Nature of the growing species

The possibility of changing the polymerisation mechanism depending on the kind of monomer as well as the catalyst, especially in binary or ternary comonomer systems, is obvious. This may also concern change in the nature of the growing species throughout the propagation of one polymer chain in the presence of both multinuclear catalysts [scheme (33)] and mononuclear catalysts [scheme (34)] ... 

Because the concentration of carbocations in a real polymerization is very low, model NMR studies have been used to obtain a deeper insight into the nature of the growing species. These experiments are restricted to sufficiently stable carbocations, such as those derived from vinyl ethers. Styrene derivatives are not stable enough and participate in Friedel-Crafts alkylation. For example, derivatives of a-methylstyrene easily deproto-nate, dimerize and then form intramolecularly indan derivatives. 

Fig. 1. The apparatus ofAbkin and Medvedev demonstrating the long-life nature of the growing species formed in polymerization of butadiene initiated by metallic sodium...

As seen, the anionic and cationic polymerizations are analogous differing mainly on the nature of the active species. The stereochemistry associated with anionic polymerization is also similar to that observed with cationic polymerization. For soluble anionic initiators at low temperatures, syndiotactic formation is favored in polar solvents, whereas isotactic formation is favored in nonpolar solvents. Thus, the stereochemistry of anionic polymerizations appears to be largely dependent on the amount of association the growing chain has with the counterion, analogous with the cationic polymerizations. 

The enhanced control can be ascribed to the dynamic stabilization of growing cations. The stabilization can be understood as an extension of the lifetime of the growing species which only for a small fraction of time are in the carbocationic form and during the vast majority of time they are in the dormant form. The nature and concentrations of various activators and deactivators define the equilibrium position and dynamics of the exchange processes. There are three general approaches to controlled systems (Section IV.B) ... 

Under these conditions, monomers of type I and type 2 add with equal facility to the growing chain, regardless of the nature of the radical species in the terminal position. The resulting copolymer is truly random and its composition is exactly the same as the feed, or the reaction mass (i.e., F = /t). 

Because of the nature of the active species, coordination polymerization has been classified as ionic polymerization, which follows the polyaddition mechanism s characteristic steps, in the growing of the polymeric chain initiation, propagation, and termination. As for the initiation step, the ionic active species is produced by the reaction between the catalyst and cocatalyst. Usually, the catalysts are actually precursor catalysts or precatalysts, which become the real cationic active species after the activation or reaction with the cocatalyst (Fig. 5.8). 

The diffusion couple consists of two reactant pieces placed in contact. In some cases, the differences in colour of reactants and the product phase can be used to observe the progress of the reaction. In other cases, thin wires of Pt can be used as markers. After some time at high temperature, the product phase nucleates and grows, and therefore separates the reactants. For relatively simple reaction mechanisms, it is possible to deduce the nature of the diffusing species from the relative growth of the new phase. 

This fact indicates the living nature of the polymerizations of p-PL initiated by 3b -g with the 100 % initiation efficiency. Thus, the observed difference in the pol3mierization rates in Figure 1 can be directly attributed to the difference in the activities of the growing species (4). 

Chain gro tvth polymerization begins when a reactive species and a monomer react to form an active site. There are four principal mechanisms of chain growth polymerization free radical, anionic, cationic, and coordination polymerization. The names of the first three refer to the chemical nature of the active group at the growing end of the monomer. The last type, coordination polymerization, encompasses reactions in which polymers are manufactured in the presence of a catalyst. Coordination polymerization may occur via a free radical, anionic, or cationic reaction. The catalyst acts to increase the speed of the reaction and to provide improved control of the process. 

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