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Ultrasonically enhanced polymer synthesis
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ELSEVIER
Ultrasonics Sonochemistry 3 (1996) $229 $238
Ultrasonically enhanced polymer synthesis G a r e t h J. Price * School of Chemistry, University of Bath, Bath, BA2 7AY, UK Received 20 December 1995; revised 22 April 1996
Abstract
Some examples of polymerization reactions with a variety of mechanisms which have been carried out under sonication are described. It is shown that they can be classified in general terms using the same reaction types proposed by Luche for low molar mass systems. The use of ultrasound allows a large degree of control over the polymer structure and hence the resulting properties, particularly in controlling the molecular weight distributions.
Keywords: Polymerization; Cavitation; Degradation; Molecular weight
1. Introduction
The first reports of the benefits achievable when applying ultrasound to chemical reactions date from the 1950's [1,2]. However, the major efforts in this area in the late 1970's and early 1980's by Henglein, Luche, Boudjouk, Suslick and others have made ultrasound a routine tool for the synthetic chemist. Among the beneficial effects are greater yields, higher product purity and rate accelerations. In some reactions, changes of stereo- and regio-selectivity have been noted as has a preferential acceleration of one mechanism where two or more possibilities exist [3-6]. A major drawback in sonochemistry until recently has been the lack of mechanistic information leading to a predictive model of which reactions are most likely to benefit from sonication. Recently, there has been a great deal of study into resolving the precise effects of cavitation and causes of chemical reactivity in terms of a 'hotspot' theory [7], the 'electrical' theory [8] or a model based on a plasma discharge [9]. Whatever the precise origin of chemical effects, there is little doubt that sonochemistry is a direct result of cavitation. The harsh conditions generated on bubble collapse lead to the production of excited states, to bond breakage and the formation of free radicals, to mechanical shocks, high shear gradients and very rapid and efficient mixing in multiphase systems. Thus, three areas of a cavitating system can be identified, as shown schematically in Fig. 1. * Fax: +44 1225 826504; e-mail: [email protected].
Large gradients of T, P and Shear
\ \
Rapid motion and taxing [
Fig. 1. Schematic representation of a cavitation event.
The centre of the bubble is where the primary chemistry involved in, for example, radical formation takes place. In contrast, the bulk liquid is relatively unaffected apart from a small conductive heating effect although species generated inside the bubble may diffuse out and react with reagents in the bulk solution. In between, the interfacial region around the bubble has very large gradients of temperature, pressure and electric field as well as rapid motion of molecules leading to very efficient mixing e.g. emulsification. Of particular note for synthetic applications is the work of Jean-Louis Luche who made the first real attempt at categorising reactions of use in chemical synthesis [5,10]. Briefly, on the basis of a large ammm, of published work, he proposed that sonochemical, v,ctions could be classified into three types: Type 1: In homogeneous solution, single electron transfer processes will occur to give raa'ral intermedi-
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ates. Ligand cleavage reactions will give coordinatively unsaturated species. Purely ionic processes should not be sensitive to sonication. Type 2: In heterogeneous (solid/liquid or liquid/ liquid) systems, ionic reactions can be stimulated by mechanical effects although the products will be the same as in the absence of ultrasound. Type 3. In heterogeneous systems which can follow an ionic or radical mechanism, sonication will favour the latter. A number of systems are known which illustrate each of these types. Although mainly empirically based, these have become known as 'Luche's rules'. Given the economic and commercial importance of polymers, it is perhaps surprising that relatively little attention has been given to applications of ultrasound in this area. In fact, polymer sonochemistry predates work in synthesis and originated in the 1930's when the change of viscosity on sonication of starch and gelatin were noted [11,12]. These effects, due to chain breakage as a result of cavitation, were extensively studied in the 1950's [13,14] and again in the 1980's when improved methods of polymer characterisation were developed [15,16]. Applications in polymer synthesis have paralleled those in low molar mass systems although there have been many fewer studies [17]. However, considerable progress has been made over recent years and this paper will describe some recent work from our laboratories and, where appropriate, from other workers. Another aim is to show that the classification outlined by Luche is equally applicable to polymer systems although there are some aspects of sonochemistry which are unique to polymer systems. A number of the effects described above are potentially useful in polymer synthesis. The enhanced production of radicals could be used as a method of initiation in pure vinyl monomers. Several polymers, for example poly(vinyl chloride), PVC, are produced in emulsion or suspension systems and the efficient mixing allowed by ultrasound could be of benefit. Heterogeneous reactions are particularly susceptible to sonication, the most obvious example in polymer chemistry being the Ziegler-Natta catalysts by which polyolefins are produced. Economic considerations are presently dictating that novel materials do not arise from new polymers but from chemical modification of existing polymers so that sonochemical methods may prove to be of increasing importance. Among previous work in polymer chemistry has been an extensive study of the chain cleavage caused by sonication [15, 16,18]. Radical initiation in vinyl monomers such as methyl methacrylate has also been studied [19,20] as have emulsion or suspension polymerizations. A small amount of work has also been published on other reactions such as ring-opening [21 ], organometallic [22] and electrochemical [23] polymerizations.
However, given the large commercial and economic interest in polymers, it is perhaps surprising that there has not been greater effort. As in the synthesis of low molar mass compounds, we are interested in the rate and mechanism of the reaction and the yield and stereochemistry of the product. However, there is an extra factor to be considered in polymer systems. Polymerization reactions occur with statistical probabilities so that a sample will contain chains with varying lengths and molecular weights. The polymer properties depend critically on both the average molecular weight and the distribution of chain lengths, or polydispersity. A further aim of this paper is to demonstrate that ultrasound has an important role to play in the control of these parameters during polymerization reactions.
2. Experimental methods In general, sonochemical polymerizations use the same types of apparatus used for other synthetic work [24]. Thus, some of the work described here has utilised a cleaning bath (a Kerry Ultrasonics Pulsatron 325 operating at 35 kHz) while most of the work has involved horn systems since these allow more ready variation of the ultrasound conditions, optimisation of which is one of our major aims. To allow this, we have used specially designed glassware, shown schematically in Fig. 2, which
rm [ ]
[ )ansOuo.
[]
Ultrasound genemtor /
N 2 inlet
Sampling syringe
Thermometer
Water out > Water jacket
Solution
I I Water in
Fig. 2. Schematic representation of experimental apparatus for sonochemical polymerization.
G.J. Price/UltrasonicsSonochemistry 3 (1996) $229 $238
allows good temperature control by the circulation of thermostatted water and has the facility to work under controlled atmosphere and to remove samples periodically without disrupting the reaction. The horn used was a 23 kHz 'Sonics and Materials VC600' equipped with a 0.5 in diameter horn. Polymer molecular weights were measured by Gel Permeation Chromatography, GPC, by reference to polystyrene standards using equipment and methods described in detail elsewhere [18,20].
3. Results and discussion
3.1. Luche type 1 systems These are processes which occur in homogeneous solution. It is suggested that "Purely ionic species should not be sensitive to sonication". A number of polymers can be produced by ionic mechanisms but as far as the author is aware, sonochemical studies of these systems are very rare and there are no examples of the study of cationic polymerization of vinyl monomers. Very recently, Schultz et al. investigated the effect of ultrasound on the anionic polymerization of styrene initiated by t-butyl lithium [25], R - acting as the initiating species. These workers found that the preparation of RLi from RBr and lithium metal was more efficient (as expected from the work of Luche and Boudjouk) but there was little or no effect on the polymerization. Radical polymerization: A second aspect of Type 1 systems is the formation of radical intermediates. It is well known that sonication of vinyl monomers such as styrene and methyl methacrylate ( M M A ) leads to radical initiation of polymerization [19,20]. This is the most common method for vinyl polymerization although a major problem caused by the high and indiscriminate reactivity of the radical is control over the molecular weight and particularly the stereochemistry of the resulting polymer. Radical polymerization is perhaps the most studied of sonochemical polymerizations and considerable progress has been made in some systems toward a complete understanding and predictive model. The primary sonochemical step is the formation of radicals by breakdown of solvent in the cavitation bubbles. These radicals then initiate the polymerization. In order to estimate the rate of initiation in MMA, we have used a radical trapping method. A solvent, methyl isobutyrate, was selected to closely mimic the physical and chemical properties of M M A so that it would react similarly in an ultrasound field and produce similar numbers of radicals on sonication. A concentration of 2,2'-diphenyl-l-picryl hydrazyl, DPPH, in excess of the radicals produced was used and its consumption monitored spectrophotometrically. It has been suggested that there are inaccuracies in this method, for example, there will be a degree of radical recombination
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and other side reactions. However, since the radical must escape the vicinity of the cavitation bubble whether to initiate polymerization or to be trapped by DPPH, we feel that the rate of trapping will closely mimic the rate of initiation. By this method we were able [20] to measure the rate constants for radical production in purified M M A using the apparatus in Fig. 2 under a range of conditions and the results are shown in Table 1. Our work showed that the results agreed well with a first order reaction although the fit was somewhat better to a zero order process, as might be expected if the sonochemical step was the rate limiting process. To compare with thermal measurements, the rate constants considered here are those calculated for a first order reaction. To further compare with more conventional experiments, solutions of 0.1% azobisisobutyronitrile, a commonly used thermal initiator, were sonicated, the results also being shown in Table 1. The rate constant for purely thermal production of radicals, estimated by extrapolation from higher temperatures is negligible compared to the sonochemical reaction. The results show that simply by using ultrasound alone, similar rates of initiation as are usually found in thermally initiated polymerizations can be achieved at 25°C. The value for AIBN is some three orders of magnitude higher than the thermal process at this temperature so that the sonication process is clearly able to accelerate the decomposition of AIBN in solution as well as producing radicals directly by decomposition of the solvent. The effect of changing the temperature is shown in Table 1 and indicates that the rate of radical production is faster at lower temperatures. The other parameter of importance is the ultrasound intensity. It was found that below an intensity of approximately 12-13 W cm-2 there was no production of radicals, presumably corresponding to the cavitation threshold in this system. Above this minimum value, there was a linear dependence of the rate of D P P H consumption on the intensity. A conversion of 2-3% h -1 was achieved at 25°C. However, above a particular conversion, cavitation in the solution essentially stopped and no further conversion to polymer occurred as demonstrated in Fig. 3. This is probably due to the increased viscosity of the Table 1 R a t e c o n s t a n t s f o r D P P H r a d i c a l t r a p p i n g in m e t h y l b u t y r a t e System
Temperature
R a t e c o n s t a n t (s 1)
(oc) Ultrasound MeOBu MeOBu MeOBu MeOBu+0.1% MeOBu+0.1%
AIBN AIBN
- 10 25 60 25 70
6.35 x 10 5 2.21x10 s 1.03 x 10 - s 9.13 x 10 s 3 . 1 0 x 10 5
Conventional
lxl0
15
2 x 10 8
G.J. Price/Ultrasonics Sonochemistry 3 (1996) $229-$238
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o~ 2o
%.
