AmericanChemicalSociety.com

Introduction

     There are essentially two parts to this project; the first is evaluation of stability of bis-siloxy arenes on iron oxide surfaces versus mono-siloxy arenes (discussed below and in Table 1).  The second part is the synthesis of chloromethyl functionalized bis-siloxy arenes, for formation of more stable coatings on metal oxides such as glass and iron-oxides. The chloromethyl group is the same used to attach reagents to “Merrifield” polymer beads to facilitate separations with low solvent consumption. Substantial  quantities of petroleum based solvents are saved every day using Merrifield polymers; but this will be significantly enhanced by the magnetic “Merrifield” supports using the bis-siloxy arenes developed in this proposal.  That is because non-magnetic Merrifield resins require sophisticated cartridges or flow-filtering systems; but magnetic-supported reagents (i.e. the supports fabricated in this research) are simply purified with a collection magnet. 

     Currently, an important topic in technology and engineering is how to graft organic compounds to metal oxides such as silicon-oxides (“window glass”), titanium oxides, or magnetite (iron oxide).  Coatings on glass or titanium oxide are often immobilized via covalent siloxane linkages, and we are investigating these conjugation groups for magnetite. 

     Metal oxide coatings covalently bonded via a single  siloxane group can degrade once the covalent bond is broken, even though bond breakage is reversible.  However, if there were 2 siloxane bonds significantly greater stabilization would occur as a result of the chelation effect, where if one bond breaks the coating is held in place by the second bond and eventually the first bond can reform.  This prediction for mono- versus bis-siloxy benzenes was studied and presented in part one of the “Results and Discussion” section. Once stability parameters have been determined, part 2 of this study is to synthesize a chloromethyl benzene substituted with the most stable siloxane derivative.  Part 3 will be the expansion of the functionalized benzene to longer conjugating chains forming long-tethered bis-alkyl-siloxy arene coatings to optimize coverage by reducing steric strain at the grafting surface. 

Results and Discussion

Part 1.  Investigation of number and type of siloxane groups for highest stability coating.

     Experiments were designed and carried out to determine optimal molecular structure characteristics of a silyl substituted benzene that would maintain the highest stability.  Scheme 1 illustrates 2 studies that were carried out on 2 different model coatings applied to iron oxide particles.  The first coating has a single silyl group attached to the benzene functionality, and the second coating has two silanes conjugating a single benzene group.  In addition, each silane can have 1-3 alkoxy groups attached to the silane  that can be either an ethoxy group (-OCH₂CH₃) or a methyl group (-CH₃).  Table 1 shows all the data for the combined series of experiments. First, thermal stability of these coatings was tested using thermo-gravimetric analysis (TGA), which gives the highest temperature the coatings can withstand before degradation.  The data for all the compounds show similar thermal degradation in approximately the range of 386-394°C. 

     Hydrolytic stability is more important with respect to shelf-life of the coating on the particles, since thermal stability for all coatings is greater than 350°C which is over three times the highest possible shelf life temperatures encountered.  On the other hand, adventitious moisture in the presence of the siloxane coatings can hydrolyze the siloxane coatings off of the metal oxide surface, and can continuously crosslink neighboring groups over time causing aggregation of  particles into agglomerates as well.  To test the different coatings for hydrolytic stability, particles were incubated in water/THF mixtures (1/99) at 30°C over the course of 20-24 hours.  Aliquots of particles were periodically removed and submitted to TGA to determine mass loss.  Plots of moles of coating lost versus time resulted in the rates of mass loss shown in the last that gave the result that the slowest hydrolysis rates were encountered for both the mono- and bis-siloxy benzenes with the methyl(trimethoxy)siloxy groups.  Furthermore, the bis-siloxy benzene  showed greater stability than the mono-siloxy benzene, concluding that the coatings with two siloxane groups gives the most stable coating. 

Part 2.  Synthesis of chloromethylbenzene with substituted with two dimethoxy-methyl silane groups.

     The synthesis for carrying out the desired bis-siloxy with the chloromethyl appendage.  Starts with 1,3-dibromo-5-methylbenzene, then first formation of the bis-Grignard; after 1 hour the methyl(trimethoxy)silane was added and the compound 1,3-bis[methyl(trimethoxy)silane]-5-methylbenzene was formed in good yield.  Currently we are working on the final step for adding a chlorine to the benzylic carbon of the compound synthesized; however, the first attempts using NCS have not been successful using AIBN.  Future synthesis will explore the use of benzoyl peroxide to initiate the radical reaction to put the chlorine moiety on the benzyl carbon.

Part 3.  Synthesis of long-tethered bis-alkyl-siloxy arenes. 

   The originally planned synthesis for the long-tethered bis-alkyl-siloxy arenes is also underway.  Before embarking on this synthesis, a model reaction is under investigation to test the “Kumada” coupling of reagents  using readily available precursors.  Because we already have 1,3-dibromo-5-methylbenzene, we have begun the Kumada coupling conditions on this starting material.  Thus far, low yields of the Kumada coupling have been achieved, and the methodology is currently being improved.