Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
  • C08G77/00—Macromolecular compounds obtained by reactions forming a linkage containing silicon with or without sulfur, nitrogen, oxygen or carbon in the main chain of the macromolecule
  • C08G77/60—Macromolecular compounds obtained by reactions forming a linkage containing silicon with or without sulfur, nitrogen, oxygen or carbon in the main chain of the macromolecule in which all the silicon atoms are connected by linkages other than oxygen atoms

Definitions

• This invention relates to improved methods and techniques for preparing polysilanes in increased yields, higher molecular weights and/or lower pdlydispersities, and for controlling the polymerization of silanes to form polysilanes.
• R 1 , R 2 ALKYL, ARYL, H, VINYL to n > 2000 ARALKYL
• Polysilanes are prepared by the reductive Wurtz-type coupling of the corresponding dichlorosilanes with an alkali metal, typically sodium (Scheme 1). Copolymers can be synthesized by using a mixture of two (or more) dichlorosilane monomers. This method generally gives a mixture of linear polymer and cyclic low molecular weight oligomers. For most of the aforementioned applications, the cyclics fraction is of little or no value. It is desirable to shift the course of the reaction to production .of as much .linear polymer as possible at the expense of the cyclics.
• this reaction normally produces a molecular weight distribution in the linear polymer fraction which is highly polydisperse and has at least two (and often more) molecular weight modes.
• This extremely high polydispersity is particularly detrimental to the proposed application of polysilanes as photoresists or imageable oxygen reactive ion etch barriers, since it can lead to low apparent photospeed, degradation of physical properties, or both.
• Polysilanes have been prepared by the reaction in Scheme 1 since the first preparation of the intractable (Ph 2 Si) n by Kipping, J.Chem.
• Patent 2,606,879 (1952) describes the preparation of various silane homopolymers as greasy or wax-like mixtures by essentially the same method using Na metal but without the autoclave. No specific molecular weight characterization is provided.
• a related disclosure is Jap. Kokai Tokyo Koho JP 58,185,628.
• Yajima, et al. (U.S. Patent 4,052,430 (1977)), disclosed the preparation of low molecular weight branched and/or polycyclic polycarbosilanes (-Si-C-Si- main chain polymers) from (Me 2 Si) n prepared essentially by the method of Burkhard.
• a similar preparation of (Me 2 Si) n as a precursor to polycarbosilanes is reported by Iwai, et al., (U.S. Patent 4,377,677 (1983)). In neither of these latter cases is information provided regarding the MW distribution of the (Me 2 Si) n prepared, but one must assume that it is similar to that reported by Burkhard since the synthetic method is the same.
• a patent by Peterson, et al. discloses an unusual variant of the Na mediated coupling of dichlorosilanes shown in Scheme 1.
• the reduction is carried out in tetrahydrofuran using Li metal as reductant and a monochlorosilane, R 1 R 2 SiHCl, as the silicon monomer.
• LiCl and H 2 gas are produced as byproducts.
• This method is claimed to be particularly useful for producing cyclic oligomeric silanes rather than high polymer. It is, thus, not applicable to the production of linear high polysilanes.
• a Yajima, et al., patent (U.S. 4,159,259) involves various procedures for the preparation of polycarbosilanes and polycarbosilanes containing various metallic species which are useful as precursors to silicon carbide.
• Several of the examples demonstrate the use of various cyclic or linear polysilanes as precursor materials.
• two different synthesis procedures for (Me 2 Si) n are given. In example 8, sodium and Me 2 SiCl 2 are reacted in an unspecified solvent to give (Me 2 Si) n as an insoluble solid of unspecified molecular weight.
• a patent to Nitzsche, et al., (U.S. 3,830,780) discloses a process for condensing linear, hydroxy-terminated polysiloxanes to higher polysiloxanes by heating in the presence of an optional aluminum catalyst. No polysilane structures are mentioned and the chemistry of the condensation process yields Si-O-Si bonds in the polymer backbone.
• the Baney, et al. processes provide only low molecular weight products by an equilibration process of disilanes rather than by Na-mediated condensation of dichlorosilanes.
