Polymerization, free-radical addition precipitation

Controlled free-radical polymerization (CFRP) has been used successfully to produce block, graft, and other controlled architecture copolymers within the last decade for a variety of free radically polymerizable monomers. The main techniques include reversible addition fragmentation and transfer (RAFT) polymerization, stable free-radical polymerization (SFRP) mediated by nitroxide/alkoxyamine based radicals, atom transfer radical polymerization (ATRP), diphenyl ethylene (DPE) mediated polymerization, and novel precipitation/emulsion polymerization based methods like free-radical retrograde precipitation polymerization (FRRPP). ... 

Finally, the use of stable free radical polymerization techniques in supercritical C02 represents an exciting new topic of research. Work in this area by Odell and Hamer involves the use of reversibly terminating stable free radicals generated by systems such as benzoyl peroxide or AIBN and 2,2,6,6-tetramethyl-l-piperidinyloxy free radical (TEMPO) [94], In these experiments, styrene was polymerized at a temperature of 125 °C and a pressure of 240-275 bar C02. When the concentration of monomer was low (10% by volume) the low conversion of PS which was produced had a Mn of about 3000 g/mol and a narrow MWD (PDI < 1.3). NMR analysis showed that the precipitated PS chains are primarily TEMPO capped, and the polymer could be isolated and then subsequently extended by the addition of more styrene under an inert argon blanket. The authors also demonstrated that the chains could be extended... 

Baxendale, Evans and coworkers reported in 1946 that the polymerization of methyl methacrylate (MMA) in aqueous solution was characterized by homogeneous solution kinetics, i.e. where mutual termination of free radicals occurred, in spite of the fact that the polymer precipitated as a separate phase. Increases in the rates of polymerization upon the addition of the surfactant cetyl trimethyl ammonium bromide (CTAB) were attributed to the retardation of the rate of coagulation of particles, which was manifested in a reduction in the effective rate constant for mutual termination,... 

Aqueous dispersions of poly(vinyl acetate) and vinyl acetate-ethylene copolymers, homo- and copolymers of acrylic monomers, and styrene-butadiene copolymers are the most important types of polymer latexes today. Applications include paints, coatings, adhesives, paper manufacturing, leather manufacturing, textiles and other industries. In addition to emulsion polymerization, other aqueous free-radical polymerizations are applied on a large scale. In suspension polymerization a water-irnrniscible olefinic monomer is also polymerized. However, by contrast to emulsion polymerization a monomer-soluble initiator is employed, and usually no surfactant is added. Polymerization occurs in the monomer droplets, with kinetics similar to bulk polymerization. The particles obtained are much larger (>15 pm) than in emulsion polymerization, and they do not form stable latexes but precipitate during polymerization (Scheme 7.2). 

The polymerization of vinyl monomers in liquid and supercritical CO2 has been studied extensively. Patents were issued in 1968 to the Sumitomo Chemical Company [81] and in 1970 to Fukui et al. [82] for the preparation of homopolymers of polystyrene, poly(vinyl chloride), poly(acrylonitrile) (PAN), poly-(acrylic acid) (PAA), and poly(vinyl acetate) (PVAc), as well as the random copolymers PS-co-PMMA and PVC-co-PVAc. Additionally, a patent was issued in 1995 to Bayer AG [83] for the preparation of styrene/acrylonitrile copolymers in SCCO2. In 1986, the BASF Corporation was issued a Canadian patent for the precipitation polymerization of 2-hydroxyethylacrylate and various N-vinylcarboxamides in compressed carbon dioxide [84]. In 1988, Terry et al. attempted to homopolymerize ethylene, 1-octene, and 1-decene in SCCO2 for the purpose of increasing the viscosity of CO2 for enhanced oil recovery [85]. These reactions utilized free-radical initiation with benzoyl peroxide and r-butylperoctoate at 71 °C and 100-130 bar for 24-48 h. Although the resulting polymers were not well characterized, they were found to be relatively... 

Free radical RAFT polymerization was used to graft linear PNIPAM chains onto PNIPAM-co-AAc 2-hydroxyethyl ester (HEA) microgels. PNIPAM/HEA miaogels prepared by precipitation polymerization were modified by initiator fragments by the addition of a-butyl acid dithiobenzoate in THF solution. Finally, the RAFT polymerization of NIPAM was initiated leading to controlled growth of linear PNIPAM chains from the... 

Some polymers are insoluble in their own monomers and therefore are precipitated during polymerization. Examples are poly(vinyl chloride) and poly(acrylonitrile). Polymerization continues in the precipitated product, but its rate is determined by the diffusion of the monomer to the free radicals and consequently by the physical structure of the coagulate. Factors such as the rate of agitation can thus affect the rate of polymerization quite considerably. The advantage of precipitation polymerization is that the polymers immediately assume solid form. For this reason, polymerizations are often carried out with the addition of substances that precipitate the polymer but that are also solvents for the monomer. 

How can we control the synthesis of drug carriers to govern the above features It has already been stated that the majority of water-soluble polymers are prepared by free-radical polymerization the mechanism of which is well-known. On the whole, the average molecular weight can be controlled relatively easily by the initiator concentration in the polymerization mixture, the polymerization temperature and conversion. In addition, the molecular weight distribution can be controlled to obtain a narrower range. If necessary, it is adjusted by fractional precipitation. 

Use of mutual solvents (e.g. alcohols) or solvent mixtures to dissolve both the water and oil-soluble monomers also has some serious limitations [4]. For example, even though a common solvent can be found for the monomers, it is unlikely that the polymer will also be soluble in such a medium. Thus, the polymer will precipitate before it has built up a sufficient molecular weight for use as a viscosifier. In addition, most of the possible water miscible solvents (e.g. alcohols, ether, acetone) are chain transfer agents for free radical polymerization, and their presence leads to low molecular weight products. 

The electrochemical conditions, the electrode material, the solvent, the coimterion and the monomer all influence the nature of the processes occurring. For example, if the applied potential is too low (imder certain conditions), the rate of polymerization will be such that no precipitate eventuates. If the solvent is nucleophilic (or contains dissolved oxygen), it will react with the free radical intermediates. If the electrode material is extremely polar, at the potential required for polymerization, deposition may be discouraged. In addition the solvent, monomer, counterion and substrate interactions are all important since they dictate the solubility and/or deposition of the resultant polymer. 

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