|Free-Radical Polymerization Reactions||Chain Initiation||Chain Propagation|
|Chain Termination||The Formation of Branched Polymers||Anionic Polymerization|
|Cationic Polymerization||Advantages of Free-radical Versus Ionic Polymerization||Coordination Polymerization|
Free-Radical Polymerization Reactions
It isn't difficult to form addition polymers from monomers containing C=C double bonds; many of these compounds polymerize spontaneously unless polymerization is actively inhibited. One of the problems with early techniques for refining gasoline, for example, was the polymerization of alkene components when the gasoline was stored. Even with modern gasolines, deposits of "gunk" can form when a car or motorcycle is stored for extended periods of time without draining the carburetors.
The simplest way to catalyze the polymerization reaction that leads to an addition polymer is to add a source of a free radical to the monomer. The term free radical is used to describe a family of very reactive, short-lived components of a reaction that contain one or more unpaired electrons. In the presence of a free radical, addition polymers form by a chain-reaction mechanism that contains chain-initiation, chain-propagation, and chain- termination steps.
A source of free radicals is needed to initiate the chain reaction. These free radicals are usually produced by decomposing a peroxide such as di-tert-butyl peroxide or benzoyl peroxide, shown below. In the presence of either heat or light, these peroxides decompose to form a pair of free radicals that contain an unpaired electron.
The free radical produced in the chain-initiation step adds to an alkene to form a new free radical.
The product of this reaction can then add additional monomers in a chain reaction.
Whenever pairs of radicals combine to form a covalent bond, the chain reactions carried by these radicals are terminated.
The Formation of Branched Polymers
At first glance we might expect the product of the free-radical polymerization of ethylene to be a straight-chain polymer. As the chain grows, however, it begins to fold back on itself. This allows an intramolecular reaction to occur in which the site at which polymerization occurs is transferred from the end of the chain to a carbon atom along the backbone.
When this happens, branches are introduced onto the polymer chain. Free-radical polymerization of ethylene produces a polymer that contains branches on between 1 and 5% of the carbon atoms. Of these branches, 10% contain two carbon atoms, 50% contain four carbon atoms, and 40% are longer side chains.
Addition polymers can also be made by chain reactions that proceed through intermediates that carry either a negative or positive charge.
When the chain reaction is initiated and carried by negatively charged intermediates, the reaction is known as anionic polymerization. Like free-radical polymerizations, these chain reactions take place via chain-initiation, chain-propagation, and chain-termination steps.
The reaction is initiated by a Grignard reagent or alkyllithium reagent, which can be thought of a source of a negatively charged CH3- or CH3CH2- ion.
The CH3- or CH3CH2- ion from one of these metal alkyls can attack an alkene to form a carbon-carbon bond.
The product of this chain-initiation reaction is a new carbanion that can attack another alkene in a chain-propagation step.
The chain reaction is terminated when the carbanion reacts with traces of water in the solvent in which the reaction is run.
The intermediate that carries the chain reaction during polymerization can also be a positive ion, or cation. In this case, the cationic polymerization reaction is initiated by adding a strong acid to an alkene to form a carbocation.
The ion produced in this reaction adds monomers to produce a growing polymer chain.
The chain reaction is terminated when the carbonium ion reacts with water that contaminates the solvent in which the polymerization is run.
Advantages of Free-radical Versus Ionic Polymerization
The initiation step of ionic polymerization reactions has a much smaller activation energy than the equivalent step for free-radical polymerizations. As a result, ionic polymerization reactions are relatively insensitive to temperature, and can be run at temperatures as low as -70°C. Ionic polymerization therefore tends to produce a more regular polymer, with less branching along the backbone, and more controlled tacticity.
Because the intermediates involved in ionic polymerization reactions can't combine with one another, chain termination only occurs when the growing chain reacts with impurities or reagents that can be specifically added to control the rate of chain growth. It is therefore easier to control the average molecular weight of the product of ionic polymerization reactions.
Ionic polymerizations are more difficult to carry out on an industrial scale than free-radical polymerizations. Ionic polymerization is therefore only used for monomers that don't polymerize by the free-radical mechanism or to prepare polymers with a regular structure.
In 1963 Karl Ziegler and Giulio Natta received the Nobel prize in chemistry for their discovery of coordination compound catalysts for addition polymerization reactions. These Ziegler-Natta catalysts provide the opportunity to control both the linearity and tacticity of the polymer.
Free-radical polymerization of ethylene produces a low-density, branched polymer with side chains of one to five carbon atoms on up to 3% of the atoms along the polymer chain. Ziegler-Natta catalysts produce a more linear polymer, which is more rigid, with a higher density and a higher tensile strength. Polypropylene produced by free-radical reactions, for example, is a soft, rubbery, atactic polymer with no commercial value. Ziegler-Natta catalysts provide an isotactic polypropylene, which is harder, tougher, and more crystalline.
A typical Ziegler-Natta catalyst can be produced by mixing solutions of titanium(IV) chloride (TiCl4) and triethylaluminum [Al(CH2CH3)3] dissolved in a hydrocarbon solvent from which both oxygen and water have been rigorously excluded. The product of this reaction is an insoluble olive-colored complex in which the titanium has been reduced to the Ti(III) oxidation state.
The catalyst formed in this reaction can be described as coordinately unsaturated because there is an open coordination site on the titanium atom. This allows an alkene to act as a Lewis base toward the titanium atom, donating a pair of electrons to form a transition-metal complex.
The alkene is then inserted into a Ti-CH2CH3 bond to form a growing polymer chain and a site at which another alkene can bond.
Thus, the titanium atom provides a template on which a linear polymer with carefully controlled stereochemistry can grow.