• a patent to Schilling, Jr., et al. (U.S. 4,472,591) describes methods for the preparation of hydrosilyl modified polycarbosilanes. Even though some Si-Si backbone bonds are formed when the various silanes are condensed with alkali metals by the procedures described in this patent, the branched polycarbosilanes obtained are only distantly related to linear high molecular weight polysilanes.
• the nature of the solvent characteristics which will be chosen to effect the desired improvement in polymer properties will be different for systems wherein the monomer is added to a dispersion of the solid agent (normal addition) versus systems wherein the solid agent is added to a solution of the monomer (inverse addition), as will be discussed further in detail below.
• the method is fully applicable to homopolymers and copolymers.
• the solvent will be a compound solvent comprising at least two individual solvents. This invention provides a unique method for determining the ratio of components in such a solvent mixture which enables achievement of the desired control over polysilane properties.
• these objects have been achieved by providing a method of decreasing the polydispersity of a polysilane prepared by reductively condensing corresponding silane monomers on a solid surface-reacting agent dispersed in an inert solvent, comprising adding the monomers to a dispersion of the solid agent at a substantially constant rate or adding the solid agent to a solution of the monomers at a substantially constant rate.
• This aspect of the invention is applicable to any order of addition of the reaction components.
• these objects have been achieved by providing a method of preparing a polysilane comprising polymerizing the corresponding monomers by adding an effective solid surface-reacting agent to a solution of the monomers in a compound solvent which is a mixture of at least two component solvents, the relative amounts of which are chosen to minimize the compatibility of the growing polymers therein without causing the polymer to precipitate; or a method of preparing a polysilane comprising polymerizing the corresponding monomers by adding them to a dispersion of a surface-reacting agent in a compound solvent which is a mixture of at least two component solvents, the relative amounts of which are chosen to maximize the compatibility of the growing polymer therein.
• the method involves calculating the solubility parameter of the polysilane, adding to said solubility parameter said optimum solubility parameter differential to obtain a desired solvent solubility parameter, and determining the relative amounts of said two individual solvents in said mixture by calculating the corresponding composition whose solubility parameter matches said desired solvent solubility parameter.
• these objects have more generally been achieved by providing a method of controlling the yield of a polymer of a desired molecular weight and/or polydispersity prepared in a heterogeneous reaction wherein solid effective surface-reacting agent is added to a solution of the corresponding monomers, comprising adjusting reaction conditions to minimize access of the monomers to the polymer chains growing at the surface of the solid agent when it is desired to increase molecular weight and/or decrease polydispersity, or to maximize access of the monomers to the polymer chains growing at the surface of the solid agent when it is desired to decrease molecular weight and/or increase polydispersity; or a method of controlling the molecular weight and/or polydispersity of a polymer prepared by adding monomers to a dispersion of solid effective agent, comprising adjusting reaction conditions to maximize access of the monomers to the polymer chains growing at the surface of the agent when it is desired to increase molecular weight and/or decrease polydispersity, or to minimize access of the monomers to the polymer chain'
• these objects have been achieved by providing a method of controlling the molecular weight and/or polydispersity of a polysilane prepared in a reductive condensation of corresponding silane monomers on a solid surface-reacting agent dispersed in an inert solvent, comprising carrying out the reaction with an effective amount of a surfactant having a polar end which can bond to said solid agent.
• Figures 1-4 show various effects of varying conditions on the nature of polysilane produced upon polymerizing the corresponding dichlorosilane
• Figure 5 shows the effect on polysilane properties of solubility parameter difference in inverse addition mode.
• this invention provides the ability of achieving simultaneously, high molecular weights, e.g., for many polysilanes, greater than 30,000 daltons in greater than 90% of the distribution; low polydispersities ( in many cases), wherein for polymers which usually give bimodal distributions, this criterion is applied separately to each mode; increased yields of polymers having such characteristics; and high reproducibility under well-defined conditions.
• N refers to normal addition mode, i.e., the addition of monomers to the sodium dispersion reductant dispersed in the reaction solvent;
• I refers to the inverse addition mode wherein the sodium dispersion is added to the monomers dissolved in the solvent of interest.
