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Tham khảo tài liệu 'thermal analysis of polymeric materials part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 226 3 Dynamics of Chemical and Phase Changes computation of the distribution of the mole fractions of molecules of length x is again based on a probability argument as displayed in the last two lines of Fig. 3.43 and can be compared to the similar equations for the step reaction in Fig. 3.36. The top equation in Fig. 3.44 lists the number and mass averages for chain reactions terminated by the combination of two growing molecule-radicals as described in Fig. 3.43. The derivation of the equation must be done in the same way as illustrated for the step reaction in Fig. 3.42. The next example shown in Fig. 3.44 applies to termination by chain transfer see Sect. 3.2 . In this case the termination Mass Averages for Chain Reactions For termination by chain transfer only double as many molecules are produced and p is treated as in a step reaction Without termination the averages are derivable from the Poisson distribution Mn M0 l v Mw M0 v 2 Note that the computed polydispersities Mw Mn range atp 1 from 1 to 2 only. Experimentally observed values may easily be 20 or more. Why Fig. 3.44 removes one polymer molecule but starts at the same time a new one. Again the derivations of nx and wx are done as before and the summation is the same as for the step reaction in Fig. 3.42. The final averages are those derivable from the Poisson distribution which describes living polymers. Again the limitations of the just derived equations must be kept in mind. 3.4 Copolymerization and Reactions of Polymers The multiplicity of possible linear macromolecules is enormously increased by copolymerization i.e. by polymerizing more than one monomer. Because it is possible to vary concentration as well as placement of the various monomers as discussed in Sect. 1.2 there are unlimited numbers of different polymers and it is impossible to ever produce all polymers of promise to check their properties and pick optimum materials. One must develop the ability to predict polymer properties before .
