Mark G. Looney

The chemistry of novolac resins: 3. 13C and 15N n.m.r. studies of curing with hexamethylenetetramine

The reactions between novolac resins and hexamethylenetetramine (HMTA) which occur on curing have been studied by C-13 and N-15 high-resolution n.m.r. in both solution and the solid state. Strong evidence for the existence of many curing intermediates is obtained. New curing intermediates are reported along with experimental data to support previously postulated intermediates. The initial curing reactions between novolac and HMTA produce various substituted benzoxazines and benzylamines. Thermal decomposition/oxidation and further reactions of these initial intermediates generate methylene linkages between phenolic rings for chain extension and cross-linking. Among the three kinds of methylene linkages, the para-para methylene linkages are formed at relatively lower temperatures. Various imine, amide and imide side-products also concurrently appear during the process. The initial amount of HMTA plays a critical role in the curing reactivity and chemical structures of the cured resins. The lower the amount of HMTA, the lower the temperature at which curing occurs, and the lower the amount of the nitrogen-containing side-products in the finally cured resins. The ortho-linked intermediates are relatively stable, and can remain in the cured resins up to higher temperatures. The study provides an extensive description of the curing reactions of novolac resins

Graft copolymerisation studies Part 1. Models related to polyolefins

Abstract The chemistry of free radical graft copolymerisation, initiated with t -butoxy radicals, has been investigated using 3-methylpentane and 2,4-dimethylpentane as models for LLDPE and PP, respectively. The tertiary C–H reaction site of 3-methylpentane is twice as reactive as the secondary position. However, the site of grafting from the polymer is determined by the concentration of tertiary and secondary reaction sites and the relative reactivity at these positions and hence grafting occurs most frequently from the secondary C–H reaction sites. Abstraction from 2,4-dimethylpentane is solvent dependent and occurs most readily at the tertiary position. Therefore, grafting will be predominantly concentrated at the tertiary C–H site. The models also indicate that PP is less reactive than LLDPE. Competition reactions between 3-methylpentane and the monomers methyl methacrylate, styrene or 4-vinylpyridine suggest that the nucleophilicity of the monomer influences the competing abstraction–monomer initiation reactions. Evidence for grafting methyl methacrylate from 3-methylpentane is also presented. This study also provides evidence of the incorporation of peroxide linkages into the graft even when standard deoxygenating techniques are used.

Studies towards the synthesis of obtusenyne : a claisen rearrangement approach to unsaturated nine-membered lactones

Abstract An advanced intermediate for the synthesis of the Laurencia oxonane natural product obtusenyne 1 . namely the unsaturated nine-membered lactone 3 , was efficiently prepared in seven steps from ( E )-3-hexenoic acid 7. The key transformation was the Claisen rearrangement of the vinyl ketene acetal 4 , which represents novel methodology for the preparation of such unsaturated nine-membered lactones.

Studies towards the synthesis of obtusenyne. Synthesis of the hexahydrooxonin nucleus

Abstract The advanced intermediate (the 2,3,4,7,8,9-hexahydrooxonin)3 for the synthesis of the Laurencia oxonane natural product, obtusenyne1 was prepared in 8 steps from the previously reported lactone4. The key transformations were the stereoselective enolate hydroxylation of the lactone4 and a hydroxyl-directed intramolecular hydrosilation of the enol ether12.

Determination of molecular weight distributions of novolac resins by gel permeation chromatography

The number- and weight-average molecular weigths of several statistical and high ortho novolac resins were determined using gel permeation chromatography (GPC). The standards used were pure compounds having between 2 and 12 phenol units bridged via methylene linkages. Three series of compounds were studied: (i) those with methylene linkages substituted in only the ortho positions relative to the phenolic hydroxyl group; (ii) those in which all para positions, together with sufficient ortho positions, were used to synthesize the compounds; and (iii) those in which the methylene linkages were substituted at a mixture of ortho and para positions. Such compounds, having known molecular architecture and units similar to the segments of industrial novolac resins, provide for a more exact measurement of the molecular weight than do the commonly used poly(styrene) standards. Using these new standards the number average molecular weights of the resins determined by GPC were in good agreement with the average molecular weight obtained by 1H-NMR spectra of the resins, particularly for low molecular weight resins. GPC analysis of higher molecular weight resins tends to underestimate the molecular weights because of complications introduced by hydrogen bonding.

Chemistry of novolac resins. II. Reaction of model phenols with hexamethylenetetramine

It is proposed that the reactions of hexamethylenetetramine (HMTA) with 2,4-xylenol and with 2,6-xylenol occur by different pathways. The rate of reaction and the final product distribution depend on the initial xylend : HMTA ratio and are different in the two systems. Measured by HMTA consumption, with 2,4-xylenol the reaction rate increased with increasing xylenol : HMTA ratios, whereas with 2,6-xylenol the rate of reaction decreased with increasing 2,6-xylenol : HMTA ratio. In systems which contain both 2,4- and 2,6-xylenol, a strong preference for reaction of the HMTA with the ortho site of 2,4-xylenol was noted. This preference was apparent even in mixtures in which 2,6-xylenol was present in greater amounts than 2,4-xylenol.

SYNTHESIS OF (-)-GLOEOSPORONE

Abstract The homochiral lactone ( 5 ) and the oxazolidinone ( 9 ), together with the phosphoniumsalt ( 15 ), serve as key building blocks for the enantioselective synthesis of the ( Z )-bis(silyloxy) ester ( 17 ), whose photochemical conversion into the ( E )-isomer ( 18 ) constitutes a formal synthesis of the antifungal macrocyclic lactone (−)-gloeosporone ( 2 ). The building blocks ( 5 ), ( 9 ), ( 15 ), and ( 17 ) are used to construct the enantiomerically pure ester ( 18 ) thus constituting a formal synthesis of (−)-gloeosporone ( 2 ).

The chemistry of novolac resins. Part 4. The strategic synthesis of model compounds

Abstract An ion assisted ortho-specific phenol-formaldehyde condensation process, incorporating selective protecting-deprotecting methodology has been adapted to prepare a range of model novolac compounds which are ideal for studying crosslinking with hexamethylenetetramine.

Enolisation of αβ-unsaturated esters : regio- and geometrical control

Abstract LDA-Mediated γ-deprotonation of simple αβ-unsaturated esters occurs ( Z )- to the ester group; silylation then leads to oxy butadienes with strong regio- and geometrical selectivity.

Approaches to the controlled formation of network polymers: 1. Synthesis and evaluation of monomers with vinyl differentiation

Abstract The synthesis of novel crosslinking agents N -acryloyl, N -methacryloyl methylenediamine ( 1 ) and N -acryloyl, N ′-methacryloyl ethylenediamine ( 2 ) with non-identical vinyl functionalities is described. When incorporated into crosslinked networks, the resultant polymers exhibit a higher degree of porosity than the more conventional systems where the vinyl functionalities are identical, as evidenced by protein mobility studies using electrophoresis and water permeability experiments. This may be attributed to the formation of a bimodal pore size distribution arising from a crosslinking mechanism which is dictated by the relative reactivities of the differentiated vinyl units within the novel monomers.