0
100
200
300
400
500
Sonication time / rain
Fig. 3. Sonochemical polymerization of methyl methacrylate at 25°C. Solid elipses: bulk M M A ; open triangles: 50% solution of M M A ; solid rectangles: M M A + 0.1% AIBN.
solution restricting movement of the solvent molecules and suppressing cavitation, hence preventing formation of radicals. The solution polymerization gives higher conversion due to the lower viscosity of the reaction mixture. One of the most important properties of a polymer is the molecular weight and Fig. 4 shows that high molecular weight polymer is formed at very early stages of the reaction but that the value falls at longer times. This is not the same plot as found with conventionally initiated radical polymerization due to other processes occurring simultaneously as discussed below. One of the goals of sonochemical polymerization is to use variation of the reaction conditions to control the properties of the resulting polymers. This is illustrated in Table 2. Changing the ultrasound intensity at constant temperature not only varies the amount of 4 < O
-I Ii' I! 0.~
'
'
,
1O0
200
300
polymer produced but also allows systematic variation of the final molecular weight. The final factor which has been investigated for these systems is the stereochemical arrangement of the monomer units along the chain. In polymer chemistry, this is described by the tacticity of the polymer. Chains which have all of the substituents arranged on the same side are referred to as isotactic while those which have an alternating arrangement are syndiotactic. Polymers with a random arrangement are termed atactic or heterotactic. The tacticity in PMMA is readily determined using NMR spectroscopy and Table 3 shows the relative degrees of tacticity in sonochemically produced PMMA together with Literature results for more conventional polymerizations. It is clear that conventional initiation using a peroxide initiator leads to predominantly syndiotactic polymers. At high temperatures sonication and thermal initiation both produce polymers with similar stereochemistry suggesting that sonication has little or effect on the propagation reaction. Polymerizations using an anionic mechanism using, for example n-butyl lithium as the initiator give 60-70% isotactic polymer because the rate of propagation is slow compared with the initiation rate. In sonochemically promoted reactions at low temperatures, the initiation is speeded up while the propagation rate is slowed compared with the thermal polymerization and hence the proportion of syndiotacticity along the chain is raised. Steric hindrance between the bulky ester groups makes syndiotactic addition thermodynamically more favourable, although the energy difference is small. As the temperature is lowered, the propagation rate is slowed and there is more chance of the favoured addition taking place. Hence, variation of the experimental conditions such as temperature and the ultrasound conditions such as intensity allows a great deal of control over the structure (and hence properties) of the resulting polymer. While it is not envisaged that sonochemical methodology will ever be adopted for large-scale production of vinyl polymers, there are some situations where low temperature, site specific initiation could be useful. The
400
Sonieation time / min Fig. 4. Variation in molecular weight during sonochemical polymerization at 25°C. Solid triangles: methyl methacrylate; open elipses: n-butyl methacrylate; solid rectangles: styrene. Table 2 Properties of P M M A produced by 6 h sonication of pure m o n o m e r Temperature (°C)
Intensity ( W cm -z)
Molecular weight
Polydispersity
Conversion (%)
25 25 25 25
13 29 35 58
144000 131000 97000 35000
2.3 1.9 1.8 1.5
2 10 14 12
Table 3 Stereochemical tacticity ratios for radically initiated P M M A . i = isotactic; a = atactic; s = syndiotactic Polymerization conditions
Monomer" 100°C Solution" 50°C Ultrasound 60°C Ultrasound 40°C Ultrasound 25°C Ultrasound 0°C Ultrasound - 10°C ~Using benzoyl peroxide as initiator.
Ratio i
a
s
8.9 6.3 4.3 4.0 2.8 1.7 0.8
37.5 37.6 41.0 40.4 34.3 33.8 25.6
53.9 56.0 54.7 56.4 62.9 64.6 73.6
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ability to work at low temperatures with monomers which have low ceiling temperatures and the ability to control molecular weights without the need to add additional agents as well as the possibility of switching off the initiation during the reaction to control heat build-up are all advantages which may be exploited in particular cases. Sonochemical polymer degradation: Radical polymerization clearly corresponds to a Luche type 1 system. Another homogeneous process which occurs with polymers lies outside this classification as it is unique to macromolecular systems. If chains are sufficiently long, they are stretched out in the solvent flows around collapsing cavitation bubbles and the resulting shock waves. The shear gradients generated in this way are sufficient to cleave the polymer backbone wheras low molar mass compounds are too small for a sufficient force to operate. The basic effects [26] of sonicating a polymer solution are shown in Fig. 5 using 1% solutions in toluene of polystyrenes with varying molecular weights at 25°C as an example. The degradation proceeds more rapidly for higher molecular weight materials and approaches a limiting value, Mlim, below which no further degradation takes place, in this case ca. 30000. Polymer chains with this or lower values are too small to be affected under these conditions. Since the rate of degradation is molecular weight dependent, longer chains are removed from the sample and the polydispersity of the polymer is changed. These effects appear to be universal in that they have been seen for a wide range of organic polymers in organic solvents, for inorganic polymers and for aqueous systems including polypeptides, proteins and DNA. In order to develop predictive models for the degradation, behaviour under a wide range of conditions has been measured. This has been documented in detail elsewhere [26,15,16] and it is sufficient here to summarise the main trends in the results. In summary, the degradation proceeds faster and to lower molecular weights at lower temperatures, in more dilute solutions
and in solvents with low volatility, in accord with the expected effect of these parameters on cavitation in the solvent [27]. Other factors which have been quantified are the ultrasound intensity and the nature of dissolved gases. Again, by suitable manipulation of the experimental conditions, we can exert a great deal of control over the process, exploitation of which allows the modification of existing polymers into new materials. At its most straightforward level, the degradation can be used as an additional processing parameter to control the molecular weight distribution. For example, Fig. 6 shows GPC chromatograms of a polyalkane in solution undergoing sonication [28]. The degradation of the higher molecular weight species narrows the distribution markedly with consequent modification of the physical properties of the polymer. A potential application would be to have a sonochemical step as the final processing stage during manufacture so as to 'fine tune' the polydispersity and molecular weight distribution to give the desired material properties of the polymer. There are examples where this has been applied to commercial processes on at least pilot-plant scale. A second application of the degradation utilises the macromolecular radicals produced on chain breakage as initiating species in the preparation of copolymers and end-capped materials. A large number of workers have sonicated mixtures of two polymers dissolved in a common solvent, cross reaction between the two types of radicals forming a block copolymer. The drawback here arises from the difficulty in recovering the products by selective precipitation and also in controlling their structure. The block copolymers formed can act [29] as 'in situ' generated compatibilisers in otherwise incompatible mixtures by acting as a 'detergent' helping to dissolve the polymers in each other. Our approach to
SONICATION TIME / hr
t%12
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\
200
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0
,
r
100
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i
200
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i
300
,
i
400
UJ
;~"
U.I
':~.