• this invention can be discussed in terms of the following mechanism. It is theorized that a major factor in controlling the nature of the polysilane which results from reductive condensation of silanes, typically dichlorosilanes, is the compatibility of the growing polymer chain with the reaction solvent. It is theorized that this compatibility affects the yield and molecular weight of the polymer by controlling the conformation of the growing chains and their degree of coverage of the active surfaces whereby diffusion of the monomer to the surface is controlled.
• the optimum solvent for achieving a desired combination of yield, molecular weight and/or polydispersity of a polysilane can be determined by selecting a solvent solubility parameter which optimizes the achievement of the desired polymer characteristics.
• a preferred method of determining the necessary solubility characteristics for the solvent and for determining a corresponding solvent identity is as follows.
• the variation in polymer properties as a function of the solubility characteristics of the solvent is determined, e.g., as shown in Figure 5 below.
• a convenient means for experimentally varying the solubility properties of a solvent is to employ a solvent mixture and to vary the relative amounts of the components. As can be seen from Figure 5, there is an optimum solvent at which high molecular weight and high yield can be achieved in the particular system employed in the reported experiment.
• the solubility parameter ( ⁇ ) corresponding to this solvent can be routinely calculated using well-known procedures. Similarly, the solubility parameter can be calculated for the particular polysilane being produced.
• the optimum solubil ity parameter difference using the systems of the above references is in the range of about 0.4 to about 0.9, preferably, 0.5 to 0.8, and most preferably 0.6 to 0.7. (If other models are used, a corresponding differential, of course, can be calculated.) This reflects the mentioned balance between low compatibility and the need to avoid precipitation. Given this value, the optimum polymerization solvent for any dichlorosilane/homosilane or mixture of dichlorosilanes/copolysilanes can be readily calculated by a simple procedure.
• the solubility parameter is calculated for the polymer or copolymer involved, e.g., using the tabulated data in the references mentioned above enabling use of routine procedures to calculate the parameter from known group molar attraction constants.
• the preselected optimum solubility parameter difference for the combination of properties desired is then added to the polymer solubility parameter. Where high yield of high molecular weight polymer is desired, a value of 0.6 to 0.7 or other selected differential would be added to the calculated polymer solubility parameter.
• solvents of interest are chosen in accordance with the usual considerations for choosing solvents for the preparation of polysilanes.
• a mixture of solvents will be chosen. Again using standard data and calculation methods the relative amounts of the components of the mixture can be selected to provide an overall solvent solubility parameter equal to that calculated above.
• a dispersing agent used for the solid agent e.g., sodium
• its contribution to the calculated solubility parameter should be taken into account. This can have a significant effect on the calculated values where the amount of dispersant (e.g., mineral oil or mineral spirits) is, e.g., about 2% or more in typical situations. This will be particularly important for N addition modes, since the dispersant is present in its full amount from the beginning of the reaction.
• these steps are carried out with a computer using a corresponding program straightforwardly carrying out the necessary calculations.
• the solvents/cosolvents utilizable in conjunction with this invention include all solvents which are employable in conjunction with polysilane polymerizations.
• the solvents must solubilize both the polymer and the corresponding monomer and must not solubilize the solid agent, e.g., sodium.
• the solvent must also not participate in chain transfer reactions and must be inert to sodium or other agent.
• Suitable solvents include hydrocarbons such as arenes, e.g., toluene, xylene, tetralin, benzene, etc., and alkanes, e.g., hexane, heptane, octane, nonane, decane, up to about tetradecane. Both straight chain and branched alkanes are employable.
• Other compatible solvents include ethers such as glyme, diglyme, triglyme, tetrahydrofuran, etc., and others having no active hydrogen atoms.
• Choice of the identity per se of the solvent is not critical provided the solvent is inert and does not participate in chain transfer reactions as mentioned; it is the overall solubility parameter of the resultant solvent which is most important in accordance with this invention as explained above.
• solvents with the same solubility parameters might provide improvements of varying degrees, e.g., due to system-specific factors.
• the active surface is normally provided by solid particles; however, the principles of this invention will be generically applicable to any similar two phase system.
