer Elimination Reactions... 

In this paper we present results on the polymer redox elimination reaction used in the synthesis of the polymers in Figure 6. Preliminary results on electrochemical redox elimination on precursor polymers are also presented. A mechanism of the polymer elimination reaction is proposed. Related recent experimental observations at other laboratories that can be described within the framework of the scheme of Figure 4 are discussed. 

Extraction of hemiceUulose is a complex process that alters or degrades hemiceUulose in some manner (11,138). Alkaline reagents that break hydrogen bonds are the most effective solvents but they de-estetify and initiate -elimination reactions. Polar solvents such as DMSO and dimethylformamide are more specific and are used to extract partiaUy acetylated polymers from milled wood or holoceUulose (11,139). Solvent mixtures of increasing solvent power are employed in a sequential manner (138) and advantage is taken of the different behavior of various alkaUes and alkaline complexes under different experimental conditions of extraction, concentration, and temperature (4,140). Some sequences for these elaborate extraction schemes have been summarized (138,139) and an experimenter should optimize them for the material involved and the desired end product (102). 

Synthesis. The first hiUy alkyl/aryl-substituted polymers were reported in 1980 via a condensation—polymeri2ation route. The method involves, first, the synthesis of organophosphine-containing alkyl or aryl substituents, followed by the ready oxidation of the phosphine to a phosphorane with leaving groups suitable for a 1,2-elimination reaction. This phosphorane is then thermally condensed to polymers in which all phosphoms atoms bear alkyl or aryl substituents. This condensation synthesis is depicted in Eigure 2 (5—7,64). 

In Europe, interest has centered particularly on polyhydroxybutyrate, which can be made into films for packaging as well as into molded items. The polymer degrades within 4 weeks in landfills, both by ester hydrolysis and by an ElcB elimination reaction of the oxygen atom p to the carbonyl group. The use of polyhydroxybutyrate is limited at present by its cost—about four times that of polypropylene. 

It has been shown that the imidoyl chloride moiety of 2(lff)-pyrazinones can imdergo an easy addition/elimination reaction with alkyl amines [24], while reactions with anilines proceed under harsher conditions. Ullmann coupling [109-113] of 2(lff)-pyrazinones with substituted anilines could open the way to the libraries of physiologically active compounds useful in inhibiting HIV replication [7]. Polymer-bound pyrazinone was successfully... 

Figure 4. General scheme of irreversible redox elimination reaction on the class of nonconjugated polymers containing alternating -C(R)H- and conjugated sections in the main chain yielding conjugated polymers.

In order to follow progress of elimination, reactions were also performed on thin films in a special sealed glass cell which permitted in situ monitoring of the electronic or infrared spectra at room temperature (23°C). Typically, the infrared or electronic spectrum of the pristine precursor polymer film was obtained and then bromide vapor was introduced into the reaction vessel. In situ FTIR spectra in the 250-4000 cm-- - region were recorded every 90 sec with a Digilab Model FTS-14 spectrometer and optical absorption spectra in the 185-3200 nm (0.39-6.70 eV) range were recorded every 15 min with a Perkin-Elmer Model Lambda 9 UV-vis-NIR spectrophotometer. The reactions were continued until no visible changes were detected in the spectra. 

The preparation of poly(m-carborane-siloxane) polymers has also been successfully achieved directly from the carborane monomer.22 The reaction used is shown in scheme 9. Here, the direct salt elimination reaction between dilithiocarborane and a dichlorosiloxane (e.g., 1,5-dichlorohexamethyltrisiloxane) results in the formation of linear polymers with a molecular-weight (M ) typically of 6800 dalton. However, the reported literature detailing this approach is very limited indeed, and the reaction has not found significant use. This is most probably because only relatively low molecular-weight polymers can be produced, ultimately restricting the flexibility to produce materials of controlled mechanical properties. 

This unique approach has also been used to find a polymer catalyst for an elimination reaction (62). In this case, the catalysis was observed without the presence of a transition metal. 

Cyclic ketene acetals, which have utility as co-polymers with functional groups capable of cross-linking, etc., have been prepared by the elimination of HX from 2-halomethyl-l,3-dioxolanes. Milder conditions are used under phase-transfer conditions, compared with traditional procedures, which require a strong base and high temperatures. Solid liquid elimination reactions frequently use potassium f-butoxide [27], but acceptable yields have been achieved with potassium hydroxide and without loss of any chiral centres. The added dimension of sonication reduces reaction times and improves the yields [28, 29]. Microwave irradiation has also been used in the synthesis of methyleneacetals and dithioacetals [30] and yields are superior to those obtained with sonofication. 

Involvement of a-elimination reactions for in situ prepared catalysts from WC16 and Me4Sn was demonstrated by the use of 13C in tetramethyltin. The norbomene polymers formed contained the 13CH2 alkene moiety as the end-group. Also unstable C14W=CH2 and Cl4W=13CH2 species were observed by H NMR spectroscopy [14],... 

Monomer/Oligomer Synthesis. The first two steps in the four step reaction sequence of Figure 1 are capable of producing both monomer and oligomer. The first step, aromatic nucleophilic substitution, is a polymer forming reaction under the correct stoichiometric conditions. In order to favor the formation of monomer with a small amount of oligomer, the substitution was carried out at a 4 1 ratio of diol to dichlorodiphenyl sulfone. This led to a predominantly monomeric product (IV) with only the requirement that the excess diol be removed from the product to eliminate the potential presence of low molecular weight species in later reactions. 

The preparation of polymers from heterocyclic monomers that contain polymerizable functional groups undoubtedly constitutes the most common method of incorporating heterocycles into polymeric materials. Polymer-forming reactions are of two possible types addition reactions and condensation reactions. Addition monomers in general contain a site of unsaturation, i.e. a double or triple bond, through which polymerization occurs by successive single bond formation from one monomer to the next. With condensation monomers a bond is formed between two monomers with concomitant elimination of a... 

Biochemical displacement reactions include all of the hydrolytic reactions by which biopolymers are broken down to monomers as well as most of the reactions by which the monomers are linked together to form polymers. Addition reactions are used to introduce oxygen, nitrogen, and sulfur atoms into biochemical compounds and elimination reactions often... 

Extraction of hemicellulose is a complex process that alters or degrades hemicellulose in sonic manner. Alkaline reagents that break hydrogen bonds arc the most effective solvents hut they de-esterily and initiate /(-elimination reactions. Polar solvents such as DMSO and dimethyllormumidc are more specific and arc used to extract partially acctylated polymers from milled wood or holoeellulnse. 

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