Polymerization, activation ionic

The initiation step of chain growth creates a reactive site that can react with other monomers, starting the polymerization process. Before the monomer forms the reactive site, the initiator ( ) (which maybe either a radical generator or an ionic species) first creates the polymerization activator (A) at a rate defined by the rate constant kv This process can be represented as shown in Eq. 4.7. 

Both the initiation step and the propagation step are dependent on the stability of the carbocations. Isobutylene (the first monomer to be commercially polymerized by ionic initiators), vinyl ethers, and styrene have been polymerized by this technique. The order of activity for olefins is Me2C=CH2 > MeCH=CH2 > CH2=CH2, and for para-substituted styrenes the order for the substituents is Me—O > Me > H > Cl. The mechanism is also dependent on the solvent as well as the electrophilicity of the monomer and the nucleophi-licity of the gegenion. Rearrangements may occur in ionic polymerizations. 

The position of the monomer is a schematic representation of relative isotactic polymerization activity of the monomer by the catalysts. Isotactic steric control is found only in a narrow range of cationidty of vinylether catalysts. Atactic polymer is produced toward the more ionic side and no polymer toward the less ionic range. 

Flere MAO first generates the dimethyl complex 6.26 from 6.25. This reaction, of course, can also be brought about by Me3Al. It is the subsequent reaction (i.e., the conversion of 6.26 to 6.27 that is of crucial importance. The high Lewis acidity of the aluminum centers in MAO enables it to abstract a CH3 group from 6.26 and sequesters it in the anion, [CH3-MAO]. Although 6.27 is shown as ionically dissociated species, probably the anion, [CH3-MAO], weakly coordinates to the zirconium atom. It is this coordinatively unsaturated species, 6.27, that promotes the alkene coordination and insertion that are necessary for polymerization activity. 

The formation of aggregates with low to zero polymerization activity is quite general in ionic polymerizations. This statement can be further documented by the observation of centre aggregation during anionic polymerization of oxirane [100-103]... 

Decay of ionic and coordination centres always leads to the formation of some end groups and centre residues. The centres usually lose their polymerizing activity on contact with atmospheric humidity. A residue of very active centres, which are rare, is usually not removed from the polymer (e.g. of the order of one ppm of the transition metal in low-pressure polyethylene). Larger residues have to be washed out (some types of polypropylene are still washed at the present time). 

Epoxide resins can be crosslinked by polyaddition of active hydrogen-containing coumpounds, e.g. carboxylic acids, anhydrides (via intermediate ester-acid steps), amines, phenols, etc. or by polymerization via ionic mechanisms These reactions are generally started by application of heat. 

In recent years, a large number of mono- and dicationic lanthanide alkyl complexes have been found to be efficient catalysts for ethylene polymerization, and in some cases, the dicationic lanthanide derivatives show higher activity and selectivity than their monocationic counterparts. Ionic radii of lanthanide metals also affect the catalytic behavior, and polymerization activity often increases with ionic radius [5, 76],... 

Another approach was proposed by Lutz et al.2122 m-Diisopropenylbenzene (DIB) was polymerized an-ionically under such conditions that the second double bond remained unaffected. Linear polymers having molecular weights between 3000 and 10 000 and pendent double bonds were prepared. The remaining double bonds were reacted with cumyl-potassium to create active sites along the PDIB chain. The polymerization of ethylene oxide was initiated from these active sites to produce PEO stars. 

Anionic polymerizations must be carried out in the absence of water, oxygen, carbon dioxide, or any other impurities that may react with the active ionic centers. Glass surfaces carry layers of adsorbed water which react with carbanions. It is thus necessary to take special precautions, such as flaming under vacuum, to remove this adsorbed water in laboratory polymerizations. The monomer itself should be very pure and free from inhibitors. 

These esters were prepared from aqueous solutions of tri-ethanolamine and PE-61.PE-62 and PE-64 block copolymers. When higher monoesters of the surface active agents were utilized, excellent antiwear properties were observed. Again, the configuration of the ether linkage in relationship to the rest of the molecule plays a role in this compound s effectiveness. Table 2.4 shows properties of cutting fluids from ricinoleic acid oligomer esters with triethanolamine. The same values from fluids made from polymeric non-ionic surfactants are shown in Table 2.5. 

Dordick and coworkers [84] have shown that polyphenols can be successfully polymerized using ionic liquids as solvent and certain peroxidases as catalyst. They reasoned that RTILs would provide a threefold advantage over conventional solvents. First, the RTIL provides a nonaqueous environment that should show good solubility of the monomer and polymer second, the catalyst activity should... 

Butyllithium was also known to polymerize ethylene (4, 5, 6), but it was less active than triethylaluminum. Conceptually it was felt that BuLi should be more active because of the smaller cation and more ionic metal-carbon bond, but the low polymerization activity could be caused by greater difficulty in breaking down the strong aggregates. Solvation by TMED was visualized to give dimer and monomer structures which were directly related to AlEt3, shown at the bottom of Figure 1. At the same time, solvation of lithium by TMED was expected to further increase the ionic character of the Li-C bond. TMED was used rather than ethers because it was expected to be less reactive toward BuLi. 

There are some cases in homogeneous anionic polymerization in which the initiator dissociates completely with quantitative transformation into the active ionic form and the process is also virually instantaneous (stoichiometric polymerization). This is the case, for example, when one uses, as initiators, alkali organic... 

It is apparent that, as a result of the extremely rapid propagation, if aU chain ends were ionized and grew simultaneously, monomer would disappear at such a high rate that the polymerization would be uncontrollable. In hving cationic polymerization, therefore, a dynamic equilibrium must exist between a very small amount of active ionic and a large pool of inactive dormant species. The expression of controlled polymerization is sometimes used to describe, perhaps questionably, such polymerizations with reversible deactivation of the chain carriers. 

The existence of a dynamic equilibrium between dormant (covalent) and active (ionic) species in controlled carbocationic polymerizations had been debated for years. It has been argued that under certain conditions, polarized covalent species can directly react with monomer examples are the pseudocationic mechanism proposed for the polymerization of styrene initiated by perchloric acid (123,124) (Fig. 5) or the two-component group transfer polymerization proposed for the polymerization of isobutylene initiated by the dicumylacetate/BCls system (125) (Fig. 6). Recent results and theoretical considerations support the now generally accepted view that the true active species are ions, and the dormant species serve as a reservoir from which the propagating ion pairs are formed (126-131). The existence of a dynamic equilibrium between dormant and active species and the ability to suppress the formation of free ions made possible the synthesis of pol5miers with controlled molecular architecture via carbocationic polymerization. 

Actually, many cationic metallocene catalysts have been prepared, and counter-anions of these ionic metallocenes are PFg, BF4, and AICI4 [19]. Because these counter-anions are tightly bound to metallocene cations, these complexes show no olefin polymerization activities. 

In cation-like catalytic centers (III), the counteranion is MAO and methyl anion derived from metallocene. Imuta et al. have pointed out that a stable ionic compound like (V) is the key structure for high polymerization activities, and proposed that the sulfonate anion from zirconium sulfonate (IV) forms stable cation-like catalytic centers (V) [Eq. (9)] [33]. This zirconium sulfonate (IV) and MAO shows higher activities compared to the corresponding chloride system ... 

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