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The application of ultrasound to the synthesis of poly(organosilanes)
Eur. Pot.vm.J. Vol. 32, No. II, PP. 1289-1295,1996
PII: sm4-3057(%)oow-5
Copyright 0 1996Elsevier ScienceLtd Printed in Great Britain. All r&hts reserved COM-3057/96 $15.00+ 0.00
THE APPLICATION OF ULTRASOUND TO THE SYNTHESIS OF POLY(ORGANOSILANES) G. J. PRICE* and A. M. PATEL School of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, U.K. (Received 4 September 1995; accepted in final form 26 October 1995)
Abstract-The effect of carrying out Wurtz type polymerizations of dichlorodiorganosilanes to poly(organosiIanes) under the influence of ultrasound has been investigated. The sonochemical reaction produced faster polymerizations and higher yields in a range of systems. Previous reports of modification of molecular weight distributions have been confirmed and the results used to elucidate the mechanism of the sonochemical enhancement. The control of molecular weight distributions by variation of the ultrasound properties is also illustrated. Copyright 0 1996 Elsevier Science Ltd
INTRODUCTION
There is rapidly growing interest in polymers with inorganic backbones [l, 21, among which silicon containing polymers are perhaps the major group. Polysiloxanes [3] have been used for some time as low viscosity, low surface energy fluids and in silicone elastomers. More recently, poly(organosilanes) have attracted considerable interest due to their potential as electro- and photoactive materials [4, 51. Delocalization of u-bond electrons in the main chain leads to strong absorption into the UV and after doping, to conductivity. Their photosensitivity has led to use as photoinitiators and as positive working microlithographic photoresists. They have also been used as precursors to ceramic materials although early promise in this area has not been fulfilled. This has largely been due to a lack of methodology for reproducibly synthesising the polymers on a reasonable scale. The usual method of synthesis [6-91 for poly(organosilanes) is a reductive, Wurtz type coupling of dichlorodiorganosilanes using molten sodium in refluxing toluene.
Na. Toluene
and > 105. The complex distribution has been attributed to concurrent reaction mechanisms although it has recently also been explained in terms of the heterogeneity of the system and the solubility characteristics of the components [lo]. More reproducible Wurtz reactions can be obtained by adding components such as diglyme or crown ethers to the solvent [l 1, 121, although this often limits the molecular weights achievable. Alternative methods for synthesising poly(organosilanes) which have been developed recently include the use of homogeneous reducing agents [ 131, dehydrogenative coupling of RR’H* using organotransition metal catalysts [14, 151, the anionic ring opening of strained cyclic organosilanes [ 161and the anionic polymerization of masked silylenes [ 171. However, while these can yield poly(organosilanes) with controlled structures, this is again usually at the expense of the chain length which remains relatively low. A good yield of poly(methy1 phenyl silane) with a monomodal molecular weight of N 65,000 was obtained at lower temperature by Jones et al. by reaction in refluxing diethyl ether [9]. More recent work has shown that thermal degradation to cyclic oligomers occurs around 150°C and this is
)
Reflux, 11O’C / sonicate, 25’C However, the reactions are irreproducible and the yields are rather low, -SO-55% at best, depending on the nature of R and R’. Also, the polymers often have a very wide, usually bi- or tri-modal, molecular weight distribution consisting of a low molecular weight (- 1000) cyclic and oligomeric fraction together with fractions in the region of 10,00@30,000 *To whom all correspondence should be addressed.
accelerated in the presence of alkali metals such as sodium [18, 191.Both of these facts suggest that better molecular weight distributions would be obtained by using low temperatures although the rate would be slow. Over the past two decades, the use of high intensity ultrasound has become a common technique in synthetic chemistry [20-221. A number of reactions, especially those in heterogeneous systems, have been
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shown to give enhanced rates and yields under ultrasound, leading to a branch of chemistry sometimes termed sonochemistry. Polymer sonochemistry in fact predates synthetic uses, the
irreversible cleavage of polymer chains when irradiated in solution having been reported in the 1930s [23]. More recently, ultrasound has been applied to polymer synthesis with some success [24, 251. In addition to rate and yield enhancements, perhaps the major advantage of performing sonochemical polymer synthesis is that it provides an additional means of molecular weight control. The main effects of sonication are due to cavitation or the growth and explosive collapse of microscopic bubbles on a microsecond timescale [26]. This can result in the formation of relatively high concentrations of excited species such as radicals [27] which can diffuse into solution and react. The rapid movement of solvent around the molecule also sets up shear gradients which stretch out and break the chains in a non-random, molecular weight dependent process resulting in a lowering of the molecular weight and polydispersity [28, 291. A second effect of this enhanced molecular motion is very efficient mixing and dispersion of multi-phase systems [20]. An extra effect occurs near a solid surface when cavitational collapse is asymmetric and results in a microjet of solvent impinging on the solid [21]. This acts to continually clean and refresh the surface as well as increasing the rate of transfer of reactants and products to and from the surface. The principle of applying ultrasound to producing poly(organosilanes) arose from the discovery in the early 1980s of the facile sonochemical coupling of chlorosilanes, RSiCl, over lithium metal to give R$iSiR, [30] a reaction which has been extended by using R2SiClz to give the polymeric materials. There are a number of effects which could be of potential benefit in using sonochemical methods for this polymerization. Firstly, there could be a preferential enhancement of one of the competing mechanisms to give more control over the molecular weight distribution. Secondly, the rate of polymerization should be faster, allowing the use of milder conditions such as lower temperatures. In addition, the control of molecular weight associated with the degradation could also be of use. The first reports of ultrasound being applied to the synthesis of poly(organosilanes) was by Kim et al. [31, 321. They described the production of materials with a single molecular weight distribution and polydispersity as low as 1.2, albeit in relatively low yields of I l-15% by carrying out the reaction at 60°C in toluene although the method worked only for organosilanes with aromatic substituents. Homopolymerization of dialkyl silanes was only possible in more polar solvent systems. An in depth study of the effect of organic substituent, solvent system and temperature was recently reported [33]. Sonochemitally assisted polymerization was also investigated by Miller et al. [34, 351 who reported somewhat conflicting results in that monomodal polymers were only obtained if diglyme or 1S-crown-S were added to the solvent. In their absence, bimodal distributions were obtained in which the high (_ 150,000) component comprised about 65% of the polymer, the
