Improving polystyrene production with continuous flow chemistry

Dramatically improving decades-old chemical processes is the stuff legends are made of. From improving efficiency to reducing cost and waste, chemists are looking for new ways to improve the efficiency of polystyrene production and continuous flow polymerization could well be the answer.

Rather than switching to larger scale bulk process to improve efficiency, moving to tiny tubes and glass microreactor chips may seem counter-intuitive – but the extra control that researchers are achieving with lab scale flow chemistry equipment provides a compelling argument to think differently about scale up.

What is polystyrene?

Polystyrene is a widely used plastic for a range of applications thanks to its inertness and resilience. It plays a particularly large role in the packaging industry; we’ve all had to dig through layers of polystyrene peanuts to retrieve an exciting new gadget from a box!

Polystyrene production and the need to improve existing styrene polymerization methods

The polymerization of styrene is typically performed by mixing the styrene monomer with an inhibitor such as peroxide or benzoyl. The inhibitor decomposes to form radicals; these radicals attack the double carbon bond of styrene and cause polymerization.

The key aims of polystyrene production methods are high molecular weight, narrow molecular weight distribution, good productivity, and high conversion rates of the styrene monomer to polymer. The molecular weight distribution of styrene polymerization is one of the most important factors as this property affects the end product’s impact and tensile strength, brittleness, hardness, and softening temperature. Optimizing the molecular weight distribution – and maintaining it in scale-up – is particularly important to ensure reproducible polystyrene production at large scales.

Polystyrene production methods have remained largely unchanged since they were first used commercially in 1931. Traditionally, batch emulsion, solution, or suspension polymerization methods have been the preferred choice for polystyrene manufacture. These methods offer limited control in molecular weight, molecular weight distribution, and the conversion of monomer to polymer but have the benefit of enabling easy scale-up. Relying on easy scale-up is no longer a viable tactic, however, due to the introduction of newer lower cost competing materials and this has driven new efforts to improve the efficiency of the polystyrene production process.

How is continuous flow polymerization helping to improve the efficiency of polystyrene production?

Using a Syrris Asia Flow Chemistry System, Professor Ardson dos Santos Vianna (Department of Chemical Engineering, São Paulo State University) enabled a new polystyrene production technique that offers good conversion of the monomer into a high molecular weight polymer, a narrow and reproducible molecular weight distribution, and improved productivity compared to traditional batch methods. This technique produces high-quality polystyrene in a more efficient and reproducible way than other techniques.

The process optimization compromise – and how continuous flow helps

Professor Ardson performed several experiments using a 4 mL fluoropolymer tube reactor and a 250 mL glass microreactor to determine the effect of temperature, concentration, residence time, and inhibitor mass on the overall polymerization reaction. The results of the experiments revealed that process optimization of styrene polymerization is a compromise as not all parameters complement each other; improving the molecular weight distribution may result in a reduction in the conversion of styrene monomer to polymer, for example.

• Aggressive reaction parameters, such as high temperatures (115 ^oC), high monomer concentration, and reduced solvent use led to increased conversion rates (up to 66.8%)
• Increasing incidence time of reaction and initial mass of initiator led to increased conversion
• However, the greatest molecular weight was achieved with a conversion rate of just 27.9%, while the largest conversion rate of 66.8% yielded significantly lower molecular weights

By switching to continuous flow methods, Professor Ardson was able to dramatically improve upon decades-old chemistry. The level of reaction parameter control that continuous flow chemistry technology offers enabled fine-tuning of the chemistry well beyond what is possible with traditional batch chemistry reactors. Accurately pushing the limits of each reaction parameter enabled Professor Ardson to determine greatly improved reaction conditions to perform styrene polymerization with a high molecular weight and narrow molecular weight distribution, good productivity, and good conversion of monomer to polymer.

The promising future of continuous flow polymerization

Professor Ardson has demonstrated continuous flow polymerization as a promising alternative to traditional batch chemistry methods.

Continuous flow glass micro/millireactors offer significant advantages over traditional batch reactors for styrene polymerization, including;

• High surface-to-volume ratios, which minimize temperature fluctuations
• Laminar flow, offering reproducible mixing
• Increased safety due to smaller reagent volumes
• Accurate maintenance of pressures

The paper demonstrated that both microreactors and millireactors offer greater productivity (0.1 kg/m³/s) compared to tubular reactors (0.029 kg/m³/s) and batch reactors (0.019 kg/m³/s). Microreactors offer the greatest productivity, but clogging is a concern as the monomer and polymer are insoluble, so millireactors seem like the most promising choice for future studies, with one suggestion being to connect multiple millireactors in parallel to dramatically increase the throughput while maintaining a high conversion rate.

Much work is still needed to discover the ultimate compromise between conversion rates, molecular weights, and molecular weight distribution before the polystyrene manufacturing industry can fully benefit, but it’s clear that improving the polystyrene production process may well be achieved with continuous flow manufacture.

Further reading on styrene polymerization in continuous flow reactors

• Solution styrene polymerization in a millireactor (Professor Ardson and Luna Fullin’s paper)
• Continuous Flow Enzyme-Catalyzed Polymerization in a Microreactor
• Dynamic behavior of a continuous-flow polymerization reactor

Talk to Syrris about your chemistry

You may be surprised at the types of chemistry that can be performed – and improved – using continuous flow technology. Speak to a Syrris flow chemistry expert today to discuss your chemistry.