Polymerization

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Polymerisation means joining together of lots of monomers to create long, repeating, chain polymers.

Addition Polymerisation

Addition polymerisation means polymerization with radical mechanism. mechanism. Monomers are molecules, usually with a double bond which breaks to bond with another monomer This repeats until long chains are formed. With two monomers, copolymerisation occurs, meaning different monomers alternating in a long chain. Free-radicals, atoms or molecules with unpaired electrons initiate this. These are very reactive causing chain-reactions with more and more monomers joining the chain.

Polymerization of ethene

Initiation:

Polymerization involves organic peroxide radicals, molecules containing an O-O single bond. These can be created by oxygen reacting with ethene. They are unstable and easily break down into two radicals. In an ethene monomer, one electron pair is held securely between the two carbons in a sigma bond. The other is more loosely held in a pi bond.

The free radical uses one electron from the pi bond to form a more stable bond with the left carbon atom. The other electron returns to the right carbon atoms:

Propagation:

The new molecule, has the unpaired electron from the C=C bond, so a larger radical has been created. This can react with other ethenes to go on reproducing.

Termination:

The growth only stops when two free-radical polymer chains collide, producing a long non-radical poly(ethene) chain.

This reaction occurs at high temperatures and pressures, such as 300 oC and 2000 At.

Control in methods of polymerisation:

High pressure/temperature polymerisations

High temperatures and pressures originally caused violent, hard to control, reactions, creating inconsistent products and even explosions.

The radical mechanism encouraged variably branched chains. Also chain-length was hard to control, as termination occurred randomly when two radical chains happened to meet.

Ziegler-Natta catalysts

The trouble with polymers produced with a radical-mechanism (see above) is there was usually a large degree of branching. This occures during propagation when the chain curls back on itself and then breaks - leaving chains coming out from the main carbon backbone. This made the polymers less dense meaning a relatively low tensile strength.

For ethene polymers Ziegler-Natta catalysts (triethylaluminium in the presence of a metal (IV) chloride) largely solved the branching problem. Instead of a radical reaction the ethene monomer inserts between the aluminium atom and one of the ethyl groups. Because the polymer chain grows out from the aluminium atom it is almost unbranched.

However, polymerisation of propene has complications: syndiotactic, isotactic and atactic polymer chains were produced. Different metal chlorides allow selective production of one form. However, if the catalyst is poisoned or damaged then the chain stops growing. Reaction of a growing polymer chain with a second catalyst particle can cause branching. Also, Ziegler-Natta monomers could be only small, with few variations.

Metallocenes

Due to their structure, metallocene catalysts, have less premature chain termination and branching. However the first metallocene catalysts were unable to selectively produce different poly(propene) stereoisomers, though later work saw development of metallocene catalysts capable of directing the stereochemistry of the growing polymer chain.