Wolf D. Habicher

Temperature and pH dependent solubility of novel poly(N-isopropylacrylamide)-copolymers

Temperature and pH sensitive polymers were prepared by the copolymerization of N-isopropylacrylamide (NIPAAm) with varying amounts of acrylamide derivatives bearing carboxylic groups attached to spacers with different chain length (C n AAm). Aqueous solutions of the copolymers show lower critical solution temperature behaviour (LCST). The LCSTs of the aqueous solutions of these copolymers decrease with increasing comonomer content and spacer chain length. Through the use of suitable amounts of the monomers it was possible to accurately predict the phase transition temperature and values were obtained between 10 and 32°C. The LCSTs also strongly depend on the pH-value of the solution. An increase of the pH-value leads to a significant increase in LCST due to the formation of a more hydrophilic copolymer. Through altering the pH it is possible to obtain phase transition temperatures between 10 to 50°C. The LCST behaviour was investigated by means of DSC as a standard method which allows the measurement of the phase transition over a wide range of temperatures and pH-values. Comparison of these results with those from other methods (mainly turbidimetric and viscosimetric measurements) shows a two step mechanism for the phase separation. Dependence of the phase transition temperature of aqueous P(NIPAAm co C 2 AAm) copolymer solutions on the pH-value (lines were drawn to make the diagram clearer).

Antioxidant interaction between organic phosphites and hindered amine light stabilizers: effects during photoxidation of polypropylene—II

Abstract The behaviour of mixtures of hindered amine light stabilizers (HALS), their nitroxyl radical, and alkylamine analogues, with aromatic and aliphatic phosphites during photoxidation of polypropylene (PP) has been examined. The photostabilizing performance of the above mixtures of stabilizers was compared with that of the individual antioxidants and with the performance of a number of synthesized bifunctional stabilizers containing aromatic phosphite or phosphonite and either of the above HALS or related functions in the same molecule (referred to as ‘HALS-Phosphites’ or ‘HALS-Phosphonite’). Combinations of the hindered aryl phosphite Irgafos 168, with the hindered amine Tinuvin 770, the tertiary amine Tinuvin 292 and the bis-nitroxyl-analogue of Tinuvin 770, gave antagonistic effects at most molar ratios examined. Furthermore, in all cases, the extent of antagonism was reduced with increasing the amount of Irgafos 168 in the mixtures. Combination of the aliphatic phosphite, trilaurylphosphite, TLP, with Tinuvin 770 in PP, on the other hand, has shown synergistic effect under photoxidative conditions. The HALS-Phosphites and HALS-Phosphonite examined have shown better stabilizing efficiency than the corresponding mixtures of the individual stabilizers.

Discoloration of polymers by phenolic antioxidants

Part of colour development in aged polyolefins stabilized with phenolic antioxidants can be attributed to the formation of conjugated diene compounds, arising as a consequence of sacrificial trapping of alkylperoxy radicals by phenolics. The discoloration depends on the structure and concentration of the phenol transformation products. Peroxycyclohexadienones are formed transiently in low concentrations and themselves do not discolour the polymer matrix. However, some products of their thermolysis are discolouring and accumulate slowly in the polymer matrix. The principal contribution to polymer discoloration is due to formation of quinone methides. Their discolouring effect is reduced in propionate-type phenolics, as a consequence of intramolecular rearrangement of a part of the primarily formed quinone methide, and is due to oxidative dimerization accounting for nonconjugated dimeric quinone methides. Attention was paid to transformations of octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoate and related quinone methides and 4-hydroxycinnamates.

Novel tocopherol compounds I. Bromination of α-tocopherol — reaction mechanism and synthetic applications

Abstract Bromination of α-tocopherol is shown to proceed as a two-step process including the occurrence of an ortho-quinone methide, not as a radical-chain reaction, contrary to earlier reports. The ortho-quinone methide intermediate is also produced by oxidation of a-tocopherol with silver oxide and can be trapped in the presence of electron-rich dienophiles in a hetero-Diels-Alder reaction to form novel dichromanes in high yields.

Chain-breaking antioxidant activity of phosphite esters

Abstract The inhibition of the autoxidation of hydrocarbons and polypropylene by aliphatic, aromatic, sterically hindered and cyclic phosphites has been studied by means of volumetric and 31P-NMR techniques. The antioxidant activity of phosphites depends on the rate of their reactions with peroxyl radicals and on the way they react with alkoxyl radicals. Only those phosphites which react by substitution to give free aryloxyl radicals are effective as chain-breaking antioxidants. The reaction modes of various phosphites with various peroxyl and alkoxyl radicals have been studied in some model reactions and the relationship between structure, reaction mechanism and antioxidant activity has been elucidated.

