Hydroxytelechelic polymers synthesis

The hydroxytelechelic polymers synthesis involving a free-radical mechanism employs polymerization initiators which are cleaved into free radicals bearing hydroxyl substituents, by heat, light or redox systems. These radicals initiate polymerization of monomers and can give hydroxyl-terminated polymers by recombination. 

Hydroxytelechelic polymer synthesis with redox systems requires hydrogen peroxide as an oxidizing agent and, generally, takes place in aqueous media (to solubilize the salts). This kind of polymerization is possible at lower temperatures compared to polymerizations initiated by thermal decomposition of H202. Therefore, the less frequent transfer reactions improve the polymer functionality and its polydispersity. 

Besides some particular cases such as ozonolysis2,3) or ring-opening polymerization of ketene-acetal type monomers4), the hydroxytelechelic polymers can be synthesized also by anionic polymerization. This process leads to polymers with smaller polydispersity and to a theoretical functionality of two free-radical polymerizations are easier to carry out, cheaper and, therefore, of industrial importance. Several reviews deal with the synthesis of functionally terminated polymers s>6 7, while this paper concerns only radical processes leading to hydroxytelechelic polymers. 

Synthesis of Hydroxytelechelic Polymers by the Free Radical Process... 

Direct Synthesis of Hydroxytelechelic Polymers a) 4,4 -Azobis(4-cyano-n-pentanol) as initiator... 

The oldest and probably the most widely used initiator is 4,4 -azobis(4-cyano-n-pentanol). Synthesized from 5-keto-n-pentanol as starting material (Strecker synthesis), this initiator was used in hydroxytelechelic polystyrene, polyacrylonitrile, and polymethacrylate syntheses, 4). Dienes were also polymerized with this initiator by a different method the molecular weights of liquid hydroxytelechelic polymers are between 2000 and 20000, their functionality is usually higher than two 15,16) (Table 1.1). The polymerization has been studied in dependence on reaction time, monomer and initiator concentrations, temperature, and solvent. The molecular weight increases with decreasing initiator concentration or increasing reaction time. The yield is a function of the nature of monomer and solvent. It decreases in the series chloroprene > butadiene > isoprene and dioxane > toluene. 

Hydrogen peroxide is the simplest peroxide many studies have been performed on the synthesis of hydroxytelechelic polymers using this initiator, which will be analysed separately in Sects. 1.3. to 1.5. Other peroxides, such as molecular weight 2000-5600, functionality 1.7-2.4) 37). The transfer ability of the solvent is a very important parameter. Reactions using alkylidene hydroxyhydroperoxides as initiators have been reported38). 

Hydroxytelechelic polymers can be synthesized via a photoinitiated radical process 49,50 76 77). This reaction resembles that of the redox system because an electron transfer mechanism is operative and the synthesis is carried out in aqueous solution. The reactive species is a complex ion such as Fe3+, X (OH-, Cl-, N". ..). The light absorption (hv) by the ionic species results in an electron transfer reducing the cation oxidation of the anion leads to a free radical X which initiates the polymerization. 

In the section concerning the synthesis of hydroxytelechelic polymers initiated by thermally or photochemically decomposed hydrogen peroxide, the molecular weight distribution of polymers has been found to be dependent on solution homogeneity. A unimodal distribution of molecular weights is observed in vinyl acetate polymerization (true solutions), a bimodal one was found for polydienes, and sometimes a tri-modal one for poly(methyl methacrylate) (non-regular solutions). 

The aim of this review is to summarize the developments concerning the synthesis of hydroxyl-terminated polymers by a radical process. Hydroxytelechelic , a term also used for denominating this type of polymers, means the presence of one OH group at each end of the macromolecular chain. These reactive ends can be further used for chain extension or network build-up if a multifunctional reagent is used, but these must be distinguished from macromonomers as pointed out by Rempp and Franta 1). 

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