Examples of polymerization in the following topics:
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- Treatment of a cold THF solution of styrene with 0.001 equivalents of n-butyllithium causes an immediate polymerization.
- This is an example of anionic polymerization, the course of which is described by the following equations.
- Only monomers having anion stabilizing substituents, such as phenyl, cyano or carbonyl are good substrates for this polymerization technique.
- A practical application of anionic polymerization occurs in the use of superglue.
- When exposed to water, amines or other nucleophiles, a rapid polymerization of this monomer takes place.
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- Polymerization of isobutylene (2-methylpropene) by traces of strong acids is an example of cationic polymerization.
- This process is similar to radical polymerization, as demonstrated by the following equations.
- Monomers bearing cation stabilizing groups, such as alkyl, phenyl or vinyl can be polymerized by cationic processes.
- At low temperatures, chain transfer reactions are rare in such polymerizations, so the resulting polymers are cleanly linear (unbranched).
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- Indeed, cases of explosively uncontrolled polymerizations have been reported.
- It is useful to distinguish four polymerization procedures fitting this general description.
- Radical Polymerization The initiator is a radical, and the propagating site of reactivity (*) is a carbon radical.
- Cationic Polymerization The initiator is an acid, and the propagating site of reactivity (*) is a carbocation.
- Anionic Polymerization The initiator is a nucleophile, and the propagating site of reactivity (*) is a carbanion.
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- An efficient and stereospecific catalytic polymerization procedure was developed by Karl Ziegler (Germany) and Giulio Natta (Italy) in the 1950's.
- In the case of ethylene, rapid polymerization occurred at atmospheric pressure and moderate to low temperature, giving a stronger (more crystalline) product (HDPE) than that from radical polymerization (LDPE).
- Polymerization of propylene through action of the titanium catalyst gives an isotactic product; whereas, a vanadium based catalyst gives a syndiotactic product.
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- Virtually all of the monomers described above are subject to radical polymerization.
- When radical polymerization is desired, it must be started by using a radical initiator, such as a peroxide or certain azo compounds.
- By using small amounts of initiators, a wide variety of monomers can be polymerized.
- One example of this radical polymerization is the conversion of styrene to polystyrene, shown in the following diagram.
- The relative importance of these terminations varies with the nature of the monomer undergoing polymerization.
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- At this point, our discussion will be limited to an application of anionic polymerization.
- In the anionic polymerization of styrene, a reactive site remains at the end of the chain until it is quenched.
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- Radical polymerization gives a statistical copolymer.
- However, the product of cationic polymerization is largely polystyrene, and anionic polymerization favors formation of poly(methyl methacrylate).
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- The two open bonds remaining at the ends of the long chain of carbons (colored magenta) are normally not specified, because the atoms or groups found there depend on the chemical process used for polymerization.
- Many polymeric materials having chain-like structures similar to polyethylene are known.
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- In one type of polymerization reaction, a series of condensation steps takes place whereby monomers or monomer chains add to each other to form longer chains.
- This is termed "condensation polymerization," or "step-growth polymerization," and occurs in such processes as the synthesis of polyesters or nylons.
- The condensation of a diacid and diamine is an example of A-B polymerization.
- Recognize the chemical principles of condensation reactions as they relate to polymerization.
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- In this respect, it is useful to distinguish several ways in which different monomeric units might be incorporated in a polymeric molecule.