• the amount of sodium is typically in slight excess, e.g., up to about a 20% excess.
• the sodium or other reductant can be employed as available commercially, e.g., as a dispersion in light oil, mineral spirits or paraffin (commonly 40% for Na). The latter can be directly suspended in the reaction solvent.
• the dispersion can be conveniently generated in the reaction vessel from chunk sodium.
• concentration of monomers employed is typically limited by the final viscosity which results. In the normal addition mode, it is generally preferred to have a monomer concentration which is as high as possible. Typically, values of 30-60 weight percent of monomer are used. In the inverse addition mode, typically, amounts of monomer of 20-35% are utilized.
• the monomers can be added to the suspension of sodium neat or preferably as a solution in the chosen solvent. Typically, the monomer solution is added to a sodium dispersion containing a minimum amount of solvent, the final reaction medium containing the mentioned 30-60% of monomer.
• the term "monomer” has a broad meaning herein. For example, it includes not only Cl-Si (RR')-Cl compounds but also corresponding dimers, trimers, and other oligomers and prepolymers which may be used as starting materials or formed in situ. All corresponding bifunctional species which can be polymerized to form the desired end polymer such as a polysilane are included.
• the reaction is generally conducted at a temperature from above the melting point of sodium up to the boiling point of the solvent, e.g., from about 90°C to about 250°C. Pressure is not critical but may be increased where the chosen solvent boils at a relatively low temperature, e.g., lower than the melting point of sodium.
• the time of reac tion is also not critical. In many instances, after addition of monomer/catalyst is complete, the reaction will be completed in a few minutes, e.g., often 5-15 minutes. However, it is preferred to continue the reaction at reflux for 0.5 to 2 hours.
• reaction it is very important to maintain a purity which is as high as possible during the reaction, not only to obtain as pure a product as possible but also because the reaction is sensitive to impurities. Since the latter tend to contaminate the sodium surface, their effects are magnified. As a result, it is preferred that the reaction be conducted under an inert atmosphere, e.g., nitrogen, argon, etc, and that all reagents be anhydrous and of the highest purity. Typically, the reaction is stirred with standard mechanical means sufficient to ensure sufficient mixing of the reactants to provide good contact between the catalyst and the reacting species. Overly vigorous stirring is to be avoided, e.g., the use of ultrasound is deleterious.
• fractionation can be fully conventionally achieved, e.g., using chromatographic, precipitation or other techniques.
• the overall yields of polysilanes produced in accordance with the improved process of this invention are improved by factors of 3-5.
• yields are 30-70% or higher, depending on the mode of addition and particular polymer involved.
• Lower or higher yields are within the scope of this invention also.
• this invention has particularly advantageous applicability to the preparation of highly hindered polysilanes which are difficult to prepare due to the effects of steric hindrance but which are highly photosensitive and thus very useful as photoresists. For such polymers even seemingly low yields which might be enabled by this invention would represent a major advance over the prior art.
• the yield of a desired fraction of molecular weight and/or polydispersity can be significantly increased over those heretofore achievable. This is especially true in situations where high molecular weight polysilanes are desired since high yields and high molecular weights typically are simultaneously achieved, along with lowered polydispersity. Conversions are essentially 100% in all cases.
• a very broad range of polysilanes can be prepared using the improved process of this invention. Typical polysilanes are those described in U.S. Applications Serial Nos. 597,005, and 616,148 incorporated by reference above.
• polysilanes can also be prepared by the process of this invention including those wherein the side chains are a wide variety of hydrocarbon and heterocyclic groups, e.g., alkyl, cycloalkyl, aryl, etc., or combinations thereof, any portion of which can be substituted by a wide variety of reaction compatible or blocked substituents, e.g., including alkyl, alkoxy, hydroxy, acyl, etc.
• the side chains can be acyclic, cyclic or a combination thereof.
• the rate of addition should be kept within ⁇ 10% of the nominal value, preferably ⁇ 5%, most preferably ⁇ 1%.
• Such constancies can be conventionally achieved using syringe pumps or, on larger scales, commercially available peristaltic pumps.
