Length Dependence of Termination Reactions

In the literature, numerous models have been reported describing the chain-length dependence of kt. Some of these are based on careful considerations of the motions of polymer chain ends, others are based on the translational diffusion of polymer coils as a whole and yet others use more empirical approaches. Not only are the models conflicting regarding their predictions and underlying mechanisms, also literature data do not yield an unambiguous picture about the chain-length dependence. Besides, because of the limitation of experimental techniques to the measurement of macroscopic termination rates, the validation of models is a cumbersome and difficult task. 

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Fischer and Paul [31] have studied the termination rate coefficient for small radicals in solution by means of ESR. The different values of kt measured for these species were closely modelled by the Smoluchowski equation. Although the chain-length dependence of termination reactions involves much larger radicals, the study clearly demonstrates that kt will... 

In the previous chapter it has been shown that single-pulse PLP experiments can be used for the model-independent determination of the chain-length dependence of termination reactions. When employing on-line conversion data from TR-SP-PLP experiments, k f can be determined directly from the monomer concentration versus time trace and its first and second derivatives, according to ... 

The second kinetic analysis that can be used to study the chain-length dependence of termination reactions, discussed in section 3.3, is based on a careful analysis of a quantitatively determined number MWD. Equations 4.2 and 4.3 summarize this kinetic tool. 

At chain lengths larger than 100, the inaccurate scaling makes it difficult to draw any conclusions about the chain-length dependence of termination reactions in this region. 

The qualitative discussion in section 4.4.3 of the chain-length dependence of termination reactions for the three acrylates has already revealed a lot of information regarding this process. In this section, we will focus on a more quantitative level for a detailed comparison of the three acrylates. Furthermore, we will investigate the extent to which the new data agree with results reported in literature. 

In this thesis the chain-length dependence of termination reactions during free-radical polymerization has been investigated. Interest in the kinetic parameters describing this process, primarily arises from the profound effect that termination reactions have on the overall kinetics of polymerization processes, on the MWD of the final product and therefore also on the final product properties. 

The approach used in this thesis to study the chain-length dependence of termination reactions can be characterized as empirical and model-independent. The basic thought behind this study is that when using single-pulse pulsed-laser initiation, the radical chain-length distribution is so narrow that a monodispersity assumption for this radical population seems to be justified. In this line of thought, two novel kinetic analyses have been presented which should enable the direct model-independent measurement of these microscopic chain-length... 

The investigations presented in this thesis have revealed that the MWD method is certainly a method with great potential for studying the chain-length dependence of termination reactions. However, the leap from the theoretical study presented in chapter 3 to the experimental one presented in chapter 4, introduces two additional complications (i) the correct shape of the number MWD must be determined and (ii) this number MWD must be scaled correctly. 

Summarizing the above, it can be concluded that single-pulse pulsed-laser polymerization techniques are amongst the most powerful techniques that are nowadays available to study the chain-length dependence of termination reactions. When using a dedicated kinetic analysis, as presented in this thesis, model-independent data for the chain-length dependence of kt can be obtained. These data allow for better predictions for the MWD of the final polymer product and so of the final product properties. 

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