Recent developments in sonochemical polymerisation
Introduction
High intensity ultrasound has been used to enhance polymerisation reactions for a number of years [1], [2]. Most published work refers to the radical polymerisation of vinyl monomers where sonication can obviate the need for thermal initiators and allow some control over the molecular weight, tacticity and polydispersity [3], [4]. A number of other polymerisation mechanisms have been investigated [1].
Considering the large number of industrially important polymers and plastics prepared via step-growth reactions (including condensation reactions) there have been relatively few publications dealing with the use of ultrasound in this area. Among these are by Long [5] who described reactors with vibrating walls which was used to control when and where polymerisation took place for several polyurethane systems. There has also been some interest in ring-opening reactions. For example, Stoessel [6] has also reported the use of ultrasound at very high intensities to promote the polymerisation of small cyclic polycarbonate oligomers. Other ring-opening reactions involve the polymerisation of cyclic siloxanes to silicones [7].
Most sonochemical effects can be attributed to cavitation [8], the growth and explosive collapse of microscopic bubbles as the sound wave propagates through the fluid. This can result in extreme conditions of temperature (>2000 K) and pressure (>500 bar) on a microsecond timescale [9] leading to the formation of reactive intermediates such as radicals. The motion of fluid around the bubbles is rapid resulting in very efficient mixing and the formation of liquid jets. The rapid motion can result in effective shear degradation of polymer chains in the vicinity of cavitation bubbles [10] as long as they are over a certain molecular weight. Thus, there are a number of effects which may be exploited.
Recent work in the author’s laboratory has been concerned with step-growth reactions and two classes will be used to illustrate the results. Firstly, the polymerisation of cyclic lactones to give aliphatic polyesters will be described. These materials have a number of applications since they are biodegradable to relatively harmless products and hence have potential as biomaterials. In these reactions, the monomer conversion and the ultimate chain length are limited by a ring-chain equilibrium so it was of interest to determine whether operating under ultrasound could influence the yields and the achievable molecular weights. A major impetus for this study comes from the work of Ragaini [11] who showed that ultrasound enhanced the ring opening of ε-caprolactam to form nylon-6, allowing a single step polymerisation. High molecular weight materials with narrower distributions were formed in shorter reaction times than when using the conventional process. Secondly, a preliminary investigation into the effect of ultrasound on the formation of polyurethanes is described. Polyurethanes are amongst the most widely applied polymers in use [12]. Variation of the diisocyanates and diols used together with the inclusion of various chain extenders allows a huge range of properties to be achieved. Again, the rates, yields and molecular weights were of interest as was the possibility of using ultrasound to control the reaction.
Section snippets
Sonication techniques
The main sources of ultrasound used were a Fisons ‘Soniprep 150’ or a Sonics and Materials VC50 sonic horn system, both operating at 23 kHz and used in the usual configuration whereby the horn was immersed to a depth of ∼1.5 cm in the reaction mixture. Thermostatting around ambient temperature was achieved to ±1 °C by circulating water through a jacket surrounding the reaction vessel although this degree of control could not be achieved during some highly exothermic polymerisations. At higher
Results and discussion
Polyurethanes are formed from the reactions of diisocyanates and di- or poly-functional alcohols. An example involving H12MDI and an aliphatic diol is shown in Scheme 2, these polymers being used in a range of surface coatings. To exemplify the initial part of the work to survey the potential for ultrasound [14] to influence diisocyanate/diol systems used in commercial polyurethane production, Fig. 1 shows the time taken for this system to form solid polymer under a variety of conditions. In
Further discussion
One explanation for the rate acceleration seen in both polymerisation systems would be the heating caused by sonication. However, the bulk temperature of the reaction mixture never rose to temperatures greater than ∼50 °C and the sonochemical rates at low temperatures were faster than the “silent” versions at this temperature.
Among the chemical effects due to cavitation is the formation of radicals due to breakdown of the vapour entering the cavitation bubbles. However, this is unlikely to be
Conclusions
This work has shown the rates of reaction in step growth reactions can be accelerated by the use of high intensity ultrasound. The source of the effect seems to be related to local heating around collapsing cavitation bubbles together with the enhanced mass transfer caused by the fluid motion but it is likely that an effect takes place to modify the mode of action of the catalysts in these systems. This is currently under further investigation to ascertain the precise mode of action.
Acknowledgments
This work was carried out in collaboration with Emma Lenz, a postgraduate student at Bath and was funded by the award of a EPSRC research studentship together with CASE funding from Smith and Nephew Ltd. from whom Dr. Chris Ansell made significant contributions to the work.
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