Mauritz - Inorganic-Organic Monomers

Organic-Inorganic Monomers

Inorganic-Organic monomers are molecules containing a metal core such as a silicon bonded to reactive alkoxy groups and/or organic groups. These monomers, when assembled together, form Organic-Inorganic siloxane materials. There are two distinctive types of Inorganic-Organic monomers: Organically Modified Silicate (ORMOSIL) monomers and Silsesquioxane-Type monomers. These monomers are typically used in the synthesis of controlled porosity materials.

1. ORMOSIL Monomers

In the mid-1980s a flurry of research was conducted in the field of ORMOSIL chemistry. 10-19 ORMOSILs are derived from tetrafunctional silicone alkoxides such as tetraethylorthosilicate (TEOS) shown in Figure 1. Organic modification may take


Figure 1: Tetraethylorthosilicate

place at any one of the reactive alkoxide arms as shown in Figure 2. Here, n is representative of the number of organic moiety connected to the silicon molecule and f is representative of the number of reactive alkoxy groups connected to the


Figure 2: The ORMOSIL Functionality Spectrum

silicon molecule. For a mono-functional silicon alkoxide (f=1, n=3), the monomer 'terminates' the polymer chain since there is only 1 reaction site on the molecule. A di-functional silicon alkoxide (f=2, n=2) behaves as a 'bridging' agent, connecting molecules in a linear fashion. A tri-functional silicon alkoxide (f=3, n=1) behaves as an 'crosslinker', allowing for branching in the network. A tetra-functional silicon alkoxide (f=4, n=0) behave as a 'networking' agent, allowing for complete connectivity between all functional groups of the molecule.

Traditional glasses require temperatures in excess of 1000°C in order to have densification of the siloxane network. 20 These temperatures prohibit the organic modification of traditional glasses due to the high temperature instability of the Si-C and C-H bonds. Sol-gel derived glasses, however, do not require high temperatures for formation, making organic modification possible (See Figure 3). Upon sol-gel


Figure 3: Incorporating Porosity in Sol-Gel Networks

polymerization of tetra-functional silicon alkoxides, network formation and connectivity is extensive. Drying of this highly connected system promotes removal of excess water and alcohol, allowing for complete vitrification of the network. With organic modification of the silicon, sol-gel polymerization limits both the connectivity (by inclusion of monomers of lower functionality) and the chemical densification (by creation of specific free volume or porosity in the sol-gel network). The functional group, represented by R, on an organically modified silicate can be any organic moiety including methyl, vinyl, or benzyl. Figure 4 depicts ORMOSIL connectivity achieved through sol-gel polymerization. It is reasonable to expect that variation of the number and type of organic moieties included on the silicon monomer, results in a variety of pore sizes that


Figure 4: ORMOSIL Connectivity Through Sol-Gel Chemistry

can be created in the sol-gel network. The pore size corresponds to the physical size of the organic substituent. These pores, due to their organic nature, can attract or divert various organic or gas molecules. In essence, ORMOSILs form well defined porous networks, each possessing unique physical and chemical properties.

2. Silsesquioxane-Type Monomers

Similar to ORMOSIL monomers, silsesquioxane monomers are useful for creating sol-gel glasses with well defined porosities. The general structure of silsesquioxane-type monomers are shown in figure 5. Here the organic substituent behaves as a spacer unit (bridging functionality) that resides between two reactive di- or tri-alkoxysilane


Figure 5: Silsesquioxane-Type Monomers

endgroups. These spacer units have traditionally been methyl or benzyl groups stranded together but can be synthesized to have any number of organic spacer functionalities. Upon initial sol-gel network formation, porosity is limited due to the presence of the bridged organic groups. Limiting porosity through usage of a spacer unit, with its specific organic character, can be beneficial for specific gas or liquid permeation. However, limited porosity systems can be modified to form well defined porous systems through removal of the spacer units by ozonolysis (See Figure 6). 21,22 By removing the spacer units after the network formation, well defined pores corresponding to the organic spacer are built into the sol-gel network.


Figure 6: Formation of Precise Porosity Through Usage of Bridged Aryl Silsesquioxanes

References:

10. Yoldas, B. E. J. Non-Cryst. Sol. 1984, 63, 145-154.
11. Scholze, H. J. Non-Cryst. Sol. 1985, 73, 669-680.
12. Schmidt, H. J. Non-Cryst. Sol. 1985, 73, 681-691.
13. Schmidt, H.; Seiferling, B. Mat. Res. Soc. Symp. Proc. 1986, 73, 739-750.
14. Sakka, S.; Tanaka, Y.; Kokubo, T. J. Non-Cryst. Sol. 1986, 82, 24-30.
15. Chen, K. C.; Thorne, K. J.; Chemseddine, A.; Babonneau, F.; Mackenzie, J. D. Mat. Res. Soc. Symp. Proc. 1988, 121, 571-574.
16. Brinker, C. J. J. Non-Cryst. Sol. 1988, 100, 31-50.
17. Schmidt, H. J. Non-Cryst. Sol. 1989, 112, 419-423.
18. Babonneau, F.; Thorne, K.; Mackenzie, J. D. Chem. Mat. 1989, 1, 554-558.
19. Schmidt, H.; Wolter, H. J. Non-Cryst. Sol. 1990, 121, 428-435.
20. Mackenzie, J. D. J. Non-Cryst. Sol. 1982, 48, 1-10.
21. Shea, K. et. al. J. Am. Chem. Soc. 1989, 111, 3442.
22. Shea, K. et. al. J. Am. Chem. Soc. 1992, 114, 6700.

Written by: Sandra Young (Partially From Her Research Prospectus)


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