Transfer Constants to Polymer

As well as conversion, the importance of transfer to polymer depends upon the monomer system. The reaction can be important in systems with very reactive radicals such as ethylene [30-32], vinyl acetate [33-35], and acrylate [36, 37] polymerizations, but seldom occurs in styrene and methacrylate systems. Transfer to polymer usually occurs via abstraction of a methine hydrogen as shown in Scheme 4.9, but may also involve other easily abstracted H-atoms, such as the acetate methyl hydrogens on poly(vinyl acetate). Transfer constants to polymer (C ° = k /kp) are not as readily determined as other transfer constants because the process does not decrease DP . Long-chain branching (LCB) levels are usually quite low, less than 2 per 1000 repeat units, making it difficult to employ NMR. Indirect methods such as multi-detector SEC [32, 38] are often used, leading to a significant scatter in reported values [7]. like other transfer events, the relative importance increases with temperature. 

The transfer constant to one given repeat unit of a polymer is in most cases quite low, and in spite of the high number of repeat units in a polymer chain, the transfer constants to polymer are also negligible in most cases. However, there are polymers which can behave as transfer agents, especially towards unstabilized, highly reactive radical sites. 

The log CLD method can sometimes provide better quality data than the conventional Mayo method. It is less sensitive to experimental noise and has application in measuring the transfer constant to polymeric species where the distributions of the transfer agent and the polymer product partially overlap.24... 

Values of CP measured in the presence of added PMMA (for example) will depend on how the PMMA was prepared and its molecular weight (i.e. on the concentration of unsaturated ends). PMMA formed by radical polymerization in the presence of a good H-donor transfer agent (or by anionic polymerization) would have only saturated chain ends. These PMMA chains should have a different transfer constant to those formed by normal radical polymerization where termination occurs by a mixture of combination and disproportionation. This could account for some of the variation in the values of CP for this polymer... 

The chain transfer constant to the polymer was determined by the following equation (27). 

Since Cp is the value summarized all the bonds in the trank polymer, the chain transfer constant to the end and the medium... 

On the other hand, the transfer constant to the polymer is always low since the cyanoisopropyl radical has a rather weak reactivity. 

Another aspect49 is the initial presence of persistent species in nonzero concentrations [Y]o, and it will be discussed more closely in section IV. In the absence of any additional initiation, the excess [Y]o at first levels the transient radical concentration to an equilibrium value [R]s = A[I]o/[Y]o. This is smaller than that found without the initial excess and lowers both the initial conversion rate and the initially large PDI. Further, it provides a linear time dependence of ln-([M]o/[M]), which is directly proportional to the equilibrium constant. Later in the reaction course, [Y] may exceed [Y]0 because of the self-termination, then [R] is given by eq 18. If there is additional radical generation, the first stages will eventually be replaced by a second stationary state that was described above. Further effects are expected from a decay or an artificial removal of the persistent species that increases the concentration of the transient radicals and the polymerization rate (see section IV). Radical transfer reactions to polymer, monomer, or initiator have not yet been incorporated in the analytical treatments. 

One of the most striking features of CCT is the exceptionally fast rate at which it takes place. The molecular weight of a polymer can be reduced from tens of thousands to several hundred utilizing concentrations of cobalt catalyst as low as 100—300 ppm or 10 3 mol/L. The efficiency of catalysis can be measured as the ratio between the chain-transfer coefficients of the catalyzed reaction versus the noncatalyzed reaction. The chain-transfer constant to monomer, Cm, in MMA polymerization is believed to be approximately 2 x 10 5.29 The chain-transfer constant to catalyst, Cc, is as high as 103 for porphyrins and 104 for cobaloximes. Hence, improved efficiency of the catalyzed relative to the uncatalyzed reaction, CJCu, is 104/10 5 or 109. This value for the catalyst efficiency is comparable to many enzymatically catalyzed reactions whose efficiencies are in the range of 109—1011.18 The rate of hydrogen atom transfer for cobaloximes, the most active class of CCT catalysts to date, is so high that it is considered to be controlled by diffusion.5-30 32 Indeed, kc in this case is comparable to the termination rate constant.33... 

It is well-known that the chain transfer reaction to initiator is of no importance in the polymerization of MMA with AIBN. The transfer constant to the MMA monomer is of the order of 10-5 that is, one transfer reaction to the monomer occurs for 105 propagation reactions. The degree of polymerization of the polymers is less than 4 x 102 therefore, transfer to the monomer can also be neglected. Then, the number of initiator fragments, 1.15, means that 85% of the polymer molecules are formed through disproportionation reaction, and that half of them should contain a double bond at the chain end. 

The decrease in the value of a with reaction time, namely with increase in per cent grafting, is due to the steric hindrance of branches already formed on the trunk polymer. Attack of the growing polymer radicals on the nitro groups on the trunk polymer is prevented increasingly by the branches already formed, and the apparent chain transfer constant to the nitro groups on the trunk polymer is decreased. 

If a monomer B is polymerized in the presence of a polymer A, already formed, transfer reactions may occur with respect to the polymer A chain, leading to grafted copol3rmers. This type of reaction was studied by Shultz, who determined, for the first time, the transfer constant to the polymer. Other copol3rmers were also prepared by this method, e.g., vinyl chloride and aciyhc and... 

The monomer-saturated emulsifier layer (or the shell of polymer particlesX the high chain transfer constant to VC and the polymerization in the interphase should promote desorption of radicals from particles to the aqueous phase. Desorbed radicals may take part in initiation and termination or re-enter the particles. In both phases, desorbed or re-entered radicals are more eflicient in termination. In addition, the mobile radicals irreversibly diffuse to the particle core in which they are immobilized by the polymer phase and/or occluded by propagation. 

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