Products from hindered amine stabilizers

Examination of the history of antioxidants such as hindered phenols and amines shows a move from low-MW products to higher-MW products. Specifically, polymer industries have abandoned the use of, e.g., butylated hydroxy toluene (BHT) in favor of tetrakismethylene (3,5-di-f-butyl-4-hydroxydrocinnamate)methane (see Figure 15.9). Likewise, polymeric HALS, like poly-methylpropyl-3-oxy-(4(2,2,6,6-tetramethyl)piperidinyl) siloxane, replaced the low-MW hindered amine Lowilite 77 (see Figure 15.10). The next obvious step was to produce a new class of stabilizers. 

An analogous reaction can be effected for primary amine products. The similarities in the infrared spectra of the anions in the sodium and alkylammonium salts support the structures proposed for these products. The stabilities and properties of the products obtained from various amines have been described (12). When hindered amines are employed, no product is obtained, presumably because of a steric effect. By analogy with the structure of S03N202 , the following structure is proposed for the anion. 

As shown in Table I, these stable radicals showed strikingly higher light stabilization activity in polypropylene than that of the UV absorber tested. We felt that their activity was related to their radical scavenging ability. This hypothesis is supported by the observation that the coupled products (32) and (33) were obtained by the reaction of the nitroxyl radicals (2) and (27), respectively, with a C-radical derived from AIBN (10). The radical scavenging ability of the stable nitroxyl radicals is now well known to play a major role in the mechanism of light stabilization by hindered amine compounds (13). 

Most hindered amine light stabilizers have evolved from the discovery (4) that compounds containing a 2,2,6,6-tetramethylpiperidine moiety can stabilize polymers against photodegradation and this moiety has been incorporated into HALS of various types (Figure 1). Much has been published on the mechanism of action of HALS and the literature in this area has recently been critically reviewed (5,6). While the complete mechanism of action has not been fully elucidated, the high performance of HALS is generally attributed to the ability of their oxidation products to... 

Together with a shift of the proton from the a-carbon to the alkoxide oxygen, the tertiary amine is eliminated from the addition product to yield the unsaturated product 3. Early examples of the Baylis-Hillman reaction posed the problem of low conversions and slow reaction kinetics, which could not be improved with the use of simple tertiary amines. The search for catalytically active substances led to more properly adjusted, often highly specific compounds, with shorter reaction times." Suitable catalysts are, for example, the nucleophilic, sterically less hindered bases diazabicyclo[2.2.2]octane (DABCO) 6, quinuclidin-3-one 7 and quinuclidin-3-ol (3-QDL) 8. The latter compound can stabilize the zwitterionic intermediate through hydrogen bonding. ... 

There are two possible explanations for this. Either the very tiny amount of unprotonated amine reacts very rapidly with SO 3 in the para position or the reaction is reversible and the para-sulfonic acid is formed because it is stabilized by delocalization and least hindered. The product is important because the amides derived from it (sulfanilamides) were the first antibiotics, the sulfa drugs. 

---