remainder being of low molecular weight (~9700). Weidman et al. have also used sonochemical methods in the synthesis of related polysilynes by reacting RSiCI, over sodium or NaK alloy [36, 371. In this paper, further study of the sonochemical polymerization of a number of dichlorodiorganosilanes is described. In particular, the effect on molecular weight distributions has been investigated and used to elucidate the mechanism of operation. Finally, the control of molecular weight by variation of the ultrasound conditions is described. EXPERIMENTAL Sonication
methods
Two sources of ultrasound were used. Much of the published sonochemical organic synthesis has been performed by immersing the reaction vessel into an ultrasonic cleaning bath [21]. Considerably higher sound intensities and better temperature control are achievable with a ‘horn’ type apparatus where the ultrasound vibrations are coupled directly into the reaction via a titanium alloy rod which dips into the reaction medium. In this work, a Kerry Ultrasonics Pulsatron 325 cleaning bath and a MSE ‘Soniprep 150’horn apparatus were used. The ultrasound intensities on the latter system were measured calorimetrically in the usual way [38] and temperature control was achieved to +_O.l”C by circulating thermostatted water through a jacketted vessel. The apparatus has been described previously [27]. Polymer
degradation
To investigate the ultrasonic degradation, a sample of poly(methyl phenyl silane) produced during the course of this work was used. A solution of 5% (w/v) concentration was prepared in HPLC grade toluene (Aldrich). 100 cm’ of solution was placed in the jacketted vessel and allowed to attain thermal equilibrium. The sonicator was switched on and small samples (_ 0.24.3 cm’) removed periodically for analysis. Polymer
analysis
Polymer molecular weights were recorded on a Bruker LC21/41 gel permeation chromatograph using tetrahydrofuran as the eluent and ultraviolet detection. Ten polystyrene standards (Polymer Laboratories Ltd) with molecular weights between 1020 and 2.65 x IO6were used as cahbrants and all values reported below are ‘polystyrene equivalents’. Polymekation
procedure
The procedure for the preparation of poly(organosilanes) was modified from those in the literature [33. 341.Typically, 2.8 g (0.12 mol) sodium (Aldrich) was added to 50 cm’ of dry toluene under nitrogen. This was brought to reflux and stirred vigorously to disperse the metal. The dichlorodiorganosilane (0.1 mol, Petrarch Ltd) was added dropwise to avoid excessive heat release. All the glassware used for polymerizations and work-up was wrapped in aluminium foil to exclude light. The reaction was allowed to proceed for the appropriate time before being quenched by the addition of propan-2-01. In the ultrasonic experiments, the sodium was dispersed by sonicating on full power for 30 min at the temperature of the experiment prior to adding the silane. The polymers were recovered by dropwise addition of propan-2-01 to quench any unreacted sodium followed by precipitation into a large excess of ice-cold propan-2-01. The resulting solid was recovered by filtration and purified by multiple reprecipitation into propan-2-01 from toluene. RESULTS AND DISCUSSION
Reactions of the dimethyl or diphenyl produced insoluble materials, presumably
silanes highly
Poly(organosilane) Table 1. Organosilane Dihexyl Methyl hexyl Methyl cyclohexyl
synthesis using ultrasound
Effectof ultrasound on preparation of poly(dialkyl silanear
ConventionaP Yield/% M. :t 19
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11,200 3100 4900
Ultrasound’ Yield/% M.
Y 3.5 7.0 2.9
25 41 33
Y
31,000 4700 8600
3.4 9.5 2.7
‘I hr reaction in toluene containing 1% l&crown-6. “Reflux, 110°C. ‘Ultrasound bath at 253O’C.
crystalline, the ‘polymeric’ nature of which is uncertain, particularly with the diphenyl compound. Both systems showed enhanced yields of the polymer when the sodium was dispersed using ultrasound and the reaction carried out using a cleaning bath. To illustrate the effects, with dichlorodiphenyl silane, the yields were 30% from a 5 hr relIux at 110°C in toluene compared with 55% from a room temperature reaction for 1 hr under ultrasound. The corresponding values with the dimethyl compound were 12 and 28%, respectively. Rather surprisingly in view of the insolubility of the homopolymers, polymerization of an equimolar mixture of the diphenyl- and dimethyl compounds gave a reasonable yield (-35%) of a soluble, high molecular weight polymer with a bimodal molecular weight distribution. Copolymers of both compounds with the methyl phenyl silane were also prepared. Further work concerned organosilanes which yield soluble polymers so that they could be fully characterized. Synthesis of poly(dialkyl silanes)
In common with previously published work, poly(organosilanes) with alkyl substituents such as dihexyl, methyl cyclohexyl and methyl hexyl could only be produced with very low yields (2-5%) by reaction in toluene alone, even in the presence of ultrasound. It should be noted that the polymers showed the usual wide molecular weight distribution including components with very large values. It has been reported 111, 121 that the addition of crown ethers which complex with sodium improves the distribution but gives low molecular weights. Table 1 shows the effect of ultrasound on the polymerizations
106
10s 104 Moleoulu weight
(1 hr) at 25°C in the presence of 18-crown-6 at a concentration of 1% of the silane in toluene. In each case, the sonochemical reaction gave higher yields and somewhat higher molecular weights. These results are presented for comparison purposes and are not optimized. The M. values are somewhat misleading due to the large polydispersities and the samples do contain some relatively long chains as shown in the distributions in Fig. 1. None of the three polymers had a single distribution and in particular that for polyfhexyl methyl silane) was unusually broad with two distinct polymer components. Synthesis of PMPS
Most effort has concentrated on the synthesis of poly(methy1 phenyl silane), PMPS, which polymerizes more rapidly and can be produced in higher yields. It has also been the object of most photochemical and application studies. In initial experiments, the conventional reflux method in toluene at 110°C afforded a yield of 15% after 1 hr. Carrying out the polymerization of 60°C for the same time in an ultrasound bath and with a “horn” system yielded 35 and 43%, respectively. The corresponding values at 25°C were 19 and 29%, respectively. These values are not optimized and do not indicate the best yields obtainable by either the ultrasonic or conventional methods. Higher yields could be obtained by allowing the reaction to proceed for longer or by changing the solvent system but these unoptimized results demonstrate that considerably higher yields and faster polymerizations rates can be obtained using ultrasound than by conventional methods. Perhaps of more significance are the changes in the molecular weights and distributions of the polymers,
10’
Fig. 1. Molecular weight distributions of sonochemically produced poly(dialkyl silanes): - poly(cyclohexyl methyl silane); -poly(n-hexyl methyl silane); ---- poly(di n-hcxyl silane).
