Chain transfer agent concentration

Table 2. Intramolecular crosslinking of PVS [217], Reaction conditions PVS concentration = 0.975 mass % AIBN concentration = 1.65X10 3 M temperature = 70 °C n-butylmercaptan (chain transfer agent) concentration = 20 mL/L reaction time = 25 min. The Mw and Mn were measured by light scattering and membrane osmometry respectively.

A laboratory scale, continuous process for the polymerization of acrylamide in aqueous solution is described. The reaction conditions can be held constant within narrow limits and the effect of small changes in individual variables, such as temperature, initiator concentration, and chain transfer agent concentration, can be quantitatively ascertained. Some experimental results are presented showing the effect of these factors on the molecular weight of the polymer. The data are examined vis-a-vis some theoretically derived equations. 

Fig. 6-4 Effect of chain transfer agent concentration (5) on polystyrene degree of polymerization [8].

Z]p chain transfer agent concentration inside particle -ab- linkage in condensation aPP atactic polypropylene Os specific area of surfactant comp/y/rj composition distribution / initiation efficiency or functionality fi mole fraction of monomer i / function in comb polymer function in comb polymer... 

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) ... 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

In mbber production, the thiol acts as a chain transfer agent, in which it functions as a hydrogen atom donor to one mbber chain, effectively finishing chain growth for that polymer chain. The sulfur-based radical then either terminates with another radical species or initiates another chain. The thiol is used up in this process. The length of the mbber polymer chain is a function of the thiol concentration. The higher the concentration, the shorter the mbber chain and the softer the mbber. An array of thiols have subsequendy been utilized in the production of many different polymers. Some of these apphcations are as foUow ... 

Various techniques have been studied to increase sohds content. Hydroxy-functional chain-transfer agents, such as 2-mercaptoethanol [60-24-2], C2HgOS, reduce the probabihty of nonfunctional or monofunctional molecules, permitting lower molecular-weight and functional monomer ratios (44). Making low viscosity acryhc resins by free-radical initiated polymerization requires the narrowest possible molecular-weight distribution. This requires carehil control of temperature, initiator concentration, and monomer concentrations during polymerization. 

Ethylbenzene, generally in 6- 15% concentration (Z 26/ 27) is used as a diluent with the feed, most likely to reduce viscosity and thereby facilitate melt pumping and heat transfer. It also serves as a chain transfer agent (30 ). ... 

Concentrations in Water and Particles. In order to obtain the rates of reaction, the concentrations of the two monomers and the chain transfer agent in the water and polymer phases were calculated using equilibrium partition coefficients (H). ... 

Usually, reactions 1 and 2 take place in the aqueous jiiase, yttiile all the other kinetic events can occur both in the aqueous and in the polymer phases. Note that Pj,n indicates the concentration of active polymer chains with nTronaner units and tenninal unit of type j (i. e. of monomer j) Hi is the concentration of monomer i and T is the concentration of the chain transfer agent. Reactions 4 and 5 are responsible for chain desorption from the polymer pjarticles reactions 6 and 7 describe bimolecular temination by conJoination and disproportionation, respiectively. All the kinetic constants are depsendent upon the last monomer unit in the chain, i. e. terminal model is assumed. 

Morton and Salatiello have deduced the ratio kpp/kp for radical polymerization of butadiene by applying the above described procedure, appropriately modified for the emulsion system they used. The primary molecular weight was controlled by a mercaptan acting as chain transfer agent, as in the experiments of Bardwell and Winkler cited above. Measurement of the mercaptan concentration over the course of the reaction provided the necessary information for calculating % at any stage of the process, and in particular at the critical conversion 6c for the initial appearance of gel. The velocity constant ratios which they obtained from their results through the use of Eq. 

The most common poly(alkenoic acid) used in polyalkenoate, ionomer or polycarboxylate cements is poly(acrylic acid), PAA. In addition, copolymers of acrylic acid with other alkenoic acids - maleic and itaconic and 3-butene 1,2,3-tricarboxylic acid - may be employed (Crisp Wilson, 1974c, 1977 Crisp et al, 1980). These polyacids are prepared by free-radical polymerization in aqueous solution using ammonium persulphate as the initiator and propan-2-ol (isopropyl alcohol) as the chain transfer agent (Smith, 1969). The concentration of poly(alkenoic add) is kept below 25 % to avoid the danger of explosion. After polymerization the solution is concentrated to 40-50 % for use. 

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