• the rate of addition will be in the range of 80-640 milli-equivalents per minute, the exact rate not being critical. In general, it is desired to add the reagent as rapidly as possible while maintaining constancy of rate.
• Suitable surfactants include all those which have a polar end which is bondable to the reactive surface.
• reductants such as sodium
• anionic surfactants are most suitable; however, as long as a surfactant species bonds to the surface and provides a non-polar portion extending out from the surface, it is employable in this aspect of the invention.
• anionic surfactants such as stearic acid
• Such polysilanes are included within the term "surfactant" within the context of this invention.
• the precise amount of surfactant added can routinely be determined in accordance with this disclosure and perhaps in conjunction with a few routine preliminary experiments. Very small amounts typically will be effective, e.g., amounts in the range of 0.1-500 mg/l of reaction solvent, preferably 10-100 mg/l of reaction solvent. If the amount added is too high, yields may be reduced presumably because of the sensitivity of the polymerization reaction to trace polar impurities of any nature.
• the surfactant provides a microenvironment around active solid particles wherein non-polar chains extend out from the surface.
• addition of surfactant tends to increase molecular weight.
• inverse mode additions wherein such a microstructure is adverse to the production of high molecular weight polymers, the molecular weight correspondingly decreases.
• the molecular weight of the resultant polymer can be controlled in either mode.
• the nature of the polymer which is achieved can also be controlled by varying the particle size of the solid-reacting surface.
• the yield of high molecular weight polymer will be increased by increasing the particle size, thereby decreasing the ratio of surface area to monomer concentration.
• a decrease in particle size will increase molecular weight because it results in increased surface area and, consequently, an increase in the ratio of surface area to monomer.
• the particle size of sodium when employed will be that which is commercially available, e.g., around 5 micrometers. This is also the particle size range conventionally achieved when a sodium dispersion is prepared by rapidly stirring a melt in a solvent. Larger particle sizes can be employed in normal addition reactions for the reasons given above, as long as the reaction rate does not become unacceptably small due to the concomitant decrease in surface area.
• a sodium amalgam can be used in place of the conventional sodium metal reductant. This has no significant effect on the desirable properties of the polymer; however, the reactions are more easily controlled since they are slowed down. As a result, overall process safety is improved. Production of the amalgam is fully conventional.
• the relative content of sodium is not critical, e.g., it can be from 5:1 to 1:5, a ratio of about 1:1 being preferred (mole/mole).
• 320 meq/min refers to addition of 32% of the stoichiometric amount in 1 minute; Mn is the number-average molecular weight; Mw is the weight-average molecular weight; “polydispersity” is the ratio Mw/Mn; "modal" molecular weight is the most prevalent single molecular weight within a single peak in the total distribution; and all molecular weights are determined by size exclusion chromatography and are quoted as “polystyrene equivalent" values.
• PhMeSiCl 2 polymerization is found to be much more sensitive to the mode of addition than n-dodecyl MeSiCl 2 , at least as indicated by the molecular weight. For this reason PhMeSiCl 2 has been used extensively to probe the reaction mechanism and investigate revised reaction conditions. Tables 3 and 4 show the results for preparation of a variety of polysilane hompolymers and copolymers, respectively.
• the first through third entries are telling.
• the Na/toluene and. Na/DME (glyme) results are typical of reactions run under heterogeneous conditions.
• a soluble Na/biphenyl reductant is used, however, no polymer is formed at all, only cyclics (cf. a 28% yield of 9,000 molecular weight polymer formed with insoluble Na metal as reductant).
• the yield and molecular weight of polymer formed has been shown to be strongly dependent on the mode and rate of reagent addition and the degree of constancy of the addition rate.
• Example 3 Although tetralin or decane are considerably easier to handle than the ether solvents, they are still somewhat expensive. However, as shown in Figure 4, the effects of the expensive solvents previously discussed can be effectively mimicked by use of heptane, a cheap solvent which is one of the major components in gasoline.
• the polydispersity of the high molecular weight mode of materials prepared in 9:1 toluene/heptane is considerably less than that of material prepared in pure toluene. This reduction in polydispersity, in addition to the increased yields, is a major advantage of this solvent modification.