I
I
106
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IO'
104 Molecular weight
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Fig. 2. The effect of ultrasound sources on the molecular weight distributions of poly(methy1 phcnyl silane).
1292
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Molecular weight Fig. 3. Molecular weight distributions during sonochemical preparation of poly(methy1 phenyl silane). (The values
indicate the reaction time in minutes). examples of which are shown in Fig. 2. The conventional reflux method gives a polymer with a very wide, bimodal distribution. The amount of high molecular weight material, and hence the polydispersity, is vastly reduced under the influence of ultrasound and use of the high intensity probe system gave a monomodal, though broad, distribution at slightly higher molecular weight than the lower component of the other distributions. Note that here, only the polymeric part of the distribution is being considered. There is an additional peak in the chromatograms due to oligomeric materials which could, if desired, be straightforwardly removed by fractionation. To investigate the possible origin of these differences, the molecular weight distribution was periodically measured during a reaction performed on the ultrasound “horn”. These are shown in Fig. 3. It is clear that high molecular weight material is formed at an early stage but that it is degraded as the reaction proceeds. Notably, in addition to the reduction in the relative amount of the longer component, it also shifts to higher molecular weight. Thus, chain growth must continue throughout the
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Molecular weight Fig. 5. Effect of ultrasound intensity on the polymerization of methyl phenyl silane. (Values indicate ultrasound intensity in W cm-*).
reaction and a simple degradation does not fully explain the results. Polymerizations were then conducted in the presence of l&crown-6. The reactions were faster under sonication and produced above twice the amount of polymer for a 1 hr reaction. In common with previous work, a single, low molecular weight polymer was formed. The molecular weight distributions and the effect of ultrasound are shown in Fig. 4. As with the alkyl silanes, the sonochemical reaction produces higher molecular weights, the values in this case being M. = 7500, M, = 23750, y = 3.2 in the presence of ultrasound and M. = 4450, M, = 11680, y = 1.9 under conventional conditions. The high polydispersity in the sonochemical reaction is caused by the presence of a significant amount of high molecular weight polymer. The major difference between using an ultrasound bath and horn is that the latter gives much higher intensities. Variation of the intensity would give a method for controlling the molecular weight distribution during synthesis and also gives a possible explanation for differences in published results. Four polymerizations were carried out for 1 hr using the horn system under identical conditions except that the intensity of the ultrasound was varied. There was little variation in the yield of polymer. The molecular weight distributions of the resulting polymers are shown in Fig. 5 and clearly demonstrate that the ultrasound plays an important part in determining the course of the reaction. The most obvious explanation here is that high molecular weight material is subject to a degradation process. It is known [19] that poly(organosilanes) are degraded by a “back-biting” mechanism in the presence of sodium resulting in chain cleavage. In addition, ultrasonic degradation will occur at these molecular weights, a process known to be more efficient at higher intensity [29]. Sonochemical degradation of PMPS
Molecular waight Fig. 4. The effect of ultrasound on crown ether promoted synthesis of PMPS in toluene. - Reflux, IlOT, 1 hr; ---ultrasound, 30°C. 1 hr.
The chain breakage when polymers above a certain molecular weight in solution are exposed to ultrasound seems to be a universal phenomena, having been seen in organic, inorganic and aqueous
Poly(organosilane) synthesis using ultrasound systems [28,29]. It was recently reported for three poly(alkyl silanes) [39]. The effect is dependent on a large number of variables such as temperature, solvent composition, polymer concentration and ultrasound intensity. As an example, the molecular weights are shown for 5% PMPS solutions in toluene irradiated at 50 W cm-? in Fig. 6. The initially rapid decrease in M, which reaches a limiting value at longer times is characteristic of the sonochemical degradation. The limiting value of 32,000 is typical of a range of polymers, Kim et al. [33] having obtained a value of 50,000, although as noted above it will vary widely with the precise conditions used. Significantly, while the distributions showed the expected movement toward lower molecular weight and removal of the higher fraction, hardly any change in the proportion of oligomeric material was observed. The primary products of the degradation will be two macromolecular radicals. It is unlikely that these would play a part in the polymerization but could readily react with the solvent. Mechanistic
considerations
The detailed mechanism of the polymerization remains a topic for debate. However, it is generally agreed that it is a chain growth which proceeds via silyl anions generated at the surface of the sodium. Hence, the initiation step is the production of these anions, which is relatively slow. The lack of EPR signals during the polymerization led to the suggestion that this was a concerted process [ll, 401. More recent experiments using radical traps during the polymerization [33,35] have suggested that the initiation proceeds via two single electron transfer steps close to the surface of the sodium, the second of which is extremely rapid so that radical species do not play a part in the process. There is no evidence of radical involvement in the propagation steps. This type of heterogeneous sonochemical reaction has been the object of considerable study in low molar mass systems. It is generally accepted that purely ionic processes are unaffected by sonication [41,42] but those involving single electron transfers are preferentially accelerated. Indeed, it has been
Fig. 6. Effect of sonochemical degradation at 25°C on molecular weight distributions of PMPS. (For experimental conditions, see text. Values indicate sonication time in minutes).
