Polymers from organosilicon derivatives of 5-norbornene-2-methanol for membrane gas separation

Abstract

In this work, we have proposed a new strategy to the synthesis of norbornene-type monomers with Si–O–C structural motifs via a straightforward and selective dehydrogenative addition reaction between 5-norbornene-2-methanol and trialkylsilanes. As a result, a set of monomers with various trilalkylsiloxy groups (alkyl – CH3, C2H5, and n-C3H7) was synthesized with the yields of 89–90%. These monomers exhibited a high reactivity in both metathesis and vinyl-addition polymerization, affording high-molecular-weight products in yields up to 90%. The metathesis polymers were rubbery with glass transition temperatures from −29 to +23 °C, while related isomeric vinyl-addition polynorbornenes were glassy polymers. Permeability (P) and diffusion (D) coefficients for these polymers were systematically evaluated for permanent gases (He, H2, O2, N2, and CO2) and C1–C4 alkanes. Studying their gas transport properties allowed to establish new structure-property relationships. Particularly, the length of alkyl fragments in trilalkylsiloxy group affected differently gas permeability of metathesis (rubbery) and vinyl-addition (glassy) polymers. Besides, it was shown that the method of linking the Si–O–C-containing group to the main chains of polymers played very important role: Alk3SiO-substituted polynorbornenes are more permeable than isomeric (AlkO)Alk2Si-substituted ones. For the metathesis polynorbornenes, P(CO2) were in the range of 152–226 Barrer, and ideal selectivity (α) of CO2/N2 – 13-16; for the vinyl-addition polynorbornenes, P(CO2) = 230–500 Barrer, and α(CO2/N2) = 14–17. Among studied polymers, promising results for ethane/methane separation was demonstrated by metathesis polynorbornene bearing tri(n-propyl)siloxymethyl substituents: P(C2H6) = 138 Barrer and α(C2H6/CH4) = 2.4. The obtained polymers were also characterized by using TGA, WAXD, and nitrogen adsorption-desorption methods.