Polymerisation Mechanisms and What They Predict

The two mechanisms

The step-growth requirement for very high conversion has a practical consequence: any impurity that caps a chain end, or any imbalance in the two monomers, limits the achievable molar mass severely. Stoichiometric precision matters far more there than in chain growth.

Chain growth in detail

Free radical

Initiation generates a radical, propagation adds monomer repeatedly, and termination occurs when two radicals meet — by combination or by disproportionation. It is tolerant and widely used, but gives little control over chain architecture.

Chain transfer is worth knowing: a growing radical can abstract an atom from another molecule, ending its own growth and starting a new chain. This lowers molar mass without lowering the rate, and it is used deliberately to control chain length.

Ionic

Cationic polymerisation suits electron-rich monomers; anionic suits electron-poor ones. Anionic polymerisation can be arranged so that no termination step occurs, leaving chains that continue growing while monomer remains. Such systems give narrow molar mass distributions and allow block copolymers to be made by adding a second monomer.

Coordination

A transition metal catalyst coordinates the monomer before insertion, which imposes control over how each unit is added. This gives regular chain structure and hence highly crystalline, high-density polymers where free radical methods give branched, lower-density material.

Tacticity

Where the monomer has a substituent, its arrangement along the chain matters.

Regular tacticity allows chains to pack closely, which is what produces crystallinity. Free radical polymerisation gives largely atactic material; coordination catalysis gives stereoregular polymer. Explaining the property difference through packing is the expected reasoning.

Degree of polymerisation and how it is controlled

The number-average degree of polymerisation is the average number of monomer units per chain. In step-growth polymerisation it is governed by the fraction of functional groups that have reacted, which is why very high conversion is essential and why an excess of one monomer deliberately caps the chain length.

In chain growth the controlling factor is the ratio of propagation rate to the rates of termination and chain transfer. Raising the initiator concentration produces more chains and therefore shorter ones, which is the usual practical handle. Adding a chain transfer agent shortens chains without slowing the reaction, and it is used industrially for exactly that reason.

Copolymers

The arrangement of two monomer types along the chain — random, alternating, block or graft — strongly affects properties. Block copolymers are particularly useful because the two blocks can behave almost independently, giving materials that combine properties of both homopolymers rather than averaging them.

Frequently asked questions

Why does step growth need such high conversion?

Because chain length depends on how many functional groups have reacted. Even a few percent unreacted leaves many chain ends, capping molar mass.

Why does free radical polymerisation give branched polymer?

Because a growing radical can abstract a hydrogen from an existing chain, creating a new growth point in the middle of that chain rather than at an end.

What makes a polymerisation living?

The absence of termination and chain transfer, so every chain continues growing while monomer remains. All chains start together and grow equally, giving a narrow distribution.

Why is atactic polymer amorphous?

Because the random substituent arrangement prevents chains from packing regularly, so no crystalline regions form.