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observed [43] in some heterogeneous sonochemical reactions that only the single electron transfer pathway is involved even where the major product arises from an ionic mechanism in the absence of ultrasound. Thus, it is reasonable to assume that the initiation step in the Wurtz coupling would be similarly accelerated. This would not only account for the faster rate of polymerization but an acceleration of initiation relative to propagation would also tend to reduce the polydispersity of the resulting polymers. Having accepted that a single polymerization mechanism is involved, a number of explanations for the polymodal molecular weight distributions have been suggested. The oligomeric fraction is mainly composed of cyclic Si, species resulting from end-biting during polymerization and the back-biting degradation. Jones et al. [lo] showed that the lower molecular weight polymer fraction was largely associated with the bulk solution, while the higher fraction(s) were associated with the sodium/NaCl precipitate in the system. This was explained by low molecular weight polymer saturating the solvent during the early stages of the polymerization so that in later reactions growing chains remained in proximity to the sodium and hence grow to longer lengths. Kim et al. [33] and Miller et al. [35] showed that a larger proportion of high polymer was formed when the polymerization was conducted at low temperature. This is consistent with the “solubility model” as saturation will occur with smaller amounts of material at lower temperatures. Lower degrees of thermal and chemical degradation to low molecular weight species also occur at reduced temperature. In the absence of ultrasound, the rate of reaction is unacceptably slow since the available surface area of the solid sodium is rapidly covered with polymer and NaCl. In the sonochemical method, microjets of solvent caused by cavitation in the vicinity of a sodium particle impinge on the surface and continually regenerate a fresh surface of sodium so that the reaction can continue. Thus, ultrasound allows the reaction to proceed at a lower temperature by promoting the desired chain growth in favour of unwanted degradation side reactions. Before the solution is saturated, this action must also remove some anchored chains from the surface. Hence, saturation would be expected earlier in a sonochemical system, leading to a more homogeneous chain growth process. The physical action of ultrasound will also cause enhanced mass transfer of monomer to the surface. These effects have been unambiguously demonstrated in electrochemical systems [44] where faster transport of reactant to an electrode and a reduced diffusion layer have been observed. In this polymerization, these effects would result in a faster chain extension step and hence to more rapid development of higher molecular weights. Under anhydrous, oxygen-free conditions, the polymeric silanes in solution will retain chloro- end groups and are thus capable of further reaction. In general, their diffusion back to the surface will be relatively slow but will be more efficient under sonication, particularly in the later stages of the reaction when most of the “monomer” will have been used up. This will lead to enhanced
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G. J. Price and A. M. Pate1
coupling of shorter chains at the surface again raising the molecular weight. Thus, as the polymerization proceeds, although the relative amount of high molecular weight material decreases as a result of degradation, some redistribution of material between different chains must occur since its molecular weight increases, as shown in Fig. 3. The lower molecular weight peak in the chromatogram moves to higher values again showing that there must be some redistribution of polymer chains. These enhancements would also contribute to the higher molecular weights obtained from systems containing crown ethers. Poly(organosilanes) have been shown to undergo ultrasonic degradation under similar conditions to those used for the polymerization. This process is a result of shear degradation caused by rapid solvent motion around cavitation bubbles and causes chain cleavage around the middle of the chain. The molecular weight is thus approximately halved but no oligomeric material is formed. It thus explains the reduction in the very high molecular weight components but cannot adequately account for all the effects described above. As the ultrasound intensity increases, more cavitation bubbles are produced and the strength of their collapse is also raised. Thus, each of these processes described here is more efficient, accounting for the results in Fig. 5. Again, a simple ultrasound degradation cannot be responsible since, although it would explain the absence of high molecular weight polymer at high intensity, it cannot account for the raising of the molecular weight of the lower fraction. The enhanced electron and mass transfer effects would explain the observations. The variation of distribution with intensity might explain some of the variable results in the literature. It also gives a potential method to achieve some degree of control over the final polymer properties although considerably more work on the various effects is needed before this would be a viable method. The Wurtz type polymerization is a complex, multi-phase system even in the absence of sonication and this has made its detailed study difficult. Introduction of ultrasound induces extra complications due to the various effects which it can cause. In this case, four effects play a part in enhancing the reaction. The microjetting and continual regeneration of a fresh sodium surface allows the reaction to proceed at a reasonable rate at lower temperatures so that unwanted side-reactions can be minimised. The increase in mass transfer provides a straightforward rate enhancement but a more subtle effect is the acceleration of the initiation step of generating the silyl anions at the surface. This not only increases the rate of reaction but acceleration of initiation relative to propagation also leads to a more homogeneous chain growth and hence a narrower distribution of chain lengths. Lower polydispersity also results from the sonochemical degradation of high molecular weight species. CONCLUSlONS
This work further illustrates that ultrasound can be a useful tool in the preparation of inorganic
polymers. It further confirms its benefits when applied to heterogeneous reaction systems. The major effects in the preparation of poly(organosilanes) are an enhancement of the initiation process and mass transfer of reactants together with a more homogeneous chain growth leading to the production of more even molecular weight distributions as well as a faster rate of polymerization. The ultrasonic degradation also limits the amount of high molecular weight material produced. However, variation of the ultrasound intensity allows some control over the resulting molecular weights although the competing influence of the degradation and the enhanced chain growth make precise modelling of the reaction difficult. Ackno+&dgemenr-We are grateful to the Science and Engineering Research Council for financial support of this work (Grant GR/F52910).