!l /
3
,
4
5
o
6
7
500
Sonieation time / min
Fig. 5. Ultrasonic degradation of 1% solutions of polystyrene of differing molecular weights in toluene at 25°C.
LOG10 ( MOLECULAR WEIGHT ) Fig. 6. Gel Permeation Chromatograms of a polyalkane in toluene at varying times during sonochemical degradation.
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preparing copolymers has been to sonicate a polymer dissolved in a solution containing the second monomer such as polystyrene and methyl methacrylate as shown in Fig. 7. Using the results of degradation studies, the structure and block length of the first polymer can be controlled quite precisely. By changing the concentration of monomer in solution the block size of the second polymer can also be varied, allowing a large degree of control over the resulting material structure. A related approach is to sonicate the polymer in the presence of a species labile to radical attack where 'end capped' polymers are formed. We have used this to prepare, for example, polystyrenes and poly(alkanes) bearing fluorescent groups [28]. The degradation process is unique to large-molecule systems and offers the oportunity of controlling polymer structure. It will occur whenever polymers in solution are sonicated and must always be considered when polymer syntheses are being undertaken. 3.2. Luche type 3 systems
These are systems in which alternative mechanisms operate in parallel and where sonication can preferentially accelerate one of them, leading to different products from the conventional reaction. As far as the author is aware, no such examples of 'sonochemical switching' have been found in polymer systems.
studied in our laboratory will be described to illustrate the effects. Synthesis of siloxanes: Poly(dimethyl siloxane), PDMS, is the base polymer from which silicone resins and rubbers are prepared. Its usual method of synthesis is a cationic ring-opening reaction of the cyclic tetramer, catalysed by a small amount of acid (see Scheme 1 ). In a typical procedure [33], 20 cm 3 of octamethyl cyclotetrasiloxane, OMCTS, 10 cm 3 diethyl ether and ca. 0.5 cm 3 sulphuric acid are thoroughly mixed, after which the acid is washed away and the polymer recovered by extraction with ether. The aqueous acid is insoluble in OMCTS so that the reaction is a two-phase system with polymerization catalysed by a phase transfer process. It is thus a heterogeneous liquid/liquid system and should act according to Luche Type 2 rules. Fig. 8 compares the results of this polymerization under silent and sonochemical conditions, the source of ultrasound in this case being a cleaning bath [34]. It is clear that ultrasound in this case accelerates the polymerization by a factor of at least two, although there is little diference in the final yield obtained. More surprisingly, the molecular weight is also higher in the sonochemical reactions whereas the degradation process described above might be expected to give lower values. After 8 h, the number average molecular weights and polydispersities obtained were 49500, 1.6 and 17300, 2.0 for the sonicated and silent reactions respectively.
Me ^ Me Me-S i / u ~ ~SilMe
3.3. Luche type 2 systems
These systems involve heterogeneous, multi-phase mixtures. A number of polymer systems satisfy these criteria. For example, polymerization in emulsion systems has been shown to benefit from sonication by Hatate et al. [30], Stoffer and co-workers [31] and recently by Greiser et al. [32]. Effects include more rapid formation of emulsions and initiation, control over particle size and a higher polymer molecular weight. Two examples selected from polymerization chemistry
))))))= ~
"
/ 0 \
\ /0
Me ~ 1 ~ 0 / ] ' -
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.,ooc
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e
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Scheme 1.
80 60 g "~40
1_I
E
8 2o
0 J¢¢0 ~¢~.O40A CH3 CH3 rn//
~ULTRASOUND
~o 20 ~. 2
Fig. 7. Schematicrepresentation of sonochemicalproduction of endcapped polymersand block copolymers.
4
6
Sonlcatlon Time / hr
Fig. 8. Sonochemicalpreparation of poly(dimethylsiloxane)at 25°C.
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The higher molecular weights and lower polydispersities suggest strongly that the 'initiation' of ring-opening is much more efficient. This is due to the ultrasound providing more effective mixing of the two phase system and better phase transfer of the acid catalyst. Synthesis of poly(organosilanes): A second example of a Type 2 system can also be found from the area of silicon based polymers. Poly(organosilanes) which have a backbone consisting exclusively of silicon atoms have recently been receiving attention as electro- and photoactive materials. [35] These polymers are usually produced by Wurtz coupling of dichlorodiorganosilanes with molten sodium. However, this leads to irreproducible results, low yields and hi- or tri- modal polymer distributions, largely due to the heterogeneity of the system. Boudjouk [6] showed some time ago that this type of coupling reaction was particularly susceptible to sonication. (See Scheme 2.) While the detailed mechanism remains a topic of debate, it is generally agreed [35-37] that chain growth proceeds via silyl anions generated at the surface of the sodium involving two SET steps. A number of systems have been studied [38, 39] and, in each case better yields and narrower distributions have been achieved under ultrasound. Most work has been done on the synthesis of poly(methyl phenyl silane) as this yields soluble, easily characterisable materials. To illustrate the results, Fig. 9 shows the molecular weight distributions of three of these polymers produced under reflux at 110°C, using CI
CH3
\s/"
Reflux, 110"C / sonicate, 25"0
60°C
a cleaning bath at and a horn system at 25-35°C. The first of these has a very broad, two component polymer distribution while the polymer from the horn experiment has a single component, that from the cleaning bath being intermediate between these extremes. To further characterise the effects, samples were removed periodically during a polymerization using the horn and the molecular weight distributions are shown in Fig. 10 and clearly show the relative degradation of the highest molecular weight component as the reaction proceeds, although it does move to higher values implying that there is an effect on the chain extension reactions. Finally, to demonstrate that ultrasound can be used to tailor the properties of these materials, polymerizations were carried out over a range of sound intensities, the results being shown in Fig. 11. By changing the intensity, the resulting distribution can be varied over a wide range. These and other results have led us [39] to suggest
:~20: i0
~ •
S ' ~ Si/ S ' ~ s r " ~ d "& t ,.%./n Ph CH3 Ph CH3
I
I
I
6
5
10
MoleculaWei r ght
f~
,.,,--...
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i
10
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10
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i
il Ii
I
3
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Fig. 10. Molecular weight distributions of poly(methyl phenyl silane) at varying times during sonochemical polymerization at 25°C.
-~"t
Reflux// "
I 4
10
Scheme 2.
///
."'/ "~
Ph CH 3 Ph CH3 .- Ik [,,~," ,~
Na, Toluene ~
t \0, Ph
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3
Fig. 9. Molecularweight distributions of poly(methylphenyl silane) produced under varyingconditions.
106
I
10s
I
104 MolecularWeight
i
--
103
Fig. 11. The effect of ultrasound intensity on the molecular weight distributions of sonochemically produced poly(methyl phenyl silane). [Intensities in W cm 2.]
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G.J. Price~Ultrasonics Sonochemistry 3 (1996) $229 $238
that ultrasound has four effects in this polymerization. Firstly, microjetting continually regenerates a fresh sodium surface so that the reaction proceeds at a reasonable rate at lower temperatures, minimising unwanted side reactions. This also results in enhanced mass transfer of reactants to the surface, increasing the rate of the initial anion formation and giving a more homogeneous chain growth and hence chain length distribution. Finally, the sonochemical degradation of polymer chains in solution leads to lower polydispersities. The effect of ultrasound on reactivity on and at surfaces of metals by means of microjets is well known [40,41]. Thus, again these systems conform in general terms to Luche's classification. The reactions are believed to involve ionic intermediates and are accelerated and enhanced by sonication due to 'mechanical' effects such as mass transfer while not affecting the nature of the products. However, sonication allows a large degree of control over the polymer structure in terms of their molecular weight distributions.
