Ionic polymerization copolymerization

Note-. Coordination polymerization often occurs by pseudo-ionic polymerization. copolymerization... 

Copolymerization behavior can also be used to distinguish between radical and ionic polymerizations (see Chap. 6). 

Of particular interest are certain ionic graft copolymerizations in which the polymerization reaction is initiated only on the macromolecular framework and no homopolymer is formed. An example is provided by the formation of polymeric carbonium ions from chloride-containing polymers, such as poly(vi-nylchloride), in the presence of diethylaluminum chloride ... 

There is far less information in the scientific literature about template copolymerization than about template homopolymerization. As in the case of template homopolymerization, template copolymerization can be realized according to different types of reaction stepwise (template polycondensation), copolyaddition, radical or ionic polymerization, ring-opening copolymerization, etc. 

Comparison of the Two Reactions Step-Growth Polymerization in More Detail Making PET in the Melt Interfacial Poly condensation Chain-Growth Polymerization in More Detail Free Radical Chain Polymerization Going One Step Better Emulsion Polymerization Copolymerization Ionic Chain Polymerization It Lives ... 

As an example of first block obtained by ionic polymerization, Tung et al. [118] have synthesized an a,co-polybutadiene, the end-groups of which were deactivated onto episulfide in order to generate a telechelic polybutadiene dithiol. Such a compound was successfully used to initiate the copolymerization... 

In copolymerization, several different combinations of initiation and termination mechanisms are possible, giving rise to a variety of different polymerization rate equations. Only two cases will be singled out here free-radical copolymerization with termination by coupling, and ionic polymerization with termination by chain transfer to a deactivating agent or impurity. For other combinations, the derivation of rate equations follows along the same lines. 

The lifetime of propagating polymer ion increases with decreasing temperature therefore, the higher molecular weight at the lower polymerization temperature may be reasonably explained. This is additional support for the ionic polymerization mechanism in the copolymerization at low temperatures. 

The propagation rates in ionic polymerizations are influenced by the polarity of the monomers in free-radical reactions, the relative reactivity of the monomers can be correlated with resonance stabihty, polarity, and steric effects we shall consider only radical copolymerizations. 

Another example of ionic graft copolymerization is a reaction carried out on pendant olefinic groups using Ziegler-Natta catalysts in a coordinated anionic-type polymerization. The procedure consists of two steps. In the first, diethylaluminum hydride is added across the double bonds. In the second the product is treated with a transition metal halide. This yields an active catalyst for polymerizations of a-olefms. By this method polyethylene and polypropylene can be grafted to butadiene styrene copolymers. Propylene monomer polymerization results in formations of isotactic polymeric branches ... 

Ionic copolymerizations differ characteristically from free radical copolymerizations. Random copolymers are mostly formed in free radical polymerizations alternating copolymers and block polymers are produced quite rarely. The situation is exactly the reverse for ionic copolymerizations. Thus, ionic polymerizations give rise to quite different copolymerization parameters from those of free radical copolymerizations (Table 22-15). Consequently, copolymerization experiments can be used to determine whether unknown initiators act by a free radical, a cationic, or an anionic mechanism (see also Table 22-16). From such experiments it is found that boroalkyls are free radical initiators, but lithium alkyls are anionic in the... 

It appears to hold quite generally that the polarity of monomer or macroions is more important than their resonance stabilizations. The reverse is true for free radical copolymerizations. Since cations and anions exhibit opposed polarities (electronegativities), an r > re in cationic copolymerizations lead to an ta re in anionic copolymerizations, and vice versa (Table 22-15). In most ionic copolymerization cases, one copolymerization parameter is always greater than unity and the other is less than unity (Tables 22-15 through 22-17). Thus, ionic copolymerizations cannot be carried out azeotropically. The product mostly has a value of about unity for the ionic copolymerization of two resonance-stabilized monomers or non-resonance-stabilized monomers that is, more or less ideal nonazeotropic copolymerizations occur. On the other hand, ionic polymerization of a resonance-stabilized monomer with a non-resonance-stabilized monomer often yields rA B values that are much greater than unity. In such cases, an accentuated tendency toward block polymerization is expected and observed. 

When specific functionalities are desired, copolymerization or grafting reactions are common ways to add functionalities to polystyrenes. Other techniques used to chemically modify polystyrenes are grafting reactions, ionic polymerization, alkylation, amination, condensation reactions and ctosdinking reactions. Copolymerization sometimes involves the laborious synthesis of functionalized styrene molecules through conventional oiganic synthetic methods. In future years it will be difficult and expensive for companies to introduce new monomers due to tougher n ulations. 

In ionic polymerizations, the polarity of the monomers or the ions is far more important than resonance stabilization, while in free radical copolymerizations the reverse is true. For example, if > r2 in cationic copolymerization, then, by contrast, in anionic copolymerization r2 (Table 22-13). If the polarities are very different, then it is no longer possible to have either cationic or anionic copolymerization. The styryl anion, for example, still adds butadiene, but the butadienyl anion does not add styrene. Only monomers with almost identical polarity can undergo true copolymerizations (with r r2 < 1) unless complexes are formed between the active growing end and the monomer. 

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