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Copyright 0 1996Elsevier ScienceLtd Printed in Great Britain. All r&hts reserved COM-3057/96 $15.00+ 0.00
THE APPLICATION OF ULTRASOUND TO THE SYNTHESIS OF POLY(ORGANOSILANES) G. J. PRICE* and A. M. PATEL School of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, U.K. (Received 4 September 1995; accepted in final form 26 October 1995)
Abstract-The effect of carrying out Wurtz type polymerizations of dichlorodiorganosilanes to poly(organosiIanes) under the influence of ultrasound has been investigated. The sonochemical reaction produced faster polymerizations and higher yields in a range of systems. Previous reports of modification of molecular weight distributions have been confirmed and the results used to elucidate the mechanism of the sonochemical enhancement. The control of molecular weight distributions by variation of the ultrasound properties is also illustrated. Copyright 0 1996 Elsevier Science Ltd
INTRODUCTION
There is rapidly growing interest in polymers with inorganic backbones [l, 21, among which silicon containing polymers are perhaps the major group. Polysiloxanes [3] have been used for some time as low viscosity, low surface energy fluids and in silicone elastomers. More recently, poly(organosilanes) have attracted considerable interest due to their potential as electro- and photoactive materials [4, 51. Delocalization of u-bond electrons in the main chain leads to strong absorption into the UV and after doping, to conductivity. Their photosensitivity has led to use as photoinitiators and as positive working microlithographic photoresists. They have also been used as precursors to ceramic materials although early promise in this area has not been fulfilled. This has largely been due to a lack of methodology for reproducibly synthesising the polymers on a reasonable scale. The usual method of synthesis [6-91 for poly(organosilanes) is a reductive, Wurtz type coupling of dichlorodiorganosilanes using molten sodium in refluxing toluene.
Na. Toluene
and > 105. The complex distribution has been attributed to concurrent reaction mechanisms although it has recently also been explained in terms of the heterogeneity of the system and the solubility characteristics of the components [lo]. More reproducible Wurtz reactions can be obtained by adding components such as diglyme or crown ethers to the solvent [l 1, 121, although this often limits the molecular weights achievable. Alternative methods for synthesising poly(organosilanes) which have been developed recently include the use of homogeneous reducing agents [ 131, dehydrogenative coupling of RR’H* using organotransition metal catalysts [14, 151, the anionic ring opening of strained cyclic organosilanes [ 161and the anionic polymerization of masked silylenes [ 171. However, while these can yield poly(organosilanes) with controlled structures, this is again usually at the expense of the chain length which remains relatively low. A good yield of poly(methy1 phenyl silane) with a monomodal molecular weight of N 65,000 was obtained at lower temperature by Jones et al. by reaction in refluxing diethyl ether [9]. More recent work has shown that thermal degradation to cyclic oligomers occurs around 150°C and this is
)
Reflux, 11O’C / sonicate, 25’C However, the reactions are irreproducible and the yields are rather low, -SO-55% at best, depending on the nature of R and R’. Also, the polymers often have a very wide, usually bi- or tri-modal, molecular weight distribution consisting of a low molecular weight (- 1000) cyclic and oligomeric fraction together with fractions in the region of 10,00@30,000 *To whom all correspondence should be addressed.
accelerated in the presence of alkali metals such as sodium [18, 191.Both of these facts suggest that better molecular weight distributions would be obtained by using low temperatures although the rate would be slow. Over the past two decades, the use of high intensity ultrasound has become a common technique in synthetic chemistry [20-221. A number of reactions, especially those in heterogeneous systems, have been
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G. J. Price and A. M. Pate1
shown to give enhanced rates and yields under ultrasound, leading to a branch of chemistry sometimes termed sonochemistry. Polymer sonochemistry in fact predates synthetic uses, the
irreversible cleavage of polymer chains when irradiated in solution having been reported in the 1930s [23]. More recently, ultrasound has been applied to polymer synthesis with some success [24, 251. In addition to rate and yield enhancements, perhaps the major advantage of performing sonochemical polymer synthesis is that it provides an additional means of molecular weight control. The main effects of sonication are due to cavitation or the growth and explosive collapse of microscopic bubbles on a microsecond timescale [26]. This can result in the formation of relatively high concentrations of excited species such as radicals [27] which can diffuse into solution and react. The rapid movement of solvent around the molecule also sets up shear gradients which stretch out and break the chains in a non-random, molecular weight dependent process resulting in a lowering of the molecular weight and polydispersity [28, 291. A second effect of this enhanced molecular motion is very efficient mixing and dispersion of multi-phase systems [20]. An extra effect occurs near a solid surface when cavitational collapse is asymmetric and results in a microjet of solvent impinging on the solid [21]. This acts to continually clean and refresh the surface as well as increasing the rate of transfer of reactants and products to and from the surface. The principle of applying ultrasound to producing poly(organosilanes) arose from the discovery in the early 1980s of the facile sonochemical coupling of chlorosilanes, RSiCl, over lithium metal to give R$iSiR, [30] a reaction which has been extended by using R2SiClz to give the polymeric materials. There are a number of effects which could be of potential benefit in using sonochemical methods for this polymerization. Firstly, there could be a preferential enhancement of one of the competing mechanisms to give more control over the molecular weight distribution. Secondly, the rate of polymerization should be faster, allowing the use of milder conditions such as lower temperatures. In addition, the control of molecular weight associated with the degradation could also be of use. The first reports of ultrasound being applied to the synthesis of poly(organosilanes) was by Kim et al. [31, 321. They described the production of materials with a single molecular weight distribution and polydispersity as low as 1.2, albeit in relatively low yields of I l-15% by carrying out the reaction at 60°C in toluene although the method worked only for organosilanes with aromatic substituents. Homopolymerization of dialkyl silanes was only possible in more polar solvent systems. An in depth study of the effect of organic substituent, solvent system and temperature was recently reported [33]. Sonochemitally assisted polymerization was also investigated by Miller et al. [34, 351 who reported somewhat conflicting results in that monomodal polymers were only obtained if diglyme or 1S-crown-S were added to the solvent. In their absence, bimodal distributions were obtained in which the high (_ 150,000) component comprised about 65% of the polymer, the