Organometallic coupling reactions of haloaromatics: Some of the most successful reactions in sonochemistry have involved the coupling of organic halides using alkali metals such as lithium and sodium [3 6]. One example applied to polymer synthesis is the Wurtz coupling to form poly(organosilanes) discussed above. Another range of targets has been polyphenylene materials consisting of linked, conjugated chains of aromatic rings. These have received considerable attention as prototype conducting materials but, until recently, difficulties in synthesis has prevented their widespread adoption. In addition, even at short chain lengths they become insoluble and cannot be melted so that their processing presents difficulties. Indeed, even now synthetic methods involve complex schemes involving precursor materials which are used to fabricate films and are then converted to the desired polymers. The earliest method for synthesising polyphenylenes involved the oxidative coupling of benzene using a Lewis Acid catalyst, shown in Scheme 3. We carried out this reaction using benzene as the solvent and sonication yielded double the amount of polymer over using more conventional methods. Analytical techniques (IR, elemental analysis etc.) showed no significant differences between the polymers. Thus, this reaction also seems to below to a Luche Type 2 system. Other methods for preparing polyphenylenes are based on coupling reactions of dihaloaromatics but they often give low yields and sometimes react only as far as 40 "C, N 2
Br~--
Br
Mg, Ni(bipy)CI2~ N 2, THF
Scheme4. Table 4 Effect of ultrasound on Grignard coupling of dibromobenzene
Conditions
Ultrasound
Polymer yield (%)
Reflux, 2 h Reflux, 5 h 20°C, 24 h 60°C, 2 h 20°C, 2 h 60°C, 2 h
No No No Bath Horn Horn
20 35 40 48 33 45
I•CI
+ 2 C6H13MgBr
C
Et20 Ni(dpPp)CI2 C 6 H 1 3 . ~
C6H13
I
Br2, hv, 0 °C
,,=
AICI 3 / CuCI 2 H20
Scheme 3.
the dimer or trimer. For example, attempts to couple dibromobenzene directly using lithium, sodium or potassium were all unsuccessful in generating polymers. Coupling took place but elemental analysis suggested that only trimers, tetramers and possibly pentamers were produced. Furthermore IR spectroscopy of the solid products suggested that they were not all para-substituted rings. These effects are presumably related to the insolubility of the target compounds which, as well as hindering their synthesis, makes them very difficult to characterise since they are also infusible and intractable. One of the more successful schemes in this area is due to Rehann et al. [42] using a nickel catalysed Grignard type reaction shown in Scheme 4. Ultrasound was applied to this scheme under various conditions and the results are given in Table 4. In each case, sonication gives better yields than the equivalent 'silent' conditions as well as allowing the reaction to proceed at lower temperatures. Considerably better yields were obtained in 2 h at room temperature using a sonic horn than from a 5 h reflux. However, full characterization of these materials is not possible for the reasons outlined above. One method for making these materials soluble, and hence characterisable, was suggested by Feast et al. [43] who prepared polymers substituted with hexyl- or octylgroups in order to confer solubility in common solvents. We have also attempted to use ultrasound in the preparation of these materials as shown in Scheme 5 but, while
11 C6H13
Mg, THF Ni(bipy)Br2 Scheme 5.
Br-~Br
C6H13-
C6H13
G.J. Price~UltrasonicsSonoehemistry 3 (1996) $229-$238
it was useful for the preparation of the monomers, also involving Grignard chemistry, we could not achieve polymerization. The starting material here, 2,5-di n-hexyl 1,4-dibromobenzene was prepared using a literature method. Dichlorobenzene was reacted with hexyl magnesium bromide and ultrasound gave a yield after 1 h at room temperature of 41% compared with 42% from a 12 h reflux. After conventional bromination, the polymerization was attempted. This gave a yield of 48% for a 48 h reflux in THF (comparable with literature) but a yield of only about 10% after 12 h sonication on the probe system, showing no significant improvement. We also attempted the reaction by coupling over lithium and several other reactive metals but no significant reaction took place in a Wurtz type reaction. The lack of reaction in the Wurtz systems prompted further study of the mechanism and we have rationalised our results in terms of a radical type reaction. We studied the Wurtz type coupling of bromobenzene and 2-, 3-, and 4-bromotoluene under varying conditions and have established that it proceeds via a SET mechanism with a radical generated on the ring [44]. This explains why the coupling is not all para and also the reduced reaction in the dialkyl benzenes during polymerization. Thus, although not a success from a polymer synthesis point of view, the work has been useful in extending our knowledge of sonochemical reaction mechanisms. An alternative approach to 'coupling polymerization' has been to employ Ullmann reactions utilising copper in DMF as the coupling agent. Lindley et al. [45] suggested that Ullman coupling of 2-iodonitrobenzene and 2,5-di bromo nitrobenzene proceeded up to sixty times faster and to give up to 95% yields under the influence of ultrasound. Although we have been unable to duplicate these results in polymerization systems, we have been able to synthesise new materials. For example, one reaction that we utilised is shown in Scheme 6. Under the standard Ullman conditions of 20 h reflux at 140°C in DMF, the reaction gave 95% dehalogenation and negligible polymerization. Conversely, reaction for 4 h on the ultrasonic probe yielded ca. 10-15% of a deep-brown material which although not completely characterised, contains at least four or five rings and we expect to have the structure shown. Similar results were obtained employing nitroiodobiphenyls as 'monomers'. Although the yields are as yet poor, if it could be increased, this would be a very interesting material since
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the alkyl groups would confer solubility, the backbone would be potentially conductive while the nitro groups would be very useful to perform further chemical modification to change the properties. Hence, in this new area of study, the use of ultrasound has allowed us to prepare new materials which cannot be prepared under conventional conditions. The full potential has yet to be fulfilled and this is an area which will continue to develop.
4. Conclusions This brief survey of polymer sonochemistry has illustrated that there is a wide range of systems which can benefit from sonication. Moreover, the types of reaction involved fit into the classifications involved in the sonochemistry of low molar mass compounds as suggested by Luche. While this classification is not perfect, it does provide a framework for the correlation and interpretation of a range of reactions. As with most classification systems, it can be used in too rigorous a manner but if used appropriately may be useful. Despite potential uses of sonochemistry in synthesis, to date there has been relatively little work related to sonochemical polymerization. The benefits offered in a range of reaction types certainly warrant further studies in the area in terms of acceleration of rate and increased yields but particularly in the control of molecular weight during polymerization. When any polymerization process in solution is subject to sonication, the degradation will always occur concurrently with chain growth. Perhaps the most obvious application would be in an 'add-on' to existing processes where the ultrasonic degradation can be utilised to control the final molecular weight of polymers during manufacture. However, it seems unlikely that a sonochemical process will replace current methodology unless it allows significant improvement of material properties or saving in production or energy costs. There are areas such as biomedical materials and food uses where the added value of the products would support the extra costs presently associated with sonochemical technology. The outlook is encouraging and it appears that sonochemical polymerization reactions are well placed to become commercially viable in the near future.
Acknowledgement 13H6C B r @
O2N'
NO2 Br
13H6C
NO2
Cu, DMF O2N
C6H13 Scheme6.
C6H13
It is a great pleasure to acknowledge the excellent group of students who have conducted research in sonochemistry with me over the past seven years or so. Dr Ali Patel, Paul Smith, Peter West, Andrew Clifton and Emma Lenz (all postgraduate students who now
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G.J. Price/Ultrasonics Sonochemistry 3 (1996) $229-$238
have their PhD's) and Diane Norris, Melanie Daw, Emma Wallace and Matthew Hearn (undergraduate students) have all contributed to the work described here. In addition, we grateful for funding of our work from the Science and Engineering Research Council (now the Engineering and Physical Sciences Research Council) as well as from several industrial sources.
References [1] P. Renaud, Bull. Soc. Chim. Fr. Ser. S. 17 (1950) 1044. [2] W. Slough and A.R. Ubbelhode, J. Chem. Soc. (1957) 918. [3] K.S. Suslick, Ultrasound: Its Chemical, Physical and Biological Effects (V.C.H. Publishers, New York, 1990). [4] S.V. Ley and C.R. Low, Ultrasound in Chemistry (Springer Verlag, London, 1989). [5] J.L. Luche, in: Current Trends in Sonochemistry, Ed. G.J. Price, R.S.C. Special Publication 116 (R.S.C, Cambridge, 1992) p. 34. [6] P. Boudjouk, in: Current Trends in Sonochemistry, Ed. G.J. Price, R.S.C. Special Publication 116 (R.S.C, Cambridge, 1992). p. 110. [7] K.S. Suslick, Science 253 (1991) 1397. [8] M.A. Margulis, Adv. Sonochem. 1 (1990) 39. [9] T. Lepoint and F. Mullie, Ultrasonics Sonochemistry 1 (1994) s13. [10] J-L. Luche, C. Einhorn, J. Einhorn and J.V, Sinisterra-Gaga, Tetrahedron Lett. 31 (1990) 4125. [11] E.W. Flosdorf and L.A. Chambers, J. Amer. Chem. Soc. 55 (1933) 3051. [12] A.S. Gyorgi, Nature 131 (1933) 278. [13] H.W. Melville and A. Murray, Trans. Farad. Soc. 46 (1950) 996. [14] D.W. Ovenall, G.W. Hastings and P.E.M. Allen, J. Polym. Sci. 33 (1958) 207. [15] A.M. Basedow and K. Ebert, Adv. Polym. Sci. 22 (1977) 83. [16] G.J. Price, Adv. Sonochem. 1 (1990) 231. [17] G.J. Price, in: New Methods of Polymer Synthesis, Vol. II, Eds. J.R. Ebdon and G.C. Eastmond (Blackie, Glasgow, 1995). [18] G.J. Price, P.J. West and P.F. Smith, Ultrasonics Sonochemistry 1 (1994) s51.
