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Mostkowane polisilseskwioksany : synteza, struktura i właściwości adsorpcyjne

Barczak M., Dąbrowski A.

Wiadomości Chemiczne

2008

[Z] 62, 11-12

977-998

Bridged polysilsesquioxanes (BPs) are an emerging group of organic-inorganic hybrid materials where organic moieties are built into a siloxane matrix by stable covalent carbon-silicon bonds [1, 2]. The simplest precursors of BP xerogels can be presented as bis(trialkoxysilanes) structures M-(SiX3)n, where X is the hydrolyzable group, and M denotes the organic bridge (spacer). By means of the appropriate precursors in the reaction of hydrolytic polycondensation it is possible to design on a molecular level the considered materials keeping control over their characteristics. Bridges can have a different nature, e.g. they can be representative of homologue series of saturated hydrocarbons or aromatic hydrocarbons. In fact, one of the greatest advantages is a huge range of sol-gel processable monomers with different organic bridges. As they can vary in composition, length, geometry of substitution and rigidity it makes possible to affect the chemical and physical properties of final materials, including structure-adsorption properties by the choice of the precursor. Flexibility of an organic bridge plays a key role in such a behavior: materials with long elastic bridges are more susceptible to collapsing during the last stages of the sol-gel treatment (ageing and drying). Furthermore, at the same length of an organic bridge, formation of mesoporosity materials is observed in the alkaline medium, while in the acid medium - formation of microporosity. The influence of factors such as flexibility of the bridge, monomer concentration, type of catalyst, ageing time, etc. on textural characteristics of BPs materials has been widely described in the literature [1-8] and is also shortly discussed in this article. As the formation of these solids is kinetically controlled all parameters of a synthesis are able to influence the kinetics of the sol-gel process, what in result affects both the physical and chemical properties of the final materials (19, 20, 24-29). Thus, porosity, specific surface area, pore volume or surface chemistry can be tuned during sol-gel processing by a proper choice of monomers and synthesis conditions. Some illuminative examples of influencing the structure-adsorption characteristics has been discussed here. The possibility of tailoring of the above-mentioned final properties makes bridged polysilsesquioxanes potential candidates in adsorption application. Up to date several works have been published in the literature describing such attempts. In this paper we briefly discuss the synthetic strategy, creation of porosity and some examples of the usage of BPs as adsorbents. Several research groups have been using these materials with different bridges (see Figure 10) to adsorb different metal ions such as Ag(I), Hg(II), Au(III), Pt(IV), Pd(II) and Rh(III) [68], Co(II), Cd(II), Cu(II), Ni(II), Zn(II), Mo(VI) and U(VI) [7, 69], La, Ce, Pr, Nd(III), Pm, Sm(III), Eu, Gd, Tb, Dy, Ho, Er(III), Tm(III), Yb, Lu [70], V(VI), Mo(VI), W(VI), Th(IV) and U(VI) [71]. Ethylene- and phenylene-bridged xerogels prepared via a surfactant template approach have been used as sorbents of phenols from solution [75]. Ethylene- and phenylene-bridged polysilsesquioxanes functionalized with amino and thiol groups have been successfully tested as adsorbents of volatile organic compounds such as: hexane, heptane, cyclohexane, benzene, and triethyloamine from the gas phase [76]. Examples shown in the article testify to a great potential of these group of hybrid materials as sorbents. BPs can partially fill the gap caused by a limited use of other type of sorbents due to their complex production technology, low chemical and mechanical stability, low efficiency or selectivity. Two significant advantages should be mentioned here. First is the possibility of a homogenic distribution of organic moieties accessible to the adsorbate particles. It allows the creation of adsorption sites which force selectivity of the adsorption process by promoting a particular type of adsorption force, for example donor-acceptor or ?-? interactions. Secondly, the precise control of parameters such like the specific surface area or pore diameter induces adsorbents with predetermined characteristics. Such simultaneous control over the porosity and surface chemistry is a great advantage of bridged polysilsesquioxane materials.

Silseskwioksany. Cz. II. Polisilseskwioksany

Leśniak E.

Polimery

2001

T. 46, nr 9

582-589

Przedstawiono przegląd literatury (90 pozycji) dotyczący metod otrzymywania polifenylosilseskwioksanów (PPSQ) i polimetylosilseskwioksanów (PMSQ) o strukturze drabinkowej [wzór (I)]. Omówiono też metody otrzymywania mostkowanych połisilseskwioksanów [wzory (III) i (IV)]. Przytoczono liczne argumenty świadczące o drabinkowej strukturze połisilseskwioksanów [wzory (V) i (VI), tabela 1] oraz scharakteryzowano ich niektóre właściwości fizykochemiczne, w tym odporność termiczną PPSQ i PMSQ (tabela 2). Przedstawiono też podstawowe kierunki zastosowania tych polimerów.

A review with 90 references covering methods used to prepare polyphenylsilsesquioxanes (PPSQ) and ladder-structured polymethylsilses-quioxanes (PMSQ) (formula I), viz., (i) three- or two-step hydrolytic poly-condensations of trifunctional silanes RSiX3 (R = alkyl, aryl; X = Cl, OR', OAc); (ii) stepwise coupling polymerization of RSiX3; (Hi) polymerization of polyhedral silsesquioxanes. Methods to prepare insoluble globular spherical PMSQ and bridged polysilsesquioxanes (formulas III and IV) are also reviewed. Evidence is adduced for the ladder structure of polysilsesquioxanes (formulas V and VI; Table 1). Selected physicochemical property data and thermal stability data are given. Principal polysilsesquioxane application trends are characterized.