remainder being of low molecular weight (~9700). Weidman et al. have also used sonochemical methods in the synthesis of related polysilynes by reacting RSiCI, over sodium or NaK alloy [36, 371. In this paper, further study of the sonochemical polymerization of a number of dichlorodiorganosilanes is described. In particular, the effect on molecular weight distributions has been investigated and used to elucidate the mechanism of operation. Finally, the control of molecular weight by variation of the ultrasound conditions is described. EXPERIMENTAL Sonication
methods
Two sources of ultrasound were used. Much of the published sonochemical organic synthesis has been performed by immersing the reaction vessel into an ultrasonic cleaning bath [21]. Considerably higher sound intensities and better temperature control are achievable with a ‘horn’ type apparatus where the ultrasound vibrations are coupled directly into the reaction via a titanium alloy rod which dips into the reaction medium. In this work, a Kerry Ultrasonics Pulsatron 325 cleaning bath and a MSE ‘Soniprep 150’horn apparatus were used. The ultrasound intensities on the latter system were measured calorimetrically in the usual way [38] and temperature control was achieved to +_O.l”C by circulating thermostatted water through a jacketted vessel. The apparatus has been described previously [27]. Polymer
degradation
To investigate the ultrasonic degradation, a sample of poly(methyl phenyl silane) produced during the course of this work was used. A solution of 5% (w/v) concentration was prepared in HPLC grade toluene (Aldrich). 100 cm’ of solution was placed in the jacketted vessel and allowed to attain thermal equilibrium. The sonicator was switched on and small samples (_ 0.24.3 cm’) removed periodically for analysis. Polymer
analysis
Polymer molecular weights were recorded on a Bruker LC21/41 gel permeation chromatograph using tetrahydrofuran as the eluent and ultraviolet detection. Ten polystyrene standards (Polymer Laboratories Ltd) with molecular weights between 1020 and 2.65 x IO6were used as cahbrants and all values reported below are ‘polystyrene equivalents’. Polymekation
procedure
The procedure for the preparation of poly(organosilanes) was modified from those in the literature [33. 341.Typically, 2.8 g (0.12 mol) sodium (Aldrich) was added to 50 cm’ of dry toluene under nitrogen. This was brought to reflux and stirred vigorously to disperse the metal. The dichlorodiorganosilane (0.1 mol, Petrarch Ltd) was added dropwise to avoid excessive heat release. All the glassware used for polymerizations and work-up was wrapped in aluminium foil to exclude light. The reaction was allowed to proceed for the appropriate time before being quenched by the addition of propan-2-01. In the ultrasonic experiments, the sodium was dispersed by sonicating on full power for 30 min at the temperature of the experiment prior to adding the silane. The polymers were recovered by dropwise addition of propan-2-01 to quench any unreacted sodium followed by precipitation into a large excess of ice-cold propan-2-01. The resulting solid was recovered by filtration and purified by multiple reprecipitation into propan-2-01 from toluene. RESULTS AND DISCUSSION
Reactions of the dimethyl or diphenyl produced insoluble materials, presumably
silanes highly
Poly(organosilane) Table 1. Organosilane Dihexyl Methyl hexyl Methyl cyclohexyl
synthesis using ultrasound
Effectof ultrasound on preparation of poly(dialkyl silanear
ConventionaP Yield/% M. :t 19
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11,200 3100 4900
Ultrasound’ Yield/% M.
Y 3.5 7.0 2.9
25 41 33
Y
31,000 4700 8600
3.4 9.5 2.7
‘I hr reaction in toluene containing 1% l&crown-6. “Reflux, 110°C. ‘Ultrasound bath at 253O’C.
crystalline, the ‘polymeric’ nature of which is uncertain, particularly with the diphenyl compound. Both systems showed enhanced yields of the polymer when the sodium was dispersed using ultrasound and the reaction carried out using a cleaning bath. To illustrate the effects, with dichlorodiphenyl silane, the yields were 30% from a 5 hr relIux at 110°C in toluene compared with 55% from a room temperature reaction for 1 hr under ultrasound. The corresponding values with the dimethyl compound were 12 and 28%, respectively. Rather surprisingly in view of the insolubility of the homopolymers, polymerization of an equimolar mixture of the diphenyl- and dimethyl compounds gave a reasonable yield (-35%) of a soluble, high molecular weight polymer with a bimodal molecular weight distribution. Copolymers of both compounds with the methyl phenyl silane were also prepared. Further work concerned organosilanes which yield soluble polymers so that they could be fully characterized. Synthesis of poly(dialkyl silanes)
In common with previously published work, poly(organosilanes) with alkyl substituents such as dihexyl, methyl cyclohexyl and methyl hexyl could only be produced with very low yields (2-5%) by reaction in toluene alone, even in the presence of ultrasound. It should be noted that the polymers showed the usual wide molecular weight distribution including components with very large values. It has been reported 111, 121 that the addition of crown ethers which complex with sodium improves the distribution but gives low molecular weights. Table 1 shows the effect of ultrasound on the polymerizations
106
10s 104 Moleoulu weight
(1 hr) at 25°C in the presence of 18-crown-6 at a concentration of 1% of the silane in toluene. In each case, the sonochemical reaction gave higher yields and somewhat higher molecular weights. These results are presented for comparison purposes and are not optimized. The M. values are somewhat misleading due to the large polydispersities and the samples do contain some relatively long chains as shown in the distributions in Fig. 1. None of the three polymers had a single distribution and in particular that for polyfhexyl methyl silane) was unusually broad with two distinct polymer components. Synthesis of PMPS
Most effort has concentrated on the synthesis of poly(methy1 phenyl silane), PMPS, which polymerizes more rapidly and can be produced in higher yields. It has also been the object of most photochemical and application studies. In initial experiments, the conventional reflux method in toluene at 110°C afforded a yield of 15% after 1 hr. Carrying out the polymerization of 60°C for the same time in an ultrasound bath and with a “horn” system yielded 35 and 43%, respectively. The corresponding values at 25°C were 19 and 29%, respectively. These values are not optimized and do not indicate the best yields obtainable by either the ultrasonic or conventional methods. Higher yields could be obtained by allowing the reaction to proceed for longer or by changing the solvent system but these unoptimized results demonstrate that considerably higher yields and faster polymerizations rates can be obtained using ultrasound than by conventional methods. Perhaps of more significance are the changes in the molecular weights and distributions of the polymers,
10’
Fig. 1. Molecular weight distributions of sonochemically produced poly(dialkyl silanes): - poly(cyclohexyl methyl silane); -poly(n-hexyl methyl silane); ---- poly(di n-hcxyl silane).
