What is Polymerisation?

Polymers are found almost everywhere in the world, from plastic shopping bags, to food containers, all the way through to building construction materials and functional materials for solar cell and biomedical applications. A few of these polymer materials occur naturally, but the majority are synthesised through a process known as polymerisation.

There are a number of different polymerisation processes utilised around the world today, each technique able to produce unique types of synthetic polymers and fine-tuned properties. In fact, companies manufacture a stupendous amount of polymers that reaches over 340 million tonnes each year, producing over 60,000 different polymeric materials, each with its own beneficial applications [1].

Here, you will learn about:

• The different mechanisms of polymerisation
• Polymeric materials and their applications
• Future trends in polymer chemistry

Mechanisms of Polymerisation

Polymerisation is the chemical process of covalently bonding many single molecules, called monomers, together to form these long chain molecules that we call polymers [2]. There are two main classes of polyreactions, step-growth polymerisation and chain-growth polymerisation.

Step-growth polymerisation

Step-growth polymerisation is a mechanism whereby the functional groups of the monomers react and combine to form dimers, trimers, longer oligomers and eventually polymers [2]. Therefore, to form a long chain, the monomers require at least 2 reactive functional groups. A step-growth reaction involving monomers with more than 2 reactive functional groups will introduce branching and result in a cross-linked polymer.

The step-growth process can be termed polycondensation when the chain growth results from a condensation reaction between the functional groups, or polyaddition when involving an addition reaction.

The reaction proceeds as follows:

`Monomer+Monomer \rightarrow Dimer`

`Dimer+Monomer \rightarrow Trimer`

`Dimer+Dimer \rightarrow Tetramer`

`Trimer+Dimer \rightarrow Pentamer`

`(n)mer+(m)mer \rightarrow (n+m)mer`

Chain-growth polymerisation

Chain-growth polymerisation involves a reaction at the active centre of radicalised or ionic monomers, involving 3 essential reaction steps:

1. Initiation

Initiation is the process of creating an active centre in a monomer. The initiator will depend on the process at hand: radical polymerisation uses a radical initiator, ionic polymerisation uses a cationic/anionic initiator, thermal polymerisation with heat and coordination polymerisation with transition metal complexes. This reaction step goes as follows:

`X\ X\rightarrow 2X^{\ast}`

`X^{\ast}+M\rightarrow X\ M^{\ast}`

where `X` is an initiator, `M` is a monomer, `X^{\ast}` and `M^{\ast}` are radicalised.

2. Propagation

Propagation refers to the repetitive growth of the polymer, where a monomer is added to the existing active centre to produce a longer polymer molecule with a new active centre. This reaction step goes as follows:

`X\ M^{\ast}+M\rightarrow X\ M_n\ M^{\ast}`

3. Termination

Termination propagation will continue until termination, where by any means, the active centre of the chain carrier disappears and the polymer molecule is no longer able to grow. This reaction step goes as follows:

`X\ M_n\ M^{\ast}\rightarrowX\ M_n\ M\ Y`

where `Y` is some molecule that terminates the active centre [2].

 

Table 1. One-to-one comparison between step-growth polymerisation and chain-growth polymerisation. 

Step-Growth Polymerisation	Chain-Growth Polymerisation
Reaction between monomers with two or more reactive functional groups	Reaction at active centres of unsaturated monomers
Oligomers are first formed, which then join to form the polymer	A polymer chain is formed through the attachment of one monomer at a time
Monomers are bi-functional or multi-functional	Monomers are unsaturated
May require a catalyst, but no initiators	Requires an initiator
Monomer consumed rapidly at the beginning of the process	Monomer is consumed at a steady rate throughout the process
Degree of polymerisation steadily increases throughout the process	Degree of Polymerisation increases quickly at the beginning of the process
No termination	Polymer is unable to grow after termination

Materials resulting from polymerisation

Polymers resulting from step-growth polymerisation are named according to functional group, such as polyesters and polyamides, whereas polymers resulting from chain-growth polymerisation are named depending on the monomer from which it was produced, such as polyethylene and polystyrene.

