Reaction with Solvents, Additives, or Impurities

A typical polymerization system comprises many components besides the initiators and the monomers. There will be solvents, additives (e.g. transfer agents, inhibitors) as well as a variety of adventitious impurities that may also be reactive towards the initiator-derived radicals. 

There is potential for this behavior to be utilized in devising methods for the control of the types of initiating radicals formed and hence the polymer end groups. 

---

The reactions that limit chain growth and initiate polymerization have not been defined. Is polymerization terminated by impurities Does solvent participate in transfer Does polymerization start by the reaction with a monomer or with impurities in the system These questions must be answered. For example, we noticed that successful formation of the monomodal high-molecular-weight polysilane requires the addition of a few drops of monomer to sonicated sodium dispersion prior to the addition of the main part of disubstituted dichlorosilane. 

A transfer agent (T) can be considered from two different aspects that of its chemical function or that of its role in the process. Any chemical group capable of affording a new free radical that would be more stable than the growing radical can cause chain transfer groups with labile hydrogen atoms, alkyl halides, peroxides, disulfides, and so on. Chain transfer involves all types of molecules present in the reaction medium whatever their role in the process monomer, polymer, solvent, initiator, additive, or impurity. Examples hereafter describe these two aspects. 

From the above discussion, it is obvious that response time of a system and its sensitivity are intrinsically linked. In fact they have a reciprocal relationship. As sensitivity increases it is possible to look at bimolecular reactions of species at lower and lower concentrations. In these circumstances the requirements on the response time for a system will get less and less. Of course, there are limits to how far this can be pushed, particularly with time resolved IR measurements in solutions, where absorption by the solvent is significant. Also, as indicated previously, coordination of a nascent photofragment by solvent molecules can occur on an exceedingly rapid timescale (15). Additionally, as the concentration of added reactant is diminished, reactions with impurities in the solvent or with small concentrations of atmospheric gases become a problem. Nevertheless, over a wide range of concentration there is a trade-off between minimum detectable signal and timescale. 

These centres are formed by the addition of monomer to a suitable anion. They are almost always simpler than their cationic reverse part. The counter ion is usually a metal cation able to interact with the electrons of the growing end of the macromolecule, and to bind in its ligand sphere monomer or solvent molecules or parts of the polymer chain. This changes the properties of the whole centre. Therefore, by selection of the metal, the stability of the centre, the tendency of the centres to aggregation, the position of the equilibrium between the contact and solvent-separated ion pairs and free ions, and the stereoselectivity of the centre [the ability to produce polymers with an ordered structure (tacticity, see Chap. 5, Sect. 4.1)] are predetermined. The chemical reactions of the metal cations are, however, very limited. Most solvents and potential impurities are of nucleophilic character. They readily solvate the cation, leaving the anion relatively free. The determination... 

Many catalyzed reactions are markedly influenced by co-catalysts and impurities. Co-catalysts may be added to a reaction or may be charged inadvertently with reagents or solvents. Knowing the reaction mechanism is very helpful in anticipating problems. Treatment of a reaction with conditions that disrupt the catalytic cycle—for instance, by removing a bromide through addition of a silver salt—may impede the catalytic reaction. Once the impurities (beneficial and detrimental) have been identified, the impact on the catalyst and reaction may be assessed. 

---