Microwave heating: still nothing special

For many years there has been debate over whether there is a specific microwave effect on chemical reactions or if it’s just a thermal effect. A couple of years ago I took lecture course on microwave and ultrasound chemistry. The course covered a few papers on the existence of a microwave effect and concluded that there isn’t anything special going on—microwaves just give very efficient and fast heating compared to normal convective heating in an oil bath or dry-syn block.

I found course particularly interesting, so whenever I see a paper on the subject I at least read the abstract to see if anything has changed. Angewandte Chemie have recently published a paper titled Microwave Effects in Organic Synthesis—Myth or Reality? (DOI: 10.1002/anie.201204103) by C. Oliver Kappe, Bartholomäus Pieber, and Doris Dallinger.

They looked at two recently published papers that allegedly found a specific microwave effect. Both claimed microwave irradiation significantly enhanced the reaction rate or yield in a way that couldn’t be replicated by regular heating to the same temperature.

Summarising a few pages: Kappe et al. couldn’t replicate the findings and argue that the problem lies in poor temperature management. To test the existence of a specific (non-thermal) microwave effect you need to run the same reaction twice at the same temperature, one with microwaves and the other normally (e.g. with an oil bath).

However the researchers who report a microwave effect use external infrared temperature probes, which record a lower temperature than the bulk reaction mixture. Microwaves heat more efficiently than the normal heating, so the microwave reaction will give you a higher yield and both vessels are in fact not at the same temperature. Instead you must use fibre optic temperature probes placed inside the reaction vessels. Doing this eliminates any microwave specific effect. To quote:

Importantly, we firmly believe that the existence of genuine nonthermal microwave effects is a myth, as all our attempts to verify these often claimed “magical” microwave effects during the past decade have failed.

It’s a good read and, I think, a nice example of science at its best. I’m also glad I read it because a colleague and I had, for some reason, been looking at getting a microwave flow reactor—which would be completely pointless, as all the benefits of microwaves in batch chemistry (high pressures and homogeneous heating) can be readily achieved in flow using normal convective heating. If anyone could tell me why such an apparently pointless bit of kit exists, I’d like to know…

Reference: C.O. Kappe, B. Pieber and D. Dallinger, Microwave Effects in Organic Synthesis—Myth or Reality?, Angewandte Chemie International Edition, 2012. DOI:

Making semiconducting polymers in flow

Organic electronics has a problem with batch-to-batch variability in the quality of materials, particularly the active semiconducting layer. A fellow PhD student in my office described to me the trouble he often experiences. He made one batch of solar cells last week and measured an average efficiency of 5 %. This week, another batch had only 0.5 % efficiency, depsite having the same device structure and processing conditions. Further investigation revealed that the lab had recently switched to a new batch of polymer, even though they’re supposedly identical.

(This raises the question of how many positive literature reports describing organic electronic devices are “correct” and not anomalous? But here I want to focus on this problem in the context of large scale device production.)

Batch-to-batch variation is bad enough with the sub-gram quantities used for small area device fabrication. Printing optimised large area devices—measured in square metres/minute rather than square centimetres/day—would be next to impossible. You need to know that this week’s batch of polymer is the same as last week’s because different molecular weight size distributions, chemical defects and impurity levels have a big effect on the required processing conditions and final device performance.

It’s quite difficult to produce organic semiconductors on the large scale required for device printing because the polymerisation reactions scale up poorly from round bottom flasks to large batch reactors. You have to re-optimise at each scale, which costs time and money.

James Bannock, who is a PhD student in the same group as me at Imperial, has been working on the use of droplet flow reactors to make poly(3-hexylthiophene) and get around the problem of batch scale up. He’s recently published his work in Advanced Functional Materials (DOI: 10.1002/adfm.201203014) and it’s open access so you can read it yourself for free.

I encourage you to take advantage of the paper being open access and have a look. There’s lots of photographs and figures. Briefly, activated monomer solution is injected into a narrow plastic tube containing a flowing stream of immiscible carrier fluid dispersed with the catalyst. Microlitre-sized droplets form, like tiny individual reaction flasks, and travel through the tubing. Because the droplets are small the chemical environment is highly homogeneous and each droplet is essentially identical. The tubing is heated in an oil bath and the polymerisation reaction (a Grignard metathesis) happens as the droplets travel through the tubing.

You can produce more material by using longer tubing and a higher flow rate. Crucially, this can be done independently of droplet size. So unlike batch reactors, nothing happens to the chemical environment and you get exactly the same polymer, with a low polydispersity index and high regioregularity. The plan is to apply this method to other organic semiconducting polymers. It provides a way to produce device-grade polymer on large scale required for large area fabrication, with minimal batch-to-batch variation, and will hopefully drive the industrial production of organic electronic devices.

Reference: J. H. Bannock, S. H. Krishnadasan, A. M. Nightingale, C. P. Yau, K. Khaw, D. Burkitt, J. J. M. Halls, M. Heeney & J. C. de Mello, Continuous Synthesis of Device-Grade Semiconducting Polymers in Droplet-Based Microreactors, Advanced Functional Materials, 2012. DOI:

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