The synthesis and properties of borosiloxane polymers with low boron content, based on boron compounds of different structure

Abstract

IN OO~r with polyorganosiloxanes polyboroorganosiloxanes possess the new properties of adhesion and autohesion. I t is these properties that explain the increased interest in the practical application of polyboroorganosiloxanes as self-adhering, heat-stable rubbers and in their use in hydrophobic electrical insulating and heat-resistant coatings [1-3]. Studies on the synthesis of borosiloxane polymers can be divided into two groups: 1) production of polymers with a high boron content, with the objective of increasing the heat resistance of the siloxane chain on the basis of the higher energy of the B-0 -S i bond, and 2) synthesis of borosiloxane polymers with a low boron content (B:Si= 1:200 or less) in order to obtain self-adhering rubbers. High molecular weight dimethylborosiloxane polymers with a large proportion of B--O--Si bonds hydrolyze easily in air [4], which leads to breakdown of the borosfloxane polymer before increase in heat resistance is attained. The introduction of small quantities of boron leads, however, to polymers of practical value with interesting properties [2, 5]. These polymers behave as high molecular elastomers, giving vuleanizates with distinctive physicomechanical properties. In solution however they have low molecular weights. Some authors [5,6] explain this by formation in the polymer of coordinate bonds between the electrophilie boron atom and the oxygen of the siloxane chain, these bonds breaking down in solution. The borosiloxane polymers with low boron contents reported in the literature were obtained from trifunctional boron compounds only, namely H3BO3 [5] and B(0CdH9)3 [2]. The mechanism of formation and the structure of these polymers have not been studied sufficiently. We have prepared borosiloxane polymers of low boron content from diand trifunctional boron compounds of different structure. As difunetional boron compounds we took monosubstituted organoboron acids and esters with different substituents on the boron atom, namely RB(OH)2 and RB(0R')2 , where 1~ is n-C3H7; CFaCH2CH2, C6I-I 5 and m-CF3CeH 4, and R' is C2H5 and Call 9.