Monomer transfer constants

Using the methods described, the values of Cm and Ci in the benzoyl peroxide polymerization of styrene have been found to be 0.00006 and 0.055 respectively [Mayo et al., 1951]. The amount of chain transfer to monomer that occurs is negligible in this polymerization. The chain-transfer constant for benzoyl peroxide is appreciable, and chain transfer with initiator becomes increasingly important as the initiator concentration increases. These effects are shown in Fig. 3-7, where the contributions of the various sources of chain ends are indicated. The topmost plot shows the total number of polymer molecules per 105 styrene monomer units. The difference between successive plots gives the number of polymer molecules terminated by normal coupling termination, transfer to benzoyl peroxide, and transfer to styrene. 

The monomer chain-transfer constants are generally small for most monomers—in the range 10 5 to 10-4 (Table 3-4). Chain transfer to monomer places the upper limit to the polymer molecular weight that can be obtained, assuming the absence of all other transfer reactions. Transfer to monomer does not, however, prevent the synthesis of polymers of sufficiently high molecular weight to be of practical importance. Cm is generally low because the reaction 

The very high value of Cm for vinyl chloride is attributed to a reaction sequence involving the propagating center XVIII formed by head-to-head addition [Hjertberg and Sorvik, 1983 Llauro-Darricades et al., 1989 Starnes, 1985 Starnes et al., 1983 Tornell, 1988]. Intramolecular migration of a chlorine atom (Eq. 3-114) yields the secondary radical XIX that subsequently transfers the chlorine atom to monomer (Eq. 3-115) to yield poly(vinyl chloride) 

The Cm value of vinyl chloride is sufficiently high that the maximum number-average molecular weight that can be achieved is 50,000-100,000. This limit is still reasonable from the practical viewpoint—PVC of this molecular weight ia a very useful product. 

Not only the case of vinyl chloride but also styrene shows that the observed chain transfer to monomer is not the simple reaction described by Eq. 3-112. Considerable evidence [Olaj et al., 1977a,b] indicates that the experimentally observed Cm may be due in large part to the Diels-Alder dimer XII transferring a hydrogen (probably the same hydrogen transferred in the thermal initiation process) to monomer. 

3-7 Contribution of various sources of chain termination in the benzoyl peroxide-initiated polymerization of styrene at 60°C. After Mayo et al. [1951] (by permission of American Chemical Society, Washington, DC). 

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TABLE 5-5 Monomer Transfer Constants in Cationic Polymerization of Isobutylene in CH2CI2... 

The traditional method of determining the monomer transfer constant is the Mayo method [294,295], where the inverse of the number average chain length Pn is extrapolated to zero polymerization rate. To obtain reliable values, one needs to measure rather large P values to high precision that can then be extrapolated to zero polymerization rate. In addition, linear extrapolation is not guaranteed if bimolecular termination reactions are chain-length-dependent [296]. 

The MC simulation method can be used to find the experimental conditions where the CLD method can be used to determine the monomer transfer constant. 

As indicated in the above examples, for a specific monomer the rate of exchange as well as the position of the equilibrium and, to some extent, the zero-order monomer transfer constants depend on the nature of the counter anion, in addition to temperature and solvent polarity. Therefore, initiator/coinitiator systems that bring about controlled and living polymerization under a certain set of experimental conditions are largely determined by monomer reactivity. 

The effect of pressures up to about 2500 atm (252 MPa) on the rate constants, conversion, molecular weight, and monomer transfer constants for the polymerization of vinyl chloride has been studied [95]. 

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