Evaluate advantages and disadvantages and major difference between cationic and living cationic polymerization.
Living cationic polymerization is a living polymerization technique involving cationic propagating species. It enables the synthesis of very well defined polymers (low molar mass distribution) and of polymers with unusual architecture such as star polymers and block copolymers and living cationic polymerization is therefore as such of commercial and academic interest.
In carbocationic polymerization the active site is a carbocation with a counterion in close proximity. The basic reaction steps are:
A+B− + H2C=CHR → A-CH2-RHC+----B−
Chain propagation:
A-CH2-RHC+----B− + H2C=CHR → A-(CH2-RHC)n-CH2-RHC+----B−
Chain termination:
A-(CH2-RHC)n-CH2-RHC+----B− → A-(CH2-RHC)n-CH2-RHC-B
chain transfer:
A-(CH2-RHC)n-CH2-RHC+----B− → A-(CH2-RHC)n-CH2=CR H+B−
Living cationic polymerization is characterised by defined and controlled initiation and propagation while minimizing side-reactions termination and chain transfer. Transfer and termination do occur but in ideal living systems the active ionic propagating species are in chemical equilibrium with the dormant covalent species with an exchange rate much faster than the propagation rate. Solution methods require rigorous purification of monomer and solvent although conditions are not as strict as in anionic polymerization.
Common monomers are vinyl ethers, alpha-methyl vinyl ethers, isobutene, styrene, methylstyrene and N-vinylcarbazole. The monomer is nucleophilic and substituents should be able to stabilize a positive carbocationic charge. For example, para-methoxystyrene is more reactive than styrene itself.
Cationic polymerization is a type of chain growth polymerization in which a cationic initiator transfers charge to a monomer which then becomes reactive. This reactive monomer goes on to react similarly with other monomers to form a polymer. The types of monomers necessary for cationic polymerization are limited to olefins with electron-donating substituents and heterocycles. Similar to anionic polymerization reactions, cationic polymerization reactions are very sensitive to the type of solvent used. Specifically, the ability of a solvent to form free ions will dictate the reactivity of the propagating cationic chain. Cationic polymerization is used in the production of polyisobutylene (used in inner tubes) and poly(N-vinylcarbazole) Monomer scope for cationic polymerization is limited to two main types: olefins and heterocyclic monomers. Cationic polymerization of both types of monomers occurs only if the overall reaction is thermally favorable. In the case of olefins, this is due to isomerization of the monomer double bond; for heterocycles, this is due to release of monomer ring strain and, in some cases, isomerization of repeating units
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Heterocyclic monomers that are cationically polymerized are lactons, lactams, and cyclic amines. Upon addition of an initiator, cyclic monomers go on to form linear polymers. The reactivity of heterocyclic monomers depends on their ring strain. Monomers with large ring strain, such as oxirane, are more reactive than 1,3-dioxepane which has considerably less ring strain.
Find the key differences between anionic and cationic polymerization.
Anionic polymerization and cationic polymerization are two types of chain growth polymerization reactions which are used to synthesize various types of polymers Both these reactions have the same reaction mechanism, but the reaction Difference Between Anionic and Cationic Polymerization.
Key Difference – Anionic vs Cationic Polymerizationinitiator is different. Anionic polymerization reactions are initiated by an active anionic species, whereas the cationic polymerization reactions are initiated by an active cationic species. This is the key difference between anionic and cationic polymerization. Both these polymerization reactions are sensitive to the solvent used.
Anionic polymerization is a chain growth reaction which begins by an anion. Several different types of initiators are used in anionic polymerization. This series of reactions takes place in three steps: initiation, chain propagation, and chain termination. These polymerization reactions are initiated by nucleophilic addition to the double bond of the monomer. Therefore, the initiator used in the reaction should be a nucleophile.
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