Chain transfer constants temperature effects

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) ... 

A combination of variables controls the outcome of the copolymerization of two or more unsaturated monomers by CCT free-radical polymerization.382 Of course, all of the features that control the outcome of a normal free-radical polymerization come into effect.40 426 429 These include the molar ratio of monomers, their relative reactivity ratios and their normal chain-transfer constants, the polymerization temperature, and the conversion. In the presence of a CCT catalyst, the important variables also include their relative CCT chain-transfer constants and the concentration of the Co chain-transfer agent. The combination of all of these features controls the molecular weight of the polymer and the nature of the vinyl end group. In addition, they can also control the degree of branching of the product. 

Equation (6.148), often referred to as the Mayo equation, shows the quantitative effect of various transfer reactions on the number average degree of polymerization. Note that the chain transfer constants, being ratios of the respective rate constants for chain transfer (Rtr) to the rate constant for propagation (kp), are dimensionless quantities dependent on the types of both the monomer and the material causing chain transfer as well as on the temperature of reaction. 

Problem 11.19 Discuss the effect of temperature on chain transfer constant and hence on PDI in RAFT... 

Okawara et cd. (42), reported that the addition of sodium N,N-dialkyldithiocarbamate to 1,2-dichloroethane (Scheme V) in DMF yielded a diadducl (XXVI) which could be cracked to produce S-vinyl-N,N-dialkyldithiocarbamate (XXVII). Monomer (XXVII) could be polymerized under free radical conditions, but it exhibited a high chain transfer constant so the molecular weight of the resultant polymer (XXVIII) was rather low. Furthermore, the thermal stability of the polymer was poor, which is not surprising since the polymer structure was very similar to diadduct (XXVI), which could be cracked at relatively low temperatures. Polymercaptan (XXDC) could be generated from (XXVIII) by treatment with dimethylamine. Polymers containing the dithiocarbamate moiety are report to be effective photosensitive resins (11,43). 

The effectiveness of a modifier depends on its chemical structure, concentration, temperature, and pressure. A concentration-independent measure for its effectiveness is the chain transfer constant, defined as the ratio of kinetic coefficients for the transfer reaction to this substance and radical chain propagation reaction. Usually the effectiveness of chain transfer agents is increased with rising temperature and reduced pressure. The chain transfer constant of modifiers falls from aldehydes, which are more effective than ketones or esters, to hydrocarbons. Unsaturated hydrocarbons typically have higher transfer constants than saturated hydrocarbons and a strong effect on polymer density must be considered because of the ability to copolymerize that give a higher frequency of short-chain branches in the polymer. 

The temperature effect on a chain-transfer constant has been interpreted as an activation energy. From experiments j)erformed at two temperatures, and T, the activation energy difference between chain transfer and propagation, E,—Ep, given in Table 8 (77) was obtained using Fq. (12). It can be seen from Table 8 that, as 63q)ected, compounds... 

The greater the rate constant for chain transfer, the lower the molecular weight of the polymer. One way to affect the rate constants is by changing the temperature. In general, the chain transfer rate constant is much more sensitive to temperature effects, increasing dramatically as the temperature is increased. For these reasons, there is an inverse correlation between temperature and molecular weight of polyvinyl chloride as shown in Fig. 22.3. 

Grafting Temperature Effect. Temperature can influence the reaction rates in different ways initiation, propagation, transfer, termination. For grafting reaction, the length and number of grafted chains depend on rate constant of these reactions. However for radiochemical grafting, the initiation rate is not temperature dependent (2, 3, 4). 

The molecular weights were roughly constant at a given temperature and about equal to those calculated from the chain transfer to monomer values. Some post effects were observed. 

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. 

The effect of the concentration of DTDGA on the molecular weight of CTPnBA was studied at a constant polymerization temperature of T = 108 C, Table 6. The results indicate that an Increase In the concentration of the chain transfer agent beyond 10% mole ratio of DTDGA to nBA may reduce the molecular weight of the polymer by only a very small amount. 

The use of organometallic compounds as chain-transfer catalysts in free-radical polymerization has been widely studied. One objective is the production of polymers with terminal vinyl groups and lower molecular weight components compared with polymerization in the absence of chain-transfer catalysts. Gomplexes of cobalt(ii) have been used as effective catalysts, but the instability of the intermediate cobalt hydride does not permit firm establishment of the reaction mechanism. To address this issue, several chromium compounds have been applied as catalysts for the polymerization of methylmethacrylate (MMA) and styrene. The temperature dependence of the rate constant for free-radical polymerization of MMA for catalyzed chain transfer by (GsPh5)Gr(GO)3 has been determined using the Mayo equation. ... 

Pressure exerts a marked effect on the polymerization reaction rate constant and can be used to control the reaction rate and molecular weight in addition to the more usual variables of initiator concentration and temperature. Since the number of short branches and the molecular weight are determined by chain transfer reactions which are more influenced by temperature and less by pressure than the polymerization reaction, it follows that the molecular weight decreases and the degree of short branching increases with increasing temperature (and vice versa with pressure). 

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