The use of stabilizers in organic materials normally subject to oxidative degradation is a widespread commercial practice. Such stabilizers provide enhanced stability to a variety of organic substrates in which they are incorporated and in many instances are responsible for a commercially acceptable utility for such materials. A particularly useful application for the stabilizers is in the production of commercially desirable polymers and polymeric products. Most, if not all, of the commercial polyolefins such as polypropylene or polyethylene will contain one or more stabilizers to enhance the oxidative stability of the polymers.
One class of oxidation stabilizers which has gained widespread commercial acceptance is the class conventionally referred to as hindered phenols. Such stabilizers will typically contain a phenolic hydroxyl as a substituent on an aromatic ring which also has a bulky substituent, e.g., a bulky alkyl substituent, on at least one aromatic ring carbon atom located ortho to the carbon atom on which the hydroxyl is substituted. Perhaps the most commonly used antioxidants of this type contain the 2,6-di-tert-butylphenol moiety. Numerous patents and other references, both U.S. and foreign, describe antioxidants of this type. Many are commercially employed, particularly as antioxidants for polymeric materials such as polypropylene.
The majority of hindered phenolic antioxidants are monomeric compounds. Although the monomeric antioxidants are of relatively high molecular weight and thus have relatively low volatility, there is some tendency for the hindered phenolic stabilizers to migrate from the polymeric substrate when a stabilized composition containing the hindered phenol is exposed to elevated temperatures or conditions of reduced pressure. Such conditions of temperature and pressure are frequently encountered during the processing of the polymeric or other organic substrate. Moreover, the hindered phenols are also susceptible to extraction when the stabilized composition is in extended contact with solvents.
The shortcomings of the conventional monomeric hindered phenol antioxidants are recognized and have been addressed by the art. Kline, U.S. Pat. No. 4,097,464, discloses self-stabilizing polymers obtained by copolymerization of unsaturated comonomers with 2,6-dialkyl-4-vinylphenols. The copolymers are said to have advantages because the antioxidant cannot be extracted or volatilized from the polymer. Minagawa et al, U.S. Pat. No. 4,243,581, disclose polypropylene compositions containing poly(tert-butyl-p-vinylphenol) and dialkyl thiodipropionate to provide stabilization against oxidative degradation. However, it would be of advantage to provide additional polymeric hindered phenolic compounds which serve as stabilizers for organic materials normally subject to oxidative degradation.
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