Polyborosiloxanes (PBSs), Synthetic Kinetics, and Characterization

• Published in: Macromolecules Vol. 47; no. 14; pp. 4531 - 4537
• Main Authors: Liu, Zhen, Picken, Stephen J, Besseling, Nicolaas A. M
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 22.07.2014
• Subjects: Applied sciences; boric acid; Chemical modifications; cleavage (chemistry); crosslinking; Exact sciences and technology; Fourier transform infrared spectroscopy; gel chromatography; humidity; hydrogen bonding; hydrolysis; Inorganic and organomineral polymers; moieties; molecular weight; Physicochemistry of polymers; polymerization; rheometry; viscoelasticity; Dimethylsiloxane polymer; Viscoelasticity; Molecular weight distribution; End group; Molecular structure; Telechelic polymer; Boron polymer; Thermal reaction; Experimental study; Chemical reaction kinetics; Boric acid; Chemical modification; Preparation; Reaction mechanism; Chain rupture
• ISSN: 0024-9297; 1520-5835; 1520-5835
• DOI: 10.1021/ma500632f

We synthesized polyborosiloxanes (PBSs) from poly(dimethylsiloxane) (PDMS) and boric acid (BA) by heating a mixture of these compounds to 200 °C. During the reaction the PDMS chains are subject to random scission, upon which the new chain ends get modified by −Si–O–(BO) m (OH) n moieties. This insight is based on gel permeation chromatography (GPC) results on the evolution of the molecular-weight distribution during the modification process, in combination with Fourier transform infrared (FTIR) spectroscopy to identify chemical moieties. We invented a refinement procedure in which the ill-defined polydisperse polar end groups can be converted into predominately borono, −Si–O–B(OH)2, by hydrolysis of boroxane bonds, B–O–B, and subsequent removal of released BA. The resulting PBSs form supramolecular elastomers owing to hydrogen bonding among the end groups. The viscoelastic behavior of the supramolecular elastomers was investigated by dynamic rheometry, and we found that the dynamic moduli increases dramatically upon the modification, even though the mean degree of polymerization decrease. This is explained by the strong interactions between the modified chain ends. Furthermore, we found that by severe dehydration, the boranol groups are converted into boroxane bonds. Upon exposure to ambient humidity, these cross-links are hydrolyzed, and the supramolecular elastomer is recovered.