1. A Theory of Time-Invariant Molecular Weight Distributions in Heterogeneous Ziegler-Natta Polymerizations

Publisher Summary This chapter points out that the behavior of molecular weight distributions (MWD) of polymers produced at the initial stages of polymerization, plays an important role for the mechanistic discussion of the broadness of MWDs. Supposing initial stages of polymerization where the yield and molecular sizes of produced polymers are very small, the original catalyst surface may remain unchanged and the diffusion control theory should predict a narrow MWD, because of the absence of any room to take the “multi-grain model” or “core model” of the diffusion control theory. The chain-length theory predicts the same conclusion. The nonuniform centers theory, however, predicts broad MWDs even for the initial stages of polymerization. Such initial polymerization stages appropriated for this criterion must belong to the quasi-living polymerization stage of any heterogeneous polymerization, where the average molecular size increases from a small and time-proportional value to a large stationary value. A few MWDs during quasi-living stages of heterogeneous Ziegler–Natta polymerization have been reported. The time dependency of MWDs reported are not unique but scatter, from a broader to a narrow, or vice versa, and down and up to stationary. Complex changes are assumed on the basis of changes in the number of some kinds of active centers. Thus, meaningful studies on MWDs in the course of quasi-living polymerization must be carried out together with observation of the kinetic behaviors of polymerization themselves. In this aspect, the experimental results described in the chapter are useful.