Tris(hydroxyphenyl)ethane Benzotriazole: A Copolymerizable UV Light Stabilizer,1a

Abstract

A novel UV-light stabilizer (UVLS), 3‘-(2H-benzotriazol-2‘ ‘-yl)-1,1,1-tris(4‘-hydroxyphenyl)ethane or simply tris(hydroxyphenyl)ethane benzotriazole (THPE-BZT), that can be copolymerized with engineering plastic monomers has been synthesized in three steps. The compound has two solid forms, a hydrate and an anhydrous form. Surprisingly, the hydrated form is more soluble in organic solvents such as acetone. Infrared spectra reveal that the hydrated form loses water of hydration upon heating and then forms a coplanar ring structure with a strong intramolecular hydrogen bond (IMHB) between one benzotriazole nitrogen and the adjacent phenolic OH. The NMR and UV−vis spectra in less polar organic solvents are also in accordance with the IMHB structure of THPE-BZT. In polar organic solvents such as DMSO and DMF and in MeOH/water at pH >10, the IMHB is disrupted, and the disrupted form exists in equilibrium with the planar or nondisrupted form. However, THPE-BZT copolymerized into low molecular weight polysulfones and polycarbonates retains the planar IMHB structure, as indicated by the UV absorption peak at ∼335 nm. These copolymers have greater UV stability than those without any THPE-BZT because of dissipation of photon energy by molecular vibrational relaxation in the planar form. Also, polysulfone copolymers containing THPE-BZT in the range of 0.25−1.5 mol % have greater UV light stability than polysulfone containing additive-type UVLS such as 2-(2‘-hydroxy-5‘−t-octylphenyl)benzotriazole (Cyasorb 5411). For polycarbonates, preliminary results show that copolymers of THPE-BZT have UV-light stability comparable to or only slightly less than that of polycarbonates blended with Cyasorb 5411 or THPE-BZT as additives. But the copolymers should escape the problems of surface “blooming” and leaching by extraction experienced by noncopolymerized UVLS additives. Thermogravimetric analysis indicates that THPE-BZT is more stable than the commercial benzotriazoles 2-(2‘-hydroxy-5‘-methylphenyl)benzotriazole (Tinuvin P), Cyasorb 5411, and the industrial standard2-[2‘-hydroxy-3‘,5‘-bis (1-methyl-1-phenylethyl)phenyl]benzotriazole (Tinuvin 900)and therefore might offer superior performance, even as an additive.