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What is Suspension Polymerisation? A review of the technique for PMMA

Research & Development  |   8 April 2020

Suspension polymerisation is a chain growth free-radical polymerisation technique, used for the synthesis of spherical bead polymers. The process creates high performance polymer beads, used in various specialised applications. At Makevale, we’ve refined the suspension polymerisation process to create quality materials at scale – scaling sophisticated laboratory technology for mass production. This article discusses the suspension polymerisation technique and how manufacturing innovations are enabling the creation of high performance materials at scale.

What is the suspension polymerisation methodology?

The suspension polymerisation method differs from other polymerisation techniques. To create high performance polymers, there are two suspension phases. Stage one is the organic phase, followed by an aqueous phase.

There are at least four active ingredients: the monomer, the initiator, water, and the suspending agent. The product obtained is a solid form – it can be used as it is, dissolved in a solution or it can be suspended within another material, with numerous applications.

The size of the particle produced depends on the speed of agitation, the size and shape of the agitator and the size of the reaction kit. This is where sophisticated machinery and technology fundamentally improves the outcome and quality of materials at high volumes.

The organic phase involves the monomer that contains the initiator – a free radical generator. For some applications, this will contain other reagents, such as chain transfer agents, plasticisers or even rubber modifiers or fillers. These are often introduced to create polymers with a range of characteristics.

Following the organic phase, the aqueous phase uses a solution with granulating agents and water soluble polymers.

The suspension polymerisation reaction

The reaction begins with mechanical stirring of both the monomer phase and aqueous phase. Under pressure, the granulating agent causes monomer droplets to form.

Heating this mixture increases the formation of radicals, which react with the monomer and begin to create the polymer chain. The overall reaction profile can be tightly controlled by temperature ramping and cooling. This, in combination with the mechanical stirring, offers a unique method of tailoring the reaction to achieve the desired qualities of the polymer.

The suspension polymerisation mechanism

Suspension polymerisation takes place over three distinct steps:

1. Initiation

Applying heat causes the initiator within monomer droplets to thermally decompose. This yields reactive free-radicals, instigating the polymerisation process. The highly reactive free-radicals attack the methyl methacrylate vinyl group. This forms a tertiary carbon radical, which instigates the next reaction.

2. Propagation

The tertiary carbon radical reacts with another MMA vinyl group, and the polymer chains grow. Over time, this accelerates, speeding up the propagation throughout the duration of the reaction.

3. Termination

The polymer chain will continue to grow until it is terminated. This occurs by a disproportion or combination mechanism. The final polymer is in a white powder form, ready for subsequent processing.

Control over the suspension polymerisation process

One of the major benefits of suspension polymerisation is the degree of control that chemists can exert over the process. For example, the rate of polymerisation and the amount of completion is directly influenced by the mini-reactions occurring in monomer droplets.

As the initiation and propagation steps occur, a polymer chain forms in each of the monomer droplets and the reaction rate accelerates steadily. As long polymer chains form in the monomer droplets, they can swell. Swelling reduces the termination reaction rate.

When termination reactions become more restricted, the concentration of propagating chains increases, as does the consumption of the monomer. This exponentially increasing reaction rate occurs due to auto-acceleration, known as the Trommsdorff effect. As a consequence of this effect, the reaction temperature rapidly rises, known by the term exotherm. The exotherm signals the end of the reaction and, thus, the polymerisation.

Bespoke bead chemistry: the Makevale method

Suspension polymerisation creates smooth and spherical PMMA polymer beads in powder form. This powder can be easily collected in high yields and put through any number of purification techniques.

Droplet chemistry is incredibly important in determining the consistency and quality of polymer beads. The sphere quality, for example, is defined by a combination of droplet chemistry and processing techniques. At Makevale, we create bespoke bead chemistry specifications, all made possible by the various elements of the process. We are able to tightly control:

  • The reaction temperature profile
  • Phase ratio
  • Speed, size and shape of the agitator
  • Engineering the Gibbs free energy of the bi-modal phase
  • Initiatory chemistry
  • Additive chemistry

Not only does the process offer greater control over the resulting beads, but it’s green – offering a reduction in carbon emissions and limited environmental impact over other similar processes.

The cutting-edge of bead chemistry

Makevale is continuously looking to improve PMMA bead characteristics and reaction yields. Innovation in production processes as well as reagents are yielding new reaction chemistries, enabling new characteristics and making high performance polymers for entirely new applications.

Innovation is at the heart of what Makevale does. Makevale has even successfully incorporated nano-platelet components inside the polymer bead during the suspension polymerisation process. The innovative step allows for a high degree of dispersion, hitherto unobtainable beyond laboratory scale.