Addition polymerization, water-soluble chain transfer

The drawback to the Iserson process is the lengthy polymerization required to obtain an economically viable yield. McCain, et al.,[ l developed a process in which polymerization time was reduced to 0.5-6 hours at 65-85°C and less than 6.9 MPa pressure. The catalyst was diisopropyl peroxydicarbonate and was present at 0.25-0.5% of the monomer weight. McCain, et al., discovered that the addition of a small amount of a water-soluble chain transfer agent to the reaction reduced the required amount of surfactant in the effective range (0.5-3.0%) without affecting the quality of the polymer. 

A polymeric composition for reducing fluid loss in drilling muds and well cement compositions is obtained by the free radical-initiated polymerization of a water-soluble vinyl monomer in an aqueous suspension of lignin, modified lignins, lignite, brown coal, and modified brown coal [705,1847]. The vinyl monomers can be methacrylic acid, methacrylamide, hydroxyethyl acrylate, hydroxypropyl acrylate, vinylacetate, methyl vinyl ether, ethyl vinyl ether, N-methylmethacrylamide, N,N-dimethylmethacrylamide, vinyl sulfonate, and additional AMPS. In this process a grafting process to the coals by chain transfer may occur. 

We can tentatively conclude, therefore, that the effect of chain transfer is still making itself felt in the polymerization of vinyl caproate in spite of its low water solubility. Except at the lowest particle concentrations, chain transfer is important. The polymerization in these regions is midway betwen Case I and Case II. When variables are considered separately, there is some dependence of polymerization rate on particle concentration, and also some dependence on initiator concentration. In addition, at constant organic volume, while the rate of polymerization increases as the particle concentration increases (Rp oc 2V- ), the rate per particle decreases as the particles get smaller. This shows that transferred radicals are mainly trapped in the particles, but some diffuse out and can undergo termination with other growing radicals. 

Essentially, TFE in gaseous state is polymerized via a free radical addition mechanism in aqueous medium with water-soluble free radical initiators, such as peroxy-disulfates, organic peroxides, or reduction-activation systems.15 The additives have to be selected very carefully since they may interfere with the polymerization. They may either inhibit the process or cause chain transfer that leads to inferior products. When producing aqueous dispersions, highly halogenated emulsifiers, such as fully fluorinated acids,16 are used. If the process requires normal emulsifiers, these have to be injected only after the polymerization has started.17 TFE polymerizes readily at moderate temperatures (40 to 80°C) (104 to 176°F) and moderate pressures (0.7 to 2.8 MPa) (102 to 406 psi). The reaction is extremely exothermic (the heat of polymerization is 41 kcal/mol). 

Copolymers are produced by aqueous copolymerization of the monomers in a manner similar to the homopolymerizations. Nonaqueous polymerizations of TFE with PPVE may be conducted in water or in fluorinated solvents at lower temperatures (30-60 °C) using a soluble organic initiator such as perfluoro-propionyl peroxide. The molecular weight is controlled by addition of chain transfer agents such as methanol in nonaqeuous polymerizations or hydrogen in aqeous polymerizations. 

Commercially, PFA is polymerized by free-radical polymerization mechanism usually in an aqueous media via addition polymerization of TFE and perfluoropropyl vinyl ether. The initiator for the polymerization is usually water-soluble peroxide, such as ammonium persulfate. Chain transfer agents such methanol, acetone and others are used to control the molecular weight of the resin. Generally, the polymerization regime resembles that used to produce PTFE by emulsion polymerization. Polymerization temperature and pressure usually range from 15 to 95°C and 0.5 to 3.5 MPa. 

Lowe AB, McCormick CL (2007) Reversible addition-fragmentation chain transfer (RAFT) radical polymerization and the synthesis of water-soluble (co)polymers under homogeneous conditions in organic and aqueous media. Prog Polym Sci 32 283-351... 

Table 6.1 presents a typical formulation for emulsion polymerization. The polymer is mainly made out of a mixture of hard monomers (leading to polymers with a high glass transition temperature, Tg for example, styrene) and soft" monomers (low Tgi for example, butyl acrylate) of low solubility in water. In addition, small amounts of functional monomers such as acrylic and methacrylic acids are included in the formulation as they provide some special characteristics, such as improved adhesion. Crosslinking agents and chain-transfer agents (CTAs) are used to control the molecular weight distribution of the polymer. 

A major application is the synthesis of high molecular weight water-soluble polymers (e.g., polymers and copolymers of acrylamide, acrylic acid, and its salts) for flocculants and tertiary oil recovery. Other uses are the synthesis of polyaniline/CdSe quantum dots composites [49], hybrid polyaniline/carbon nanotube nanocomposites [50], polyani-line-montmorillonite nanocomposites [51], or in reversible addition-fragmentation chain-transfer-controlled radical polymerization (RAFT) [52]. 

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