their products have variable physical properties. To this day, the systems are little understood, but the monomers (polymer starting materials) react through a number of reaction sites on the catalyst. Unfortunately, this means the polymer can grow from many sites and at different rates, leading to a very wide distribution in the molecular weight, based on the polymer chain length.

Modern life has demanded more of the humble polymer. We want polymers that are stronger than steel, lighter than aluminium, and can be dyed any colour imaginable

Metallocenes are positively charged metal ions, most commonly Titanium or Zirconium, sandwiched between two negatively charged cyclopentadienyl rings (see fig). Their big advantage over the Ziegler-Natta systems is that they catalyse the reaction of olefins through only one reactive site. Due to this “single site” reaction, the polymerization continues in a far more controllable fashion, leading to polymers with narrow ranges of molecular weight and, more importantly, predictable and desirable properties.

it has been found that changing the ligands (functional groups attached to the metal) upon the metallocene molecule can controllably affect the properties of the polymer.

The drawback of metallocene catalysts is that they are unable to polymerize polar molecules, such as common acrylics or vinyl chloride. This is due to the metallocenes’ oxophilicity – their propensity for binding to oxygen.

Catalysts using late transition metals – those metals from groups 6 and higher in the Periodic Table – have become increasingly utilized. These compounds have good polymerization activity, although slightly less than metallocenes. However, crucially they can polymerize reactions with polar monomers.

The most commercially advanced of this type of catalysts are the Brookhart catalysts [6], which are diimine complexes of palladium or nickel (see fig).

Table 7 shows that the composition varies from chains of only monomer A (homopolymer A) to chains containing only monomer B (homopolymer B).

In many of the structures no regularity can be detected, although there will be short sequences of one type of unit, and the copolymer can be regarded as completely random; such copolymers are usually said to be ideal copolymers. These possible copolymer structures are shown schematically in Table 7.

It can be shown that the rate of change of monomer concentration in any copolymerization is given by the equation where [M1] and [M2] are the concentrations of monomers 1 and 2 at any instant and r1 and r2., are reactivity ratios. The reactivity ratios represent the rate at which one type of growing chain end adds on to a monomer of the same structure relative to the rate at which it adds on to the alternative monomer. The copolymer equation can be used to predict chain structure in the three different ways, already mentioned.

An ideal copolymer will tend to form when each type of chain end shows an equal preference for adding on to either monomer. In this case, and the copolymer equation becomes Hence composition depends on the relative amounts of monomer present at any time and the relative reactivities of the two monomers.

Step growth copolymerizations produce ideal (random) copolymers since in this special case r 1 = r 2 = 1.

The main reason for copolymerizing different monomers is to adjust the physical properties of a given homopolymer to meet a specific demand. SBR elastomer, for example (Table 1), based on 24 wt% styrene monomer shows better mechanical properties and better resistance to degradation than polybutadiene alone

A second reason for copolymerization is to enhance the chemical reactivity of a polymer, particularly to aid crosslinking. Conventional vulcanization in rubbers is brought about by forming sulphur crosslinks at or near double bonds in the chain

To show the dramatic effect of copolymer structure on physical properties, consider the change from random SBR copolymer to a block copolymer of exactly the same chemical composition but where the styrene and butadiene parts are effectively homopolymer chains linked at two points: The material behaves like a vulcanized butadiene rubber without the need for chemical crosslinking since the styrene chains segregate together to form small islands or domains within the structure. Such so-called thermoplastic elastomers (TPEs) today form an important growth area for new polymers because of the process savings in manufacture that can be achieved with their use.

They are colourless, odourless liquids produced by a simple chemical reaction, whereby molecules of water are eliminated from commercially produced petrochemical products.

The most commonly used plasticisers are phthalates.

Predominantly, phthalates are used in the plastics industry to soften the popular plastic PVC. This is used to make a diverse range of cost effective products with various levels of technical performance suited to a wide range of applications. Many of these PVC products we use everyday but tend to take for granted. They include everything from lifesaving medical devices such as medical tubing and blood bags, to footwear, electrical cables; packaging, stationery, and toys.