Molecular weight methods

If the rate constant kd for spontaneous decomposition of the initiator is known, the efficiency / of initiation may be determined. (This is a refinement of the molecular weight method set forth in Sec. Ic) The spontaneous decomposition rate of benzoyl peroxide in styrene, according to the work of Swain, Stockmayer, and Clarke is 3.2X10- sec. at 60°C. Hence the efficiency of initiation of the polymerization of styrene by benzoyl peroxide at 60°C is indicated to be about 0.60. 

Transfer constants for regulators may be determined by application of the previously discussed molecular weight method, which depends on the use of Eq. (42). The concentration ratio [S]/[M] is assumed to be constant in this method, hence the conversion must be kept very low, unless C5 is near unity. For transfer constants greater than about five, impractically low conversions are required. A second method - depends on determination of the amount of regulator reacting. The rate of reaction of regulator is given by... 

An additional complication arises from the different types of molecular weight averages obtained by different methods. If the polymer comprises a broad range of species differing in chain length, as is usually the case (see Chap. VIII) unless a careful fractionation has been performed, different molecular weight methods will give substantially different values on this account. 

Starch source M M KOHa Molecular- weight method 3 Solvent Molecular weight D. P. Refer- ences... 

For polydisperse polymer samples, measurements that lead directly to the determination of the molecular weight, such as light-scattering photometry and membrane osmometry, are referred to as absolute molecular weight methods. Techniques such as viscometry are not absolute molecular weight methods because they require calibration using an absolute molecular weight technique. 

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