I
I
106
I
I
IO'
104 Molecular weight
I
10’
Fig. 2. The effect of ultrasound sources on the molecular weight distributions of poly(methy1 phcnyl silane).
1292
G. J. Price and A. M. Pate1 A
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Molecular weight Fig. 3. Molecular weight distributions during sonochemical preparation of poly(methy1 phenyl silane). (The values
indicate the reaction time in minutes). examples of which are shown in Fig. 2. The conventional reflux method gives a polymer with a very wide, bimodal distribution. The amount of high molecular weight material, and hence the polydispersity, is vastly reduced under the influence of ultrasound and use of the high intensity probe system gave a monomodal, though broad, distribution at slightly higher molecular weight than the lower component of the other distributions. Note that here, only the polymeric part of the distribution is being considered. There is an additional peak in the chromatograms due to oligomeric materials which could, if desired, be straightforwardly removed by fractionation. To investigate the possible origin of these differences, the molecular weight distribution was periodically measured during a reaction performed on the ultrasound “horn”. These are shown in Fig. 3. It is clear that high molecular weight material is formed at an early stage but that it is degraded as the reaction proceeds. Notably, in addition to the reduction in the relative amount of the longer component, it also shifts to higher molecular weight. Thus, chain growth must continue throughout the
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104
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Molecular weight Fig. 5. Effect of ultrasound intensity on the polymerization of methyl phenyl silane. (Values indicate ultrasound intensity in W cm-*).
reaction and a simple degradation does not fully explain the results. Polymerizations were then conducted in the presence of l&crown-6. The reactions were faster under sonication and produced above twice the amount of polymer for a 1 hr reaction. In common with previous work, a single, low molecular weight polymer was formed. The molecular weight distributions and the effect of ultrasound are shown in Fig. 4. As with the alkyl silanes, the sonochemical reaction produces higher molecular weights, the values in this case being M. = 7500, M, = 23750, y = 3.2 in the presence of ultrasound and M. = 4450, M, = 11680, y = 1.9 under conventional conditions. The high polydispersity in the sonochemical reaction is caused by the presence of a significant amount of high molecular weight polymer. The major difference between using an ultrasound bath and horn is that the latter gives much higher intensities. Variation of the intensity would give a method for controlling the molecular weight distribution during synthesis and also gives a possible explanation for differences in published results. Four polymerizations were carried out for 1 hr using the horn system under identical conditions except that the intensity of the ultrasound was varied. There was little variation in the yield of polymer. The molecular weight distributions of the resulting polymers are shown in Fig. 5 and clearly demonstrate that the ultrasound plays an important part in determining the course of the reaction. The most obvious explanation here is that high molecular weight material is subject to a degradation process. It is known [19] that poly(organosilanes) are degraded by a “back-biting” mechanism in the presence of sodium resulting in chain cleavage. In addition, ultrasonic degradation will occur at these molecular weights, a process known to be more efficient at higher intensity [29]. Sonochemical degradation of PMPS
Molecular waight Fig. 4. The effect of ultrasound on crown ether promoted synthesis of PMPS in toluene. - Reflux, IlOT, 1 hr; ---ultrasound, 30°C. 1 hr.
The chain breakage when polymers above a certain molecular weight in solution are exposed to ultrasound seems to be a universal phenomena, having been seen in organic, inorganic and aqueous
Poly(organosilane) synthesis using ultrasound systems [28,29]. It was recently reported for three poly(alkyl silanes) [39]. The effect is dependent on a large number of variables such as temperature, solvent composition, polymer concentration and ultrasound intensity. As an example, the molecular weights are shown for 5% PMPS solutions in toluene irradiated at 50 W cm-? in Fig. 6. The initially rapid decrease in M, which reaches a limiting value at longer times is characteristic of the sonochemical degradation. The limiting value of 32,000 is typical of a range of polymers, Kim et al. [33] having obtained a value of 50,000, although as noted above it will vary widely with the precise conditions used. Significantly, while the distributions showed the expected movement toward lower molecular weight and removal of the higher fraction, hardly any change in the proportion of oligomeric material was observed. The primary products of the degradation will be two macromolecular radicals. It is unlikely that these would play a part in the polymerization but could readily react with the solvent. Mechanistic
considerations
The detailed mechanism of the polymerization remains a topic for debate. However, it is generally agreed that it is a chain growth which proceeds via silyl anions generated at the surface of the sodium. Hence, the initiation step is the production of these anions, which is relatively slow. The lack of EPR signals during the polymerization led to the suggestion that this was a concerted process [ll, 401. More recent experiments using radical traps during the polymerization [33,35] have suggested that the initiation proceeds via two single electron transfer steps close to the surface of the sodium, the second of which is extremely rapid so that radical species do not play a part in the process. There is no evidence of radical involvement in the propagation steps. This type of heterogeneous sonochemical reaction has been the object of considerable study in low molar mass systems. It is generally accepted that purely ionic processes are unaffected by sonication [41,42] but those involving single electron transfers are preferentially accelerated. Indeed, it has been
Fig. 6. Effect of sonochemical degradation at 25°C on molecular weight distributions of PMPS. (For experimental conditions, see text. Values indicate sonication time in minutes).
