Chain length dependence techniques

On the other hand, however, it is not straightforward to calculate the MWDs for intermediate cases using the conventional approach. A notable advantage of using an MC simulation technique is that it can be applied to virtually any type of emulsion polymerization, and can account for the chain-length-dependent bimolecular termination reactions in a straightforward manner [265]. Sample simulation results for instantaneous MWDs were shown [265] that were obtained using parameters for styrene polymerization that were reported by Russell [289]. 

Fig. 9 Chain-length dependence of the self-diffusion coefficients of n-alkanes in silicalite-1 (cf. [65]) at 303 K derived from the FR technique (- -) compared with the results measured by PPG NMR [50] at 298 K (-o-), QENS [51] (-A-) at 300 K and molecular dynamic calculations [52] (-0-) at 300 K...

The collection of data and critical evaluation of possible influences of parameters on the CLD (as was carried out by Heuts et al. ) might help to elucidate the current question of the true nature of chain length dependence of the rate constant of propagation in free radical polymerization. Therefore, the investigation of the polymerization behaviour of monomers other than styrene and methyl methacrylate is necessaryf and the use of the correction procedures should eliminate the error introduced by the effect of BB. Thus, comparison of data obtained from either different research groups and/or with the aid of different techniques (MALDI, SEC) should be better feasible. 

If we could observe the ESR spectra of radicals with controlled chain length, chain length dependent phenomena could be precisely examined. In order to clarify the phenomena, model radical precursors were prepared by the ATRP technique. ATRP... 

The RAFT-CLD-T technique was exemplified on styrene and later successfully mapped the chain length dependence of ki for methyl acrylate (MA), butyl acrylate,f dodecyl acryl-ate ° and methyl methacrylate. Additionally, the simultaneous dependence of kt on radical size and monomer conversion was mapped using the RAFT-CLD-T methodology for and vinyl acet-... 

Stationary Polymerization Methods. The determination of the kinetic rate coefficients and p in their coupled form k /kt has long proceeded via measurement of the rate of polymerization and the calculation of the kp/kt via equation 56. With the advent of pulsed laser techniques that allow to obtain much more accurate and detailed information about the kinetic rate coefficients (such as chain length dependencies), these techniques became less important. Nevertheless, measurements of the rate of polymerization are still widely performed and are... 

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. 

The numerical transformation of microscopic k F values into macroscopic quantities as discussed in the previous section is, seen from a numerical perspective, quite straightforward. However, when detailed kinetic information on chain-length dependent termination is desired, the inverse route should be explored as well. Somehow macroscopic quantities must be unraveled in such a way that accurate and reliable data on the chain-length dependence can be uncovered. It will be obvious from the discussion so far, that this route is far from straightforward and requires dedicated experimental techniques. 

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. 

Figure 1.6 Chain-length dependent termination rate coefficients determined via the SP-PLP-NIR-RAFT technique for methyl acrylate and dodecyl acrylate bulk polymerization at 60 °C and 1000 bar.

By a variation of chemistry and/or chain length the different time regimes can be shifted. From a simulation point of view we are again faced by the decision what kind of information we want to get out of the simulation. If one wants to look at very local properties, depending on the local chemistry of the individual monomers, there is no way around a simulation with all chemical details. However, one should keep in mind that by such a technique it is impossible to equilibrate the system near the glass transition temperature. 

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