[19] P. Kruus, Adv. Sonochem. 2 (1991) 1. [20] G.J. Price, D.J. Norris and P.J. West, Macromolecules 25 (1992) 6447. [21] R. Carli, C.L. Bianchi, P. Gariboldi and V. Ragaini, Proc. 3rd. Europ. Sonochem. Soc., Coimbra, Portugal (1993). [22] L.D. David, U.S. Patent 4576688 (1986). [23] L. Topare, S. Eren and U. Akbulut, Polymer Commun. 28 (1987) 36. [24] T.J. Mason, Practical Sonochemistry (Ellis Horwood, Chichester, 1991). [25] D.N. Schultz, J.A. Sissano and C.A. Costello, Polym. Prepr. ACS 35 (1994) 514. [26] G.J. Price and P.F. Smith, Polymer 34 (1993) 4111. [27] T.G. Leighton, The Acoustic Bubble (Academic Press, London, 1994). [28] G.J. Price, P.J. West and P.F. Smith, manuscripts in preparation. [29] G.J. Price and P.J. West, Polymer, in press. [30] Y. Hatate, T. Ikamura, M. Shinonome and F. Nakashio, J. Chem. Eng. Japan 14 (1981) 38. [31] J.O. Stoffer, O.C. Sitton and Y.H. Kim, A.C.S. Polym. Mater. Sci. Eng. Prepr. 67 (1992) 242. [32] S. Biggs and F. Greiser, Macromolecules 28 (1995) 4877. [33] W.R. Sorenson and T. Campbell, Preparative Methods in Polymer Chemistry (Wiley Interscience, New York, 1968). [34] G.J. Price, M.P. Hearn, E. Wallace and A.M. Patel, Polymer, in press. [35] Silicon Containing Polymers, Ed. R.G. Jones (Royal Society of Chemistry, Cambridge, 1995). [36] R.D. Miller, E.J. Ginsberg and D. Thompson, Polymer J. 25 (1993) 310. [37] R.E. Benfield, R.H. Cragg, R.G. Jones and A.C. Swain, Nature 353 (1991) 340. [38] H.K. Kim, H. Uchida and K. Matyjaszewski, Macromolecules 28 (1995) 59. [39] G.J. Price and A.M. Patel, Eur. Polym. J., in press. [40] S. Doctycz and K.S. Suslick, Science 247 (1990) 1067. [41] W. Lauterborn and W. Hentschel, Ultrasonics 24 (1984) 59. [42] M. Rehann, A. Schluter and W.J. Feast, Synthesis (1988) 386. [43] M. Rehann, A. Schluter, G. Wegner and W.J. Feast, Polymer 30 (1989) 1054. [44] G.J. Price and A.A. Clifton, Tetrahedron. Lett. 32 (1991) 7133. [45] J. Lindley, J.P. Lorimer and T.J. Mason, Ultrasonics 24 (1986) 292.
ELSEVIER
Ultrasonics Sonochemistry 3 (1996) $229 $238
Ultrasonically enhanced polymer synthesis G a r e t h J. Price * School of Chemistry, University of Bath, Bath, BA2 7AY, UK Received 20 December 1995; revised 22 April 1996
Abstract
Some examples of polymerization reactions with a variety of mechanisms which have been carried out under sonication are described. It is shown that they can be classified in general terms using the same reaction types proposed by Luche for low molar mass systems. The use of ultrasound allows a large degree of control over the polymer structure and hence the resulting properties, particularly in controlling the molecular weight distributions.
Keywords: Polymerization; Cavitation; Degradation; Molecular weight
1. Introduction
The first reports of the benefits achievable when applying ultrasound to chemical reactions date from the 1950's [1,2]. However, the major efforts in this area in the late 1970's and early 1980's by Henglein, Luche, Boudjouk, Suslick and others have made ultrasound a routine tool for the synthetic chemist. Among the beneficial effects are greater yields, higher product purity and rate accelerations. In some reactions, changes of stereo- and regio-selectivity have been noted as has a preferential acceleration of one mechanism where two or more possibilities exist [3-6]. A major drawback in sonochemistry until recently has been the lack of mechanistic information leading to a predictive model of which reactions are most likely to benefit from sonication. Recently, there has been a great deal of study into resolving the precise effects of cavitation and causes of chemical reactivity in terms of a 'hotspot' theory [7], the 'electrical' theory [8] or a model based on a plasma discharge [9]. Whatever the precise origin of chemical effects, there is little doubt that sonochemistry is a direct result of cavitation. The harsh conditions generated on bubble collapse lead to the production of excited states, to bond breakage and the formation of free radicals, to mechanical shocks, high shear gradients and very rapid and efficient mixing in multiphase systems. Thus, three areas of a cavitating system can be identified, as shown schematically in Fig. 1. * Fax: +44 1225 826504; e-mail: [email protected].
Large gradients of T, P and Shear
\ \
Rapid motion and taxing [
Fig. 1. Schematic representation of a cavitation event.
The centre of the bubble is where the primary chemistry involved in, for example, radical formation takes place. In contrast, the bulk liquid is relatively unaffected apart from a small conductive heating effect although species generated inside the bubble may diffuse out and react with reagents in the bulk solution. In between, the interfacial region around the bubble has very large gradients of temperature, pressure and electric field as well as rapid motion of molecules leading to very efficient mixing e.g. emulsification. Of particular note for synthetic applications is the work of Jean-Louis Luche who made the first real attempt at categorising reactions of use in chemical synthesis [5,10]. Briefly, on the basis of a large ammm, of published work, he proposed that sonochemical, v,ctions could be classified into three types: Type 1: In homogeneous solution, single electron transfer processes will occur to give raa'ral intermedi-
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ates. Ligand cleavage reactions will give coordinatively unsaturated species. Purely ionic processes should not be sensitive to sonication. Type 2: In heterogeneous (solid/liquid or liquid/ liquid) systems, ionic reactions can be stimulated by mechanical effects although the products will be the same as in the absence of ultrasound. Type 3. In heterogeneous systems which can follow an ionic or radical mechanism, sonication will favour the latter. A number of systems are known which illustrate each of these types. Although mainly empirically based, these have become known as 'Luche's rules'. Given the economic and commercial importance of polymers, it is perhaps surprising that relatively little attention has been given to applications of ultrasound in this area. In fact, polymer sonochemistry predates work in synthesis and originated in the 1930's when the change of viscosity on sonication of starch and gelatin were noted [11,12]. These effects, due to chain breakage as a result of cavitation, were extensively studied in the 1950's [13,14] and again in the 1980's when improved methods of polymer characterisation were developed [15,16]. Applications in polymer synthesis have paralleled those in low molar mass systems although there have been many fewer studies [17]. However, considerable progress has been made over recent years and this paper will describe some recent work from our laboratories and, where appropriate, from other workers. Another aim is to show that the classification outlined by Luche is equally applicable to polymer systems although there are some aspects of sonochemistry which are unique to polymer systems. A number of the effects described above are potentially useful in polymer synthesis. The enhanced production of radicals could be used as a method of initiation in pure vinyl monomers. Several polymers, for example poly(vinyl chloride), PVC, are produced in emulsion or suspension systems and the efficient mixing allowed by ultrasound could be of benefit. Heterogeneous reactions are particularly susceptible to sonication, the most obvious example in polymer chemistry being the Ziegler-Natta catalysts by which polyolefins are produced. Economic considerations are presently dictating that novel materials do not arise from new polymers but from chemical modification of existing polymers so that sonochemical methods may prove to be of increasing importance. Among previous work in polymer chemistry has been an extensive study of the chain cleavage caused by sonication [15, 16,18]. Radical initiation in vinyl monomers such as methyl methacrylate has also been studied [19,20] as have emulsion or suspension polymerizations. A small amount of work has also been published on other reactions such as ring-opening [21 ], organometallic [22] and electrochemical [23] polymerizations.
However, given the large commercial and economic interest in polymers, it is perhaps surprising that there has not been greater effort. As in the synthesis of low molar mass compounds, we are interested in the rate and mechanism of the reaction and the yield and stereochemistry of the product. However, there is an extra factor to be considered in polymer systems. Polymerization reactions occur with statistical probabilities so that a sample will contain chains with varying lengths and molecular weights. The polymer properties depend critically on both the average molecular weight and the distribution of chain lengths, or polydispersity. A further aim of this paper is to demonstrate that ultrasound has an important role to play in the control of these parameters during polymerization reactions.
2. Experimental methods In general, sonochemical polymerizations use the same types of apparatus used for other synthetic work [24]. Thus, some of the work described here has utilised a cleaning bath (a Kerry Ultrasonics Pulsatron 325 operating at 35 kHz) while most of the work has involved horn systems since these allow more ready variation of the ultrasound conditions, optimisation of which is one of our major aims. To allow this, we have used specially designed glassware, shown schematically in Fig. 2, which
rm [ ]
[ )ansOuo.
[]
Ultrasound genemtor /
N 2 inlet
Sampling syringe
Thermometer
Water out > Water jacket
Solution
I I Water in
Fig. 2. Schematic representation of experimental apparatus for sonochemical polymerization.
