Chain-length-dependent termination, polydispersity

The inlet monomer concentration was varied sinusoidally to determine the effect of these changes on Dp, the time-averaged polydispersity, when compared with the steady-state case. For the unsteady state CSTR, the pseudo steady-state assumption for active centres was used to simplify computations. In both of the mechanisms considered, D increases with respect to the steady-state value (for constant conversion and number average chain length y ) as the frequency of the oscillation in the monomer feed concentration is decreased. The maximum deviation in D thus occurs as lo 0. However, it was predicted that the value of D could only be increased by 10-325S with respect to the steady state depending on reaction mechanism and the amplitude of the oscillating feed. Laurence and Vasudevan (12) considered a reaction with combination termination and no chain transfer. 

For a nonodisperse polymer. My = M = M for a polydisperse polymer the two kinds of averages are not equal but are related by a constant factor that depends on the distribution function F M) and the parameter a. A commonly encountered distribution function, and one that is likely to be valid for PVOH, is one that arises if the probability of a chain-termination reaction during the polymerization is constant with time and independent of the chain length already achieved. It is also the most likely function for the product resulting from cleavage with periodate if the head-to-head structures can be assumed to be randomly distributed along the PVOH chains. This distribution function is... 

Termination. With independent measures of kp available, kt can be estimated from the lumped ratio of k /kt. Specialized techniques involving PLP have also been developed to yield accurate estimates of the ratio kp/k [10]. Termination rates in FRP are always diffusion controlled such that the apparent value of kt depends on the conditions under which it has been measured, including the lengths of the radicals involved in the reaction (see Section 3.2.3). Nonetheless, the assumption of chain-length independence is usually made for modeling of commercial FRP systems, as the errors introduced are not large. The mode of termination affects the molecular architecture of the polymer formed and thus some of its properties. The instantaneous polymer polydispersity (PDI = / n )... 

Equations that describe several types of molecular weight distribution were presented in Chapter 2. One of these is the most-probable or Schulz-Flory distribution that is derived assuming that each catalyst site is equally active and that the probability of termination does not depend on chain length. This distribution has a polydispersity index of two, and... 

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