S.G. Alekseyeva

13C-NMR determination of the structure on linear-branched urea-formaldehyde resins☆

Abstract A 13 C-NMR method has been developed for the determination of the structure of urea-formaldehyde resins. The signals have been identified on the basis of spectra produced from a number of model compounds, and the dependence of the spectra on the urea formaldehyde components ratio, but also from the duration of the reaction. The signals are those of the methylene and methylate linkages in the linear chain, while they are those of the methoxymethylene groups, the radicals of tetra-, tri-, di- and mono-substituted urea, and of free urea, free methanol, and free formaldehyde (methylene glycol and polyoxymethlyene glycol oligomers) in the branches. Branches will form in the early stages of condensation under industrial scale production conditions when the urea: formaldehyde ratio is from 1 : 2 to 1 : 1·3, and the pH = 4·5−8·5, and the products will have a linear-branched structure.

A 13C-NMR study of equilibria in the urea-formaldehyde system

The composition of equilibrium reaction mixtures formed in reaction of urea with formaldehyde in neutral, aqueous solution, has been determined by 13C-NMR analysis. The equilibrium constants have been calculated for all possible methylolation reactions, involved in formation of asymmetric N,N-dimethylolurea and tetramethylolurea. The equilibrium constants for formation of hemiformal groups on secondary and tertiary urea nitrogen atoms were determined and the correlation was found between these constants and the electron density on the carbon atom of the methylol group, as characterized by the 13CH2 chemical shift. The value of the constant in formation of dimethylene-ether bridges was also determined.

Determination of the structure of cyclochain urea-formaldehyde resins by 13C NMR analysis

A method has been developed for 13C NMR analysis of the structure of urea-formldehyde resins containing imidazolidone, triazinone and uronic rings. It is shown that the reaction of urea and formaldehyde with ammonia or hexamethylene tetramine results in the formation of triazinone rings stabilized by methylol groups linked to the “amine” nitrogen of the ring. In a urea-formaldehyde system uronic ring formation takes place in an acid or alkaline medium with a large excess of formaldehyde in the system. Rings of both types add formaldehyde with formation of methylol groups and methylene and methylene ether bridges. The obtained cyclochain