G.J. Price/UltrasonicsSonochemistry 3 (1996) $229 $238
allows good temperature control by the circulation of thermostatted water and has the facility to work under controlled atmosphere and to remove samples periodically without disrupting the reaction. The horn used was a 23 kHz 'Sonics and Materials VC600' equipped with a 0.5 in diameter horn. Polymer molecular weights were measured by Gel Permeation Chromatography, GPC, by reference to polystyrene standards using equipment and methods described in detail elsewhere [18,20].
3. Results and discussion
3.1. Luche type 1 systems These are processes which occur in homogeneous solution. It is suggested that "Purely ionic species should not be sensitive to sonication". A number of polymers can be produced by ionic mechanisms but as far as the author is aware, sonochemical studies of these systems are very rare and there are no examples of the study of cationic polymerization of vinyl monomers. Very recently, Schultz et al. investigated the effect of ultrasound on the anionic polymerization of styrene initiated by t-butyl lithium [25], R - acting as the initiating species. These workers found that the preparation of RLi from RBr and lithium metal was more efficient (as expected from the work of Luche and Boudjouk) but there was little or no effect on the polymerization. Radical polymerization: A second aspect of Type 1 systems is the formation of radical intermediates. It is well known that sonication of vinyl monomers such as styrene and methyl methacrylate ( M M A ) leads to radical initiation of polymerization [19,20]. This is the most common method for vinyl polymerization although a major problem caused by the high and indiscriminate reactivity of the radical is control over the molecular weight and particularly the stereochemistry of the resulting polymer. Radical polymerization is perhaps the most studied of sonochemical polymerizations and considerable progress has been made in some systems toward a complete understanding and predictive model. The primary sonochemical step is the formation of radicals by breakdown of solvent in the cavitation bubbles. These radicals then initiate the polymerization. In order to estimate the rate of initiation in MMA, we have used a radical trapping method. A solvent, methyl isobutyrate, was selected to closely mimic the physical and chemical properties of M M A so that it would react similarly in an ultrasound field and produce similar numbers of radicals on sonication. A concentration of 2,2'-diphenyl-l-picryl hydrazyl, DPPH, in excess of the radicals produced was used and its consumption monitored spectrophotometrically. It has been suggested that there are inaccuracies in this method, for example, there will be a degree of radical recombination
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and other side reactions. However, since the radical must escape the vicinity of the cavitation bubble whether to initiate polymerization or to be trapped by DPPH, we feel that the rate of trapping will closely mimic the rate of initiation. By this method we were able [20] to measure the rate constants for radical production in purified M M A using the apparatus in Fig. 2 under a range of conditions and the results are shown in Table 1. Our work showed that the results agreed well with a first order reaction although the fit was somewhat better to a zero order process, as might be expected if the sonochemical step was the rate limiting process. To compare with thermal measurements, the rate constants considered here are those calculated for a first order reaction. To further compare with more conventional experiments, solutions of 0.1% azobisisobutyronitrile, a commonly used thermal initiator, were sonicated, the results also being shown in Table 1. The rate constant for purely thermal production of radicals, estimated by extrapolation from higher temperatures is negligible compared to the sonochemical reaction. The results show that simply by using ultrasound alone, similar rates of initiation as are usually found in thermally initiated polymerizations can be achieved at 25°C. The value for AIBN is some three orders of magnitude higher than the thermal process at this temperature so that the sonication process is clearly able to accelerate the decomposition of AIBN in solution as well as producing radicals directly by decomposition of the solvent. The effect of changing the temperature is shown in Table 1 and indicates that the rate of radical production is faster at lower temperatures. The other parameter of importance is the ultrasound intensity. It was found that below an intensity of approximately 12-13 W cm-2 there was no production of radicals, presumably corresponding to the cavitation threshold in this system. Above this minimum value, there was a linear dependence of the rate of D P P H consumption on the intensity. A conversion of 2-3% h -1 was achieved at 25°C. However, above a particular conversion, cavitation in the solution essentially stopped and no further conversion to polymer occurred as demonstrated in Fig. 3. This is probably due to the increased viscosity of the Table 1 R a t e c o n s t a n t s f o r D P P H r a d i c a l t r a p p i n g in m e t h y l b u t y r a t e System
Temperature
R a t e c o n s t a n t (s 1)
(oc) Ultrasound MeOBu MeOBu MeOBu MeOBu+0.1% MeOBu+0.1%
AIBN AIBN
- 10 25 60 25 70
6.35 x 10 5 2.21x10 s 1.03 x 10 - s 9.13 x 10 s 3 . 1 0 x 10 5
Conventional
lxl0
15
2 x 10 8
G.J. Price/Ultrasonics Sonochemistry 3 (1996) $229-$238
$232 25
o~ 2o
%.
0
100
200
300
400
500
Sonication time / rain
Fig. 3. Sonochemical polymerization of methyl methacrylate at 25°C. Solid elipses: bulk M M A ; open triangles: 50% solution of M M A ; solid rectangles: M M A + 0.1% AIBN.
solution restricting movement of the solvent molecules and suppressing cavitation, hence preventing formation of radicals. The solution polymerization gives higher conversion due to the lower viscosity of the reaction mixture. One of the most important properties of a polymer is the molecular weight and Fig. 4 shows that high molecular weight polymer is formed at very early stages of the reaction but that the value falls at longer times. This is not the same plot as found with conventionally initiated radical polymerization due to other processes occurring simultaneously as discussed below. One of the goals of sonochemical polymerization is to use variation of the reaction conditions to control the properties of the resulting polymers. This is illustrated in Table 2. Changing the ultrasound intensity at constant temperature not only varies the amount of 4 < O
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,
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200
300
polymer produced but also allows systematic variation of the final molecular weight. The final factor which has been investigated for these systems is the stereochemical arrangement of the monomer units along the chain. In polymer chemistry, this is described by the tacticity of the polymer. Chains which have all of the substituents arranged on the same side are referred to as isotactic while those which have an alternating arrangement are syndiotactic. Polymers with a random arrangement are termed atactic or heterotactic. The tacticity in PMMA is readily determined using NMR spectroscopy and Table 3 shows the relative degrees of tacticity in sonochemically produced PMMA together with Literature results for more conventional polymerizations. It is clear that conventional initiation using a peroxide initiator leads to predominantly syndiotactic polymers. At high temperatures sonication and thermal initiation both produce polymers with similar stereochemistry suggesting that sonication has little or effect on the propagation reaction. Polymerizations using an anionic mechanism using, for example n-butyl lithium as the initiator give 60-70% isotactic polymer because the rate of propagation is slow compared with the initiation rate. In sonochemically promoted reactions at low temperatures, the initiation is speeded up while the propagation rate is slowed compared with the thermal polymerization and hence the proportion of syndiotacticity along the chain is raised. Steric hindrance between the bulky ester groups makes syndiotactic addition thermodynamically more favourable, although the energy difference is small. As the temperature is lowered, the propagation rate is slowed and there is more chance of the favoured addition taking place. Hence, variation of the experimental conditions such as temperature and the ultrasound conditions such as intensity allows a great deal of control over the structure (and hence properties) of the resulting polymer. While it is not envisaged that sonochemical methodology will ever be adopted for large-scale production of vinyl polymers, there are some situations where low temperature, site specific initiation could be useful. The
400
Sonieation time / min Fig. 4. Variation in molecular weight during sonochemical polymerization at 25°C. Solid triangles: methyl methacrylate; open elipses: n-butyl methacrylate; solid rectangles: styrene. Table 2 Properties of P M M A produced by 6 h sonication of pure m o n o m e r Temperature (°C)
Intensity ( W cm -z)
Molecular weight
Polydispersity
Conversion (%)
25 25 25 25
13 29 35 58
144000 131000 97000 35000
2.3 1.9 1.8 1.5
2 10 14 12
Table 3 Stereochemical tacticity ratios for radically initiated P M M A . i = isotactic; a = atactic; s = syndiotactic Polymerization conditions
Monomer" 100°C Solution" 50°C Ultrasound 60°C Ultrasound 40°C Ultrasound 25°C Ultrasound 0°C Ultrasound - 10°C ~Using benzoyl peroxide as initiator.