Polyesters are durable, tough, chemically resistant and non-allergenic. One of the most commonly used polymers, in fact, is the polyester PET (polyethylene terephthalate), which is a clear, strong and lightweight polymer used in plastic bottles, containers and even water-resistant fabrics [3].

Polyamides have a high tensile strength, high flexibility, good resilience and high impact strength. The most common polyamides are Nylon 6,6 (polyhexamethylene adipamide) and Nylon 6 (polycaprolactam), with uses in high-strength ropes, sports clothing and parachutes [4].

Polyethylene can be categorised as high-density polyethylene (HDPE) or low-density polyethylene (LDPE). HDPE is strong, dense and crystalline, whereas LDPE is flexible, ductile and tends to stretch when strained. The applications of such materials are extensive, from cling wrap and plastic bags through to reinforcements and bullet-proofing [5].

Polystyrene is most commonly known as “Styrofoam” in its foamed state, used for packaging food and fragile goods. However, solid polystyrene also has a long list of applications, exhibiting brittle, hard and transparent properties for kitchen, bathroom and medical accessories [6].

Applications of polymeric materials

Thermoplastics

Thermoplastic materials can be categorised into commodity plastics and engineering plastics. Commodity plastics are produced in high volume and with low cost for applications such as disposable packaging and films. Contrastingly, engineering plastics are produced in lower volumes and at a higher cost. They exhibit superior mechanical properties and are more durable, looking to replace traditional glass, metals and ceramics.

Applications of commodity plastics are everywhere: a butter container out of polypropylene (PP), yoghurt container of polystyrene (PS), vegetable drawer from polycarbonate (PC), HDPE milk container and PET bottle of soy sauce. Applications of engineering plastics include acetal ball-bearings, polycarbonate shatter-proof windows and lightweight glasses lenses, polyether ether ketone (PEEK) medical implants, polyethylenimine (PEI) electrical circuit boards and polyphenylene sulphide (PPS) electrical insulation.

Fibres

Fibres have a high tensile strength and elastic modulus, with good elongation and thermal stability, making them appropriate for weaving, spinning and ironing. Examples of fibres produced from polymerisation include polyester shirts, Kevlar bullet-proof vests, olefin wallpaper, and polyethylene ropes.

Rubbers

Rubbers are classified as elastomers and properties include large reversible elongation, complete amorphism, low glass transition temperature and light cross-linking. Synthetic rubbers have extensive automotive and medical applications. Polybutadiene is used in car tyres, polychloroprene for wetsuits and automotive seals, polysiloxane for medical sealing and polyurethanes for filling foams, insulation and surface coatings/finishes.

Future Trends in Polymerisation

The current lifecycle of synthetic polymers is extremely unsustainable, causing severe polymer pollution and material value loss. To address these environmental and economic issues, the polymer industry needs to adapt to a more eco-friendly world. Plastic manufacturers are looking towards natural and durable polymer solutions, such as compostable or reusable shopping bags, and there is an increased focus on recycling processes to help clear landfills and clean the oceans.

In terms of more advanced applications, polymers will most likely be the material of this century, addressing issues of energy, health and infrastructure. The polymers of today, and of the near future, potentially offer green, sustainable, energy-efficient, high-quality, low-priced solutions to important worldwide issues we currently face. Major industries where polymers will continue to intervene include the medical, packaging, automotive, aerospace and electronics industries [7]. Notable advancements which will continue to evolve include 3D-printing, printable polymer solar cells, polymers for stem-cell biology applications and self-healing or regenerative polymer systems.

[1] PlasticsEurope, “Plastics - the Facts 2018” [Online], 2018. 

[2] G. Odian, Principles of Polymerisation, John Wiley & Sons, Inc, 2004.