• silylene trapping agent Et 3 SiH
• Et 3 SiH the highly efficient silylene trapping agent
• silylene intermediates if formed at all in these reactions, are not on the pathway to high polymer.
• silyl anions might be crucial intermediates in polymer formation, based on the fact that addition of diglyme, and other ether solvents, increases the yield of polymer, due to its ability to stabilize silyl anions by complexation of the Na counterion.
• Example 5 As discussed above, addition of a solution of PhMeSiCl 2 in toluene to a 1:1 (v:v) toluene/Na dispersion mixture improves the yield from 23 to 41% and also increases modal MW from 4,000 to 20,000.
• a revised procedure incorporating three of the improvements of this invention allows for the first time, the synthesis of high molecular weight (PhMeSi) n in high yield without crosslinking.
• Example 6 The use of an improved reaction solvent mixture has been tested for other polysilanes. The results of these tests are summarized in Table 8 and show that similar striking improvements are achieved with other monomers.
• Example 9 Na amalgam can also be used to prepare polysilanes.
• Table 10 are results for preparation of polysilanes in non-optimum solvents using Na amalgam (1:1). Reaction conditions were the same as in Example 8.
• the surface of the solid surface reacting agent used e.g., in the heterogeneous reductive condensations, be as clean and impurity- and contaminant-free as possible.
• anything which is in contact with the solid surface can impact the methods of this application, especially when applied to the preparation of poly(silylsilanes) referred to below.
• the same principles also apply to the preparation of the polysilanes discussed above, as well as to other polysilane-containing polymers, e.g., such as the interrupted polysilanes referred to below.
• Typical "contaminants" found to materially affect the performance of the methods include oxidation products and other foreign material, the surface active agents typically included in commercial dispersions of sodium and other solid agents (e.g., stearates such as aluminum stearate), etc.
• commercial sodium dispersions having as low an amount of surface active agent as possible and which have been prepared as recently as possible.
• the use of such fresh dispersions minimizes the likelihood of surface contamination by the usual oxidation products.
• the latter include the common, slightly yellowish or brownish to highly yellowish or brownish (depending on the degree of oxidation) products sometimes formed by reactions of peroxides formed on the surface of sodium or other surface reacting agents.
• the most preferred variant for applying the principles discussed above to the methods of this invention is the employment of a sodium dispersion which is essentially free from surface active agents as well as other impurities. In this way, the cleanest possible surface will be used as the starting point.
• a sodium dispersion which is essentially free from surface active agents as well as other impurities.
• the possibility will be provided of utilizing an essentially totally clean surface by simply maintaining the starting form of the solid agent.
• the desired molecular weight and/or polydispersity is not adversely affected or is even enhanced by the presence of certain molecular species at the surface, and where these species are adventitious surface "contaminants" in a given reaction medium, e.g.
• surfactants from a sodium dispersion avoidance or elimination of such "contaminants" from the surface will be less important.
• example 7 demonstrates the effect of a surface active agent on the methods of this invention where commercially available sodium dispersions are employed as the solid surface reacting agent.
• the latter data demonstrate how the presence of a surface active agent in commercial formulations of sodium can have adverse effects where the desired mode is to provide good access to the sodium surface.
• these data also demonstrate how the inherent presence of the surface active agent in the commercial formulations can beneficially affect performance under the appropriate circumstances, e.g., where it is desired to prepare lower molecular weights using inverse addition.
• the light oil data and especially the mineral spirits data also show the effect of aging on results.
• the four mineral spirits experiments were performed sequentially over a period of one week from top to bottom. Except for the passage of time, all conditions were the same. These data show that aging does play a significant role, most likely due to the increasing concentration of oxidation products over time. Mineral oil dispersions showed results similar to those shown in Table 12.
• the preferred manner of carrying out this invention is to begin with a solid surface reacting agent in the form of a freshly prepared dispersion which is essentially free of surface active agents and other contaminants or at least as free of these as possible. Most preferably, this dispersion will be freshly prepared just prior to use using fully conventional methods both for surfactant-free and surfactant-containing mixtures.

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