Polymerization reactions core model

The core first method starts from multifunctional initiators and simultaneously grows all the polymer arms from the central core. The method is not useful in the preparation of model star polymers by anionic polymerization. This is due to the difficulties in preparing pure multifunctional organometallic compounds and because of their limited solubility. Nevertheless, considerable effort has been expended in the preparation of controlled divinyl- and diisopropenylbenzene living cores for anionic initiation. The core first method has recently been used successfully in both cationic and living radical polymerization reactions. Also, multiple initiation sites can be easily created along linear and branched polymers, where site isolation avoids many problems. 

The rapid decay of activity is another drawback of the present systems. This may be due to deactivation processes as described above. It may also be due to diffusion control (vide supra). Of the three models, Rp decreases most markedly with reaction for the fixed site or polymer core model. Figure 2 suggests that this applies to the catalyst under discussion. The flow model would have the smallest change of Rp with polymerization. Therefore, a catalyst which is more readily fractured or fragmented may well have nearly constant activity. 

In this study, molecular weight of the produced polymers will not be tracked over the course of the reaction. Thus, in order to simplify the model, chain transfer mechanisms will not be considered, along with side reactions, such as the production of carbon disulfide. Each of these reactions, as well as the molecular weight, plays a significant role in the iniferter polymerizations however, to simplify the system, it is essential to examine only the core reactions which contribute significantly to the mechanism. 

According to the core-shell model, the growing particle is actually heterogeneous rather than homogeneous, and it consists of an expanding polymer-rich (monomer-starved) core surrounded by a monomer-rich (polymer-starved) outer spherical shell. It is the outer shell that serves as the major locus of polymerization and Smith-Ewart (on-off) mechanism prevails while virtually no polymerization occurs in the core because of its monomer-starved condition. Reaction within an outer shell or at the particle surface would be most likely to be operative for those polymerizations in which the polymer is insoluble in its own monomer or under conditions where the polymerization is diffusion-controlled such that a propagating radical cannot diffuse into the center of the particle. 

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