1293
observed [43] in some heterogeneous sonochemical reactions that only the single electron transfer pathway is involved even where the major product arises from an ionic mechanism in the absence of ultrasound. Thus, it is reasonable to assume that the initiation step in the Wurtz coupling would be similarly accelerated. This would not only account for the faster rate of polymerization but an acceleration of initiation relative to propagation would also tend to reduce the polydispersity of the resulting polymers. Having accepted that a single polymerization mechanism is involved, a number of explanations for the polymodal molecular weight distributions have been suggested. The oligomeric fraction is mainly composed of cyclic Si, species resulting from end-biting during polymerization and the back-biting degradation. Jones et al. [lo] showed that the lower molecular weight polymer fraction was largely associated with the bulk solution, while the higher fraction(s) were associated with the sodium/NaCl precipitate in the system. This was explained by low molecular weight polymer saturating the solvent during the early stages of the polymerization so that in later reactions growing chains remained in proximity to the sodium and hence grow to longer lengths. Kim et al. [33] and Miller et al. [35] showed that a larger proportion of high polymer was formed when the polymerization was conducted at low temperature. This is consistent with the “solubility model” as saturation will occur with smaller amounts of material at lower temperatures. Lower degrees of thermal and chemical degradation to low molecular weight species also occur at reduced temperature. In the absence of ultrasound, the rate of reaction is unacceptably slow since the available surface area of the solid sodium is rapidly covered with polymer and NaCl. In the sonochemical method, microjets of solvent caused by cavitation in the vicinity of a sodium particle impinge on the surface and continually regenerate a fresh surface of sodium so that the reaction can continue. Thus, ultrasound allows the reaction to proceed at a lower temperature by promoting the desired chain growth in favour of unwanted degradation side reactions. Before the solution is saturated, this action must also remove some anchored chains from the surface. Hence, saturation would be expected earlier in a sonochemical system, leading to a more homogeneous chain growth process. The physical action of ultrasound will also cause enhanced mass transfer of monomer to the surface. These effects have been unambiguously demonstrated in electrochemical systems [44] where faster transport of reactant to an electrode and a reduced diffusion layer have been observed. In this polymerization, these effects would result in a faster chain extension step and hence to more rapid development of higher molecular weights. Under anhydrous, oxygen-free conditions, the polymeric silanes in solution will retain chloro- end groups and are thus capable of further reaction. In general, their diffusion back to the surface will be relatively slow but will be more efficient under sonication, particularly in the later stages of the reaction when most of the “monomer” will have been used up. This will lead to enhanced
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G. J. Price and A. M. Pate1
coupling of shorter chains at the surface again raising the molecular weight. Thus, as the polymerization proceeds, although the relative amount of high molecular weight material decreases as a result of degradation, some redistribution of material between different chains must occur since its molecular weight increases, as shown in Fig. 3. The lower molecular weight peak in the chromatogram moves to higher values again showing that there must be some redistribution of polymer chains. These enhancements would also contribute to the higher molecular weights obtained from systems containing crown ethers. Poly(organosilanes) have been shown to undergo ultrasonic degradation under similar conditions to those used for the polymerization. This process is a result of shear degradation caused by rapid solvent motion around cavitation bubbles and causes chain cleavage around the middle of the chain. The molecular weight is thus approximately halved but no oligomeric material is formed. It thus explains the reduction in the very high molecular weight components but cannot adequately account for all the effects described above. As the ultrasound intensity increases, more cavitation bubbles are produced and the strength of their collapse is also raised. Thus, each of these processes described here is more efficient, accounting for the results in Fig. 5. Again, a simple ultrasound degradation cannot be responsible since, although it would explain the absence of high molecular weight polymer at high intensity, it cannot account for the raising of the molecular weight of the lower fraction. The enhanced electron and mass transfer effects would explain the observations. The variation of distribution with intensity might explain some of the variable results in the literature. It also gives a potential method to achieve some degree of control over the final polymer properties although considerably more work on the various effects is needed before this would be a viable method. The Wurtz type polymerization is a complex, multi-phase system even in the absence of sonication and this has made its detailed study difficult. Introduction of ultrasound induces extra complications due to the various effects which it can cause. In this case, four effects play a part in enhancing the reaction. The microjetting and continual regeneration of a fresh sodium surface allows the reaction to proceed at a reasonable rate at lower temperatures so that unwanted side-reactions can be minimised. The increase in mass transfer provides a straightforward rate enhancement but a more subtle effect is the acceleration of the initiation step of generating the silyl anions at the surface. This not only increases the rate of reaction but acceleration of initiation relative to propagation also leads to a more homogeneous chain growth and hence a narrower distribution of chain lengths. Lower polydispersity also results from the sonochemical degradation of high molecular weight species. CONCLUSlONS
This work further illustrates that ultrasound can be a useful tool in the preparation of inorganic
polymers. It further confirms its benefits when applied to heterogeneous reaction systems. The major effects in the preparation of poly(organosilanes) are an enhancement of the initiation process and mass transfer of reactants together with a more homogeneous chain growth leading to the production of more even molecular weight distributions as well as a faster rate of polymerization. The ultrasonic degradation also limits the amount of high molecular weight material produced. However, variation of the ultrasound intensity allows some control over the resulting molecular weights although the competing influence of the degradation and the enhanced chain growth make precise modelling of the reaction difficult. Ackno+&dgemenr-We are grateful to the Science and Engineering Research Council for financial support of this work (Grant GR/F52910).
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