Novel water-soluble tetrasulfonatomethylcalix[4]resorcinarenes

Abstract Here we present a simple, high-yielding method of preparation of the novel water-soluble tetrasulfonatomethylcalix[4]resorcinarenes. Preliminary results of complex formation between the host 1 and several aminoacids are reported.

Phosphorus-containing calixarenes

Calixarenes are macro(hetero)cyclic compounds having three-dimensional molecular cavities and are ubiquitous as synthetic receptors in supramolecular chemistry. This paper provides an overview of studies on the phosphorus-containing calixarenes and thiacalixarenes, within the context of synthesis, metal cation binding, catalysis, molecular recognition and bioactivity.

Organophosphorus antioxidants: Part X—Mechanism of antioxidant action of aryl phosphites and phosphonites at higher temperatures

Abstract By means of kinetic and product studies using 31 P-NMR-spectroscopy and HPLC it is shown that in the autoxidation of hydrocarbons (tetralin and a normal paraffin blend) inhibited by aryl phosphites and phosphonites at 150–180°C, the phosphorus acid esters are hydrolysed to give phenols and hydrogen phosphites and phosphonites respectively. Under certain conditions, these hydrogen esters hydrolyse further to form phosphorous and phosphonic acid respectively. The mixture of antioxidants thus generated is responsible for the high stabilizing efficiency of phosphite and phosphonite esters in autoxidations at these temperatures. In addition to hydrolysis, oxidation of the phosphorus compounds by hydroperoxides and peroxyl radicals takes place in the course of the reaction giving the corresponding phosphates and phosphonates. The ratio of oxidation to hydrolysis depends on the oxidizability of the particular hydrocarbon and on the reaction conditions (temperature). In the inhibited oxidation of the highly oxidizable aralkyl hydrocarbons (tetralin), oxidation of the phosphorus compounds is faster than hydrolysis, whereas in the less oxidizable paraffin hydrocarbons at 150°C, hydrolysis exceeds oxidation. In the paraffin oxidation at 180°C, the two processes take place in a 1 : 1 ratio. In the inhibited oxidation of a polyether (polyethylene-propylene oxide) at 100–150°C only oxidation and no hydrolysis of the phosphorus inhibitors occurs. Obviously, this is due to the reduced reactivity of water complexed by the polyether in this case.

Antioxidant action mechanisms of hindered amine stabilisers

Abstract Kinetic modeling and simulation studies reveal that the antioxidant effectiveness of Hindered Amine Stabilisers (HAS) cannot be accounted for by the currently accepted action mechanisms. Obviously, hindered aminoxyl radicals are capable of reacting with alkylperoxyl radicals. N -alkoxyamines and oxygen are formed in the reaction with tertiary alkylperoxyl radicals, and the free amines, ketones and oxygen in the reaction with sec -alkylperoxyls. In oxidations inhibited by hindered aminoxyls, these reactions are kinetically most pronounced when the chain propagation and termination rates are low as in the AIBN-initiated thermooxidation of cumene inhibited by 2,2,6,6-tetramethyl-pyridinoxyls (TEMPO) where a pronounced induction period of the oxygen uptake is observed which cannot be accounted for by the currently accepted action mechanisms of hindered amine stabilisers (HAS). In the inhibited oxidation of tetralin, however, where the propagation and termination rates are higher, only a retardation of the oxygen uptake but no induction period are observed. But here too, the consumption of the aminoxyl is faster and the oxygen uptake slower than computed without taking into account the reaction of aminoxyl with peroxyl radicals.

Antioxidant interaction between organic phosphites and hindered amine light stabilisers during processing and thermoxidation of polypropylene

Abstract Mixtures of hindered amine light stabilisers (HALS) and their radical analogues, with aromatic and aliphatic phosphites have been examined for their effectiveness as melt and thermal antioxidants in polypropylene (PP). Stabilisers containing two different antioxidant functions in the same molecule (aromatic phosphite and either of the above HALS or related functions), referred to as ‘HALS-phosphites’, were also examined and their antioxidant effectiveness compared with the optimum performance of the corresponding combination of mixtures of the individual antioxidants. Binary combinations of the phosphites Irgafos 168 (aromatic) and TLP (aliphatic) with Tinuvin 770, its bis-nitroxyl analogue, or with Tinuvin 292 (tertiary amine) exhibited synergistic effects at different molar ratios; the level of synergism was generally higher during thermal ageing than during melt processing. The optimum combination, leading to the most effective synergism, varied with the chemical nature of the phosphite: under thermoxidative conditions, molar excess of the Irgafos 168 is required whereas molar excess of the HALS is needed for optimum of TLP. HALS-phosphites act autosynergistically and show, in most cases, higher efficiency than their corresponding additive mixtures of stabilisers.