

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.



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.



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 CH

3-

or CH

3

CH

2-

ion.



The CH

3-

or CH

3

CH

2-

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.



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

o

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 (TiCl

4

) and triethylaluminum [Al(CH

2

CH

3

)

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-CH

2

CH

3

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.