Active species, types

In cationic polymerization the active species is the ion which is formed by the addition of a proton from the initiator system to a monomer. For vinyl monomers the type of substituents which promote this type of polymerization are those which are electron supplying, like alkyl, 1,1-dialkyl, aryl, and alkoxy. Isobutylene and a-methyl styrene are examples of monomers which have been polymerized via cationic intermediates. 

The most important appHcation of metal alkoxides in reactions of the Friedel-Crafts type is that of aluminum phenoxide as a catalyst in phenol alkylation (205). Phenol is sufficientiy acidic to react with aluminum with the formation of (CgH O)2Al. Aluminum phenoxide, when dissolved in phenol, greatiy increases the acidic strength. It is beheved that, similar to alkoxoacids (206) an aluminum phenoxoacid is formed, which is a strong conjugate acid of the type HAl(OCgH )4. This acid is then the catalyticaHy active species (see Alkoxides, metal). 

Another group of isoprene polymerization catalysts is based on alanes and TiCl. In place of alkyl aluminum, derivatives of AlH (alanes) are used and react with TiCl to produce an active catalyst for the polymerization of isoprene. These systems are unique because no organometaHic compound is involved in producing the active species from TiCl. The substituted alanes are generally complexed with donor molecules of the Lewis base type, and they are Hquids or soHds that are soluble in aromatic solvents. The performance of catalysts prepared from AlHCl20(C2H )2 with TiCl has been reported (101). 

Poisoning is operationally defined. Often catalysts beheved to be permanently poisoned can be regenerated (5) (see Catalysts, regeneration). A species may be a poison ia some reactions, but not ia others, depending on its adsorption strength relative to that of other species competing for catalytic sites (24), and the temperature of the system. Catalysis poisons have been classified according to chemical species, types of reactions poisoned, and selectivity for active catalyst sites (24). 

As mentioned, AOS is a complex mixture of anionic active substances the nature of which is dependent on the quality of the olefin feed and the manufacturing processes employed. Formulations utilizing AOS are developed according to the level and type of these active species. Analytical procedures are therefore required to define their level and nature. 

Furthermore, the reaction scheme implies that the molecular weight distribution is Poisson-like — i.e. very narrow — as it had been shown earlier on theoretical basis by Flory 8), Gold 9), and Szwarc l0>. Even though two (or more) types of active species add monomer at very different rates, the polydispersity remains narrow, provided solvation/desolvation and ionic dissociation/association processes are fast U). 

The active species are generated after refluxing the pristine CNT in HNO3 [137, 138]. Other oxidation strategies can be implemented for tuning the type and density of the oxidized catalytic functions. Resasco [139] pointed out that these results open up an avenue for tuning the density and distribution of C=0 pairs, in particular with controlled chirahties. 

Termination results in the removal of the activated species from the reaction. It involves the bimolecular reaction between the MA and a specific reactive species, D. Depending on the type of polymerization reaction, the reactive species may be a radical or an ion acceptor. The reaction, then, can be defined as Eq. 4.12. 

Most likely the flavin triplet is the photochemically active species and the flavin semiquinone is the reducing agent for the b-type cytochrome. 

---