Ratio i
a
s
8.9 6.3 4.3 4.0 2.8 1.7 0.8
37.5 37.6 41.0 40.4 34.3 33.8 25.6
53.9 56.0 54.7 56.4 62.9 64.6 73.6
G.J. Price/Ultrasonicw Sonochemistry 3 (1996) $229-$238
ability to work at low temperatures with monomers which have low ceiling temperatures and the ability to control molecular weights without the need to add additional agents as well as the possibility of switching off the initiation during the reaction to control heat build-up are all advantages which may be exploited in particular cases. Sonochemical polymer degradation: Radical polymerization clearly corresponds to a Luche type 1 system. Another homogeneous process which occurs with polymers lies outside this classification as it is unique to macromolecular systems. If chains are sufficiently long, they are stretched out in the solvent flows around collapsing cavitation bubbles and the resulting shock waves. The shear gradients generated in this way are sufficient to cleave the polymer backbone wheras low molar mass compounds are too small for a sufficient force to operate. The basic effects [26] of sonicating a polymer solution are shown in Fig. 5 using 1% solutions in toluene of polystyrenes with varying molecular weights at 25°C as an example. The degradation proceeds more rapidly for higher molecular weight materials and approaches a limiting value, Mlim, below which no further degradation takes place, in this case ca. 30000. Polymer chains with this or lower values are too small to be affected under these conditions. Since the rate of degradation is molecular weight dependent, longer chains are removed from the sample and the polydispersity of the polymer is changed. These effects appear to be universal in that they have been seen for a wide range of organic polymers in organic solvents, for inorganic polymers and for aqueous systems including polypeptides, proteins and DNA. In order to develop predictive models for the degradation, behaviour under a wide range of conditions has been measured. This has been documented in detail elsewhere [26,15,16] and it is sufficient here to summarise the main trends in the results. In summary, the degradation proceeds faster and to lower molecular weights at lower temperatures, in more dilute solutions
and in solvents with low volatility, in accord with the expected effect of these parameters on cavitation in the solvent [27]. Other factors which have been quantified are the ultrasound intensity and the nature of dissolved gases. Again, by suitable manipulation of the experimental conditions, we can exert a great deal of control over the process, exploitation of which allows the modification of existing polymers into new materials. At its most straightforward level, the degradation can be used as an additional processing parameter to control the molecular weight distribution. For example, Fig. 6 shows GPC chromatograms of a polyalkane in solution undergoing sonication [28]. The degradation of the higher molecular weight species narrows the distribution markedly with consequent modification of the physical properties of the polymer. A potential application would be to have a sonochemical step as the final processing stage during manufacture so as to 'fine tune' the polydispersity and molecular weight distribution to give the desired material properties of the polymer. There are examples where this has been applied to commercial processes on at least pilot-plant scale. A second application of the degradation utilises the macromolecular radicals produced on chain breakage as initiating species in the preparation of copolymers and end-capped materials. A large number of workers have sonicated mixtures of two polymers dissolved in a common solvent, cross reaction between the two types of radicals forming a block copolymer. The drawback here arises from the difficulty in recovering the products by selective precipitation and also in controlling their structure. The block copolymers formed can act [29] as 'in situ' generated compatibilisers in otherwise incompatible mixtures by acting as a 'detergent' helping to dissolve the polymers in each other. Our approach to
SONICATION TIME / hr
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i
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i
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i
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,
4
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6
7
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Fig. 5. Ultrasonic degradation of 1% solutions of polystyrene of differing molecular weights in toluene at 25°C.
LOG10 ( MOLECULAR WEIGHT ) Fig. 6. Gel Permeation Chromatograms of a polyalkane in toluene at varying times during sonochemical degradation.
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G.J. Price/UltrasonicsSonochemistry3 (1996) $229 $238
preparing copolymers has been to sonicate a polymer dissolved in a solution containing the second monomer such as polystyrene and methyl methacrylate as shown in Fig. 7. Using the results of degradation studies, the structure and block length of the first polymer can be controlled quite precisely. By changing the concentration of monomer in solution the block size of the second polymer can also be varied, allowing a large degree of control over the resulting material structure. A related approach is to sonicate the polymer in the presence of a species labile to radical attack where 'end capped' polymers are formed. We have used this to prepare, for example, polystyrenes and poly(alkanes) bearing fluorescent groups [28]. The degradation process is unique to large-molecule systems and offers the oportunity of controlling polymer structure. It will occur whenever polymers in solution are sonicated and must always be considered when polymer syntheses are being undertaken. 3.2. Luche type 3 systems
These are systems in which alternative mechanisms operate in parallel and where sonication can preferentially accelerate one of them, leading to different products from the conventional reaction. As far as the author is aware, no such examples of 'sonochemical switching' have been found in polymer systems.
studied in our laboratory will be described to illustrate the effects. Synthesis of siloxanes: Poly(dimethyl siloxane), PDMS, is the base polymer from which silicone resins and rubbers are prepared. Its usual method of synthesis is a cationic ring-opening reaction of the cyclic tetramer, catalysed by a small amount of acid (see Scheme 1 ). In a typical procedure [33], 20 cm 3 of octamethyl cyclotetrasiloxane, OMCTS, 10 cm 3 diethyl ether and ca. 0.5 cm 3 sulphuric acid are thoroughly mixed, after which the acid is washed away and the polymer recovered by extraction with ether. The aqueous acid is insoluble in OMCTS so that the reaction is a two-phase system with polymerization catalysed by a phase transfer process. It is thus a heterogeneous liquid/liquid system and should act according to Luche Type 2 rules. Fig. 8 compares the results of this polymerization under silent and sonochemical conditions, the source of ultrasound in this case being a cleaning bath [34]. It is clear that ultrasound in this case accelerates the polymerization by a factor of at least two, although there is little diference in the final yield obtained. More surprisingly, the molecular weight is also higher in the sonochemical reactions whereas the degradation process described above might be expected to give lower values. After 8 h, the number average molecular weights and polydispersities obtained were 49500, 1.6 and 17300, 2.0 for the sonicated and silent reactions respectively.
Me ^ Me Me-S i / u ~ ~SilMe
3.3. Luche type 2 systems
These systems involve heterogeneous, multi-phase mixtures. A number of polymer systems satisfy these criteria. For example, polymerization in emulsion systems has been shown to benefit from sonication by Hatate et al. [30], Stoffer and co-workers [31] and recently by Greiser et al. [32]. Effects include more rapid formation of emulsions and initiation, control over particle size and a higher polymer molecular weight. Two examples selected from polymerization chemistry
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0 J¢¢0 ~¢~.O40A CH3 CH3 rn//
~ULTRASOUND
~o 20 ~. 2
Fig. 7. Schematicrepresentation of sonochemicalproduction of endcapped polymersand block copolymers.
4
6
Sonlcatlon Time / hr
Fig. 8. Sonochemicalpreparation of poly(dimethylsiloxane)at 25°C.
G.J. Price~UltrasonicsSonochemistry 3 (1996) $229-$238
The higher molecular weights and lower polydispersities suggest strongly that the 'initiation' of ring-opening is much more efficient. This is due to the ultrasound providing more effective mixing of the two phase system and better phase transfer of the acid catalyst. Synthesis of poly(organosilanes): A second example of a Type 2 system can also be found from the area of silicon based polymers. Poly(organosilanes) which have a backbone consisting exclusively of silicon atoms have recently been receiving attention as electro- and photoactive materials. [35] These polymers are usually produced by Wurtz coupling of dichlorodiorganosilanes with molten sodium. However, this leads to irreproducible results, low yields and hi- or tri- modal polymer distributions, largely due to the heterogeneity of the system. Boudjouk [6] showed some time ago that this type of coupling reaction was particularly susceptible to sonication. (See Scheme 2.) While the detailed mechanism remains a topic of debate, it is generally agreed [35-37] that chain growth proceeds via silyl anions generated at the surface of the sodium involving two SET steps. A number of systems have been studied [38, 39] and, in each case better yields and narrower distributions have been achieved under ultrasound. Most work has been done on the synthesis of poly(methyl phenyl silane) as this yields soluble, easily characterisable materials. To illustrate the results, Fig. 9 shows the molecular weight distributions of three of these polymers produced under reflux at 110°C, using CI
CH3
\s/"
Reflux, 110"C / sonicate, 25"0
60°C
a cleaning bath at and a horn system at 25-35°C. The first of these has a very broad, two component polymer distribution while the polymer from the horn experiment has a single component, that from the cleaning bath being intermediate between these extremes. To further characterise the effects, samples were removed periodically during a polymerization using the horn and the molecular weight distributions are shown in Fig. 10 and clearly show the relative degradation of the highest molecular weight component as the reaction proceeds, although it does move to higher values implying that there is an effect on the chain extension reactions. Finally, to demonstrate that ultrasound can be used to tailor the properties of these materials, polymerizations were carried out over a range of sound intensities, the results being shown in Fig. 11. By changing the intensity, the resulting distribution can be varied over a wide range. These and other results have led us [39] to suggest
:~20: i0
~ •
S ' ~ Si/ S ' ~ s r " ~ d "& t ,.%./n Ph CH3 Ph CH3
I
I
I
6
5
10
MoleculaWei r ght
f~
,.,,--...
P
i
10
I
10 s
10
I
I
4
MoleculaWei r ght
10
..7~
il",,,f\ t,t
/ ,o.i I_>_,:,\ { t
'-,...... .
I
6
10
i
il Ii
I
3
10
Fig. 10. Molecular weight distributions of poly(methyl phenyl silane) at varying times during sonochemical polymerization at 25°C.
-~"t
Reflux// "
I 4
10
Scheme 2.
///
."'/ "~
Ph CH 3 Ph CH3 .- Ik [,,~," ,~
Na, Toluene ~
t \0, Ph
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3
Fig. 9. Molecularweight distributions of poly(methylphenyl silane) produced under varyingconditions.
