Solvent/transfer agent

The trichlorethylene is a solvent transfer agent used to control molecular weight. 

Explicit balances must be written for S and the extra mechanisms must be included when deriving expressions for [Ptot]> Rp and DP . As solvent/transfer agent is generally not completely consumed, the retardation effect will last the duration of the polymerization. The degree of retardation depends on the value of which can vary with monomer type ... 

In contrast, RAFT has already been used manifold to prepare (semi)fluorinated polymers and BCP with fluorinated blocks [87]. Almost all monomers for which free radical polymerization works can also be polymerized by RAFT, assumed the right choice of solvent, transfer agent, and initiator. Again, the method was mainly applied to BCP synthesis, for instance, by Laschewsky and coworkers [95-97], as depicted in Figure 11.9c. 

A different form of retardation occurs when a radical species formed from transfer (S in Scheme 4.3) reinitiates at a slow rate. In addition to the slower reaction rate with monomer to form a polymer radical, the termination of S with other radicals in the system may also need to be considered (Scheme 4.8). Explicit balances must be written for S, and the extra mechanisms must be included when deriving expressions for [Ptot], Rpoi, and DP . As solvent/transfer agent is generally not completely consumed, the retardation effect will last the duration of the polymerization (curve b in Figure 4.2). The degree of retardation depends on the value of which can vary with monomer type many carbon-centered radicals show much lower reactivity toward vinyl esters (for example, vinyl acetate) than (meth)acrylates [3]. 

In production, anhydrous formaldehyde is continuously fed to a reactor containing well-agitated inert solvent, especially a hydrocarbon, in which monomer is sparingly soluble. Initiator, especially amine, and chain-transfer agent are also fed to the reactor (5,16,17). The reaction is quite exothermic and polymerisation temperature is maintained below 75°C (typically near 40°C) by evaporation of the solvent. Polymer is not soluble in the solvent and precipitates early in the reaction. 

The molecular weight of a polymer can be controlled through the use of a chain-transfer agent, as well as by initiator concentration and type, monomer concentration, and solvent type and temperature. Chlorinated aUphatic compounds and thiols are particularly effective chain-transfer agents used for regulating the molecular weight of acryUc polymers (94). Chain-transfer constants (C at 60°C) for some typical agents for poly(methyl acrylate) are as follows (87) ... 

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

Chain transfer to solvent is an important factor in controlling the molecular weight of polymers prepared by this method. The chain-transfer constants for poly(methyl methacrylate) in various common solvents (C) and for various chain-transfer agents are Hsted in Table 10. 

The compound R X is a chain-transfer agent, with X usually H or Cl. The net effect of chain transfer is to kill a growing chain and start a new one in its place, thus shortening the chains. Mercaptan chain-transfer agents ate often used to limit molecular weight, but under appropriate conditions, almost anything in the reaction mass (solvent, dead polymer, initiator) can act as a chain-transfer agent to a certain extent. 

Pha.se-Tra.nsfer Ca.ta.lysts, Many quaternaries have been used as phase-transfer catalysts. A phase-transfer catalyst (PTC) increases the rate of reaction between reactants in different solvent phases. Usually, water is one phase and a water-iminiscible organic solvent is the other. An extensive amount has been pubHshed on the subject of phase-transfer catalysts (233). Both the industrial appHcations in commercial manufacturing processes (243) and their synthesis (244) have been reviewed. Common quaternaries employed as phase-transfer agents include benzyltriethylammonium chloride [56-37-17, tetrabutylammonium bromide [1643-19-2] tributylmethylammonium chloride [56375-79-2] and hexadecylpyridinium chloride [123-03-5]. 

Solution Polymerization. Solution polymerization of vinyl acetate is carried out mainly as an intermediate step to the manufacture of poly(vinyl alcohol). A small amount of solution-polymerized vinyl acetate is prepared for the merchant market. When solution polymerization is carried out, the solvent acts as a chain-transfer agent, and depending on its transfer constant, has an effect on the molecular weight of the product. The rate of polymerization is also affected by the solvent but not in the same way as the degree of polymerization. The reactivity of the solvent-derived radical plays an important part. Chain-transfer constants for solvents in vinyl acetate polymerizations have been tabulated (13). Continuous solution polymers of poly(vinyl acetate) in tubular reactors have been prepared at high yield and throughput (73,74). 

Chain transfer to monomer and to other small molecules leads to lower molecular weight products, but when polymerisation occurs ia the relative absence of monomer and other transfer agents, such as solvents, chain transfer to polymer becomes more important. As a result, toward the end of batch-suspension or batch-emulsion polymerisation reactions, branched polymer chains tend to form. In suspension and emulsion processes where monomer is fed continuously, the products tend to be more branched than when polymerisations are carried out ia the presence of a plentiful supply of monomer. 

Most metal carbonyls are synthesized in nonaqueous media. Reactive metals, such as sodium (85), magnesium (105), zinc (106), and aluminum (107,108), are usually used as reducing agents. Solvents that stabilize low oxidation states of metals and act as electron-transfer agents are commonly employed. These include diethyl ether, tetrahydrofiiran, and 2-methoxyethyl ether (diglyme). 

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use. 

A typical polymerization system comprises many components besides the initiators and the monomers. There will be solvents, additives (e.g. transfer agents, inhibitors) as well as a variety of adventitious impurities that may also be reactive towards the iniLiator-derived radicals. 

Chain transfer is the reaction of a propagating radical with a non-radical substrate (X-Y, Scheme 6.1) to produce a dead polymer chain and a new radical (Y ) capable of initiating a polymer chain. The transfer agent (X-Y) may be a deliberate additive (e.g. a thiol) or it may be the initiator, monomer, polymer, solvent or an adventitious impurity. 

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