What factors can influence polymerisation?

Question

What factors can influence polymerisation?

Answer

A huge number of factors can affect polymerisation reactions. These start from the basics, including the monomers involved, the type of polymerisation that they are capable of engaging in, and the type and quantity of solvent used. While some monomers will freely engage in energetic polymerisation reactions, others will need very finely-tuned reaction conditions to make their polymerisation feasible. The thermodynamics of the reaction, including the change in enthalpy and entropy determine this, as well as the effect of heat on the rate of the reaction.

Some reaction mixtures will have additional requirements in order for the reaction to begin. Some will require catalysts; the concentration and efficiency of these catalysts will naturally have an effect on the rate of reaction. Some reactions have more unusual requirements, such as the creation of free-radical containing initiator molecules. These can be created by a number of processes, including thermal decomposition, ionising radiation, or electrolysis. Biological reactions, such as the production of polysaccharides and polymeric proteins, are catalysed by enzymes, which come with a number of influences of their own; they are usually only effective within certain ranges of temperature and pH, and may also have additional requirements such as coenzymes or cofactors.

The rate of termination will also have a strong influence on the rate of reaction. Impurities and inhibitors can react with the active end of a polymer chain, resulting in ‘dead’ ends which cannot continue the reaction. Common impurities include oxygen, and derivatives of any solvents used.

Some monomer units have multiple propagation sites, potentially resulting in highly branched polymers, or high proportions of dimers or oligomers. If all of the side-chains continue to polymerise, the number of active sites could grow exponentially, possibly resulting in a dangerous acceleration of the reaction. This is known as autoacceleration, and can also be caused by a decrease in termination rate, such as encountered during increases in the viscosity of the reaction mixture.

References