106
I
10s
I
104 MolecularWeight
i
--
103
Fig. 11. The effect of ultrasound intensity on the molecular weight distributions of sonochemically produced poly(methyl phenyl silane). [Intensities in W cm 2.]
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G.J. Price~Ultrasonics Sonochemistry 3 (1996) $229 $238
that ultrasound has four effects in this polymerization. Firstly, microjetting continually regenerates a fresh sodium surface so that the reaction proceeds at a reasonable rate at lower temperatures, minimising unwanted side reactions. This also results in enhanced mass transfer of reactants to the surface, increasing the rate of the initial anion formation and giving a more homogeneous chain growth and hence chain length distribution. Finally, the sonochemical degradation of polymer chains in solution leads to lower polydispersities. The effect of ultrasound on reactivity on and at surfaces of metals by means of microjets is well known [40,41]. Thus, again these systems conform in general terms to Luche's classification. The reactions are believed to involve ionic intermediates and are accelerated and enhanced by sonication due to 'mechanical' effects such as mass transfer while not affecting the nature of the products. However, sonication allows a large degree of control over the polymer structure in terms of their molecular weight distributions.
Organometallic coupling reactions of haloaromatics: Some of the most successful reactions in sonochemistry have involved the coupling of organic halides using alkali metals such as lithium and sodium [3 6]. One example applied to polymer synthesis is the Wurtz coupling to form poly(organosilanes) discussed above. Another range of targets has been polyphenylene materials consisting of linked, conjugated chains of aromatic rings. These have received considerable attention as prototype conducting materials but, until recently, difficulties in synthesis has prevented their widespread adoption. In addition, even at short chain lengths they become insoluble and cannot be melted so that their processing presents difficulties. Indeed, even now synthetic methods involve complex schemes involving precursor materials which are used to fabricate films and are then converted to the desired polymers. The earliest method for synthesising polyphenylenes involved the oxidative coupling of benzene using a Lewis Acid catalyst, shown in Scheme 3. We carried out this reaction using benzene as the solvent and sonication yielded double the amount of polymer over using more conventional methods. Analytical techniques (IR, elemental analysis etc.) showed no significant differences between the polymers. Thus, this reaction also seems to below to a Luche Type 2 system. Other methods for preparing polyphenylenes are based on coupling reactions of dihaloaromatics but they often give low yields and sometimes react only as far as 40 "C, N 2
Br~--
Br
Mg, Ni(bipy)CI2~ N 2, THF
Scheme4. Table 4 Effect of ultrasound on Grignard coupling of dibromobenzene
Conditions
Ultrasound
Polymer yield (%)
Reflux, 2 h Reflux, 5 h 20°C, 24 h 60°C, 2 h 20°C, 2 h 60°C, 2 h
No No No Bath Horn Horn
20 35 40 48 33 45
I•CI
+ 2 C6H13MgBr
C
Et20 Ni(dpPp)CI2 C 6 H 1 3 . ~
C6H13
I
Br2, hv, 0 °C
,,=
AICI 3 / CuCI 2 H20
Scheme 3.
the dimer or trimer. For example, attempts to couple dibromobenzene directly using lithium, sodium or potassium were all unsuccessful in generating polymers. Coupling took place but elemental analysis suggested that only trimers, tetramers and possibly pentamers were produced. Furthermore IR spectroscopy of the solid products suggested that they were not all para-substituted rings. These effects are presumably related to the insolubility of the target compounds which, as well as hindering their synthesis, makes them very difficult to characterise since they are also infusible and intractable. One of the more successful schemes in this area is due to Rehann et al. [42] using a nickel catalysed Grignard type reaction shown in Scheme 4. Ultrasound was applied to this scheme under various conditions and the results are given in Table 4. In each case, sonication gives better yields than the equivalent 'silent' conditions as well as allowing the reaction to proceed at lower temperatures. Considerably better yields were obtained in 2 h at room temperature using a sonic horn than from a 5 h reflux. However, full characterization of these materials is not possible for the reasons outlined above. One method for making these materials soluble, and hence characterisable, was suggested by Feast et al. [43] who prepared polymers substituted with hexyl- or octylgroups in order to confer solubility in common solvents. We have also attempted to use ultrasound in the preparation of these materials as shown in Scheme 5 but, while
11 C6H13
Mg, THF Ni(bipy)Br2 Scheme 5.
Br-~Br
C6H13-
C6H13
G.J. Price~UltrasonicsSonoehemistry 3 (1996) $229-$238
it was useful for the preparation of the monomers, also involving Grignard chemistry, we could not achieve polymerization. The starting material here, 2,5-di n-hexyl 1,4-dibromobenzene was prepared using a literature method. Dichlorobenzene was reacted with hexyl magnesium bromide and ultrasound gave a yield after 1 h at room temperature of 41% compared with 42% from a 12 h reflux. After conventional bromination, the polymerization was attempted. This gave a yield of 48% for a 48 h reflux in THF (comparable with literature) but a yield of only about 10% after 12 h sonication on the probe system, showing no significant improvement. We also attempted the reaction by coupling over lithium and several other reactive metals but no significant reaction took place in a Wurtz type reaction. The lack of reaction in the Wurtz systems prompted further study of the mechanism and we have rationalised our results in terms of a radical type reaction. We studied the Wurtz type coupling of bromobenzene and 2-, 3-, and 4-bromotoluene under varying conditions and have established that it proceeds via a SET mechanism with a radical generated on the ring [44]. This explains why the coupling is not all para and also the reduced reaction in the dialkyl benzenes during polymerization. Thus, although not a success from a polymer synthesis point of view, the work has been useful in extending our knowledge of sonochemical reaction mechanisms. An alternative approach to 'coupling polymerization' has been to employ Ullmann reactions utilising copper in DMF as the coupling agent. Lindley et al. [45] suggested that Ullman coupling of 2-iodonitrobenzene and 2,5-di bromo nitrobenzene proceeded up to sixty times faster and to give up to 95% yields under the influence of ultrasound. Although we have been unable to duplicate these results in polymerization systems, we have been able to synthesise new materials. For example, one reaction that we utilised is shown in Scheme 6. Under the standard Ullman conditions of 20 h reflux at 140°C in DMF, the reaction gave 95% dehalogenation and negligible polymerization. Conversely, reaction for 4 h on the ultrasonic probe yielded ca. 10-15% of a deep-brown material which although not completely characterised, contains at least four or five rings and we expect to have the structure shown. Similar results were obtained employing nitroiodobiphenyls as 'monomers'. Although the yields are as yet poor, if it could be increased, this would be a very interesting material since
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the alkyl groups would confer solubility, the backbone would be potentially conductive while the nitro groups would be very useful to perform further chemical modification to change the properties. Hence, in this new area of study, the use of ultrasound has allowed us to prepare new materials which cannot be prepared under conventional conditions. The full potential has yet to be fulfilled and this is an area which will continue to develop.
4. Conclusions This brief survey of polymer sonochemistry has illustrated that there is a wide range of systems which can benefit from sonication. Moreover, the types of reaction involved fit into the classifications involved in the sonochemistry of low molar mass compounds as suggested by Luche. While this classification is not perfect, it does provide a framework for the correlation and interpretation of a range of reactions. As with most classification systems, it can be used in too rigorous a manner but if used appropriately may be useful. Despite potential uses of sonochemistry in synthesis, to date there has been relatively little work related to sonochemical polymerization. The benefits offered in a range of reaction types certainly warrant further studies in the area in terms of acceleration of rate and increased yields but particularly in the control of molecular weight during polymerization. When any polymerization process in solution is subject to sonication, the degradation will always occur concurrently with chain growth. Perhaps the most obvious application would be in an 'add-on' to existing processes where the ultrasonic degradation can be utilised to control the final molecular weight of polymers during manufacture. However, it seems unlikely that a sonochemical process will replace current methodology unless it allows significant improvement of material properties or saving in production or energy costs. There are areas such as biomedical materials and food uses where the added value of the products would support the extra costs presently associated with sonochemical technology. The outlook is encouraging and it appears that sonochemical polymerization reactions are well placed to become commercially viable in the near future.
Acknowledgement 13H6C B r @
O2N'
NO2 Br
13H6C
NO2
Cu, DMF O2N
C6H13 Scheme6.
C6H13
It is a great pleasure to acknowledge the excellent group of students who have conducted research in sonochemistry with me over the past seven years or so. Dr Ali Patel, Paul Smith, Peter West, Andrew Clifton and Emma Lenz (all postgraduate students who now
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G.J. Price/Ultrasonics Sonochemistry 3 (1996) $229-$238
have their PhD's) and Diane Norris, Melanie Daw, Emma Wallace and Matthew Hearn (undergraduate students) have all contributed to the work described here. In addition, we grateful for funding of our work from the Science and Engineering Research Council (now the Engineering and Physical Sciences Research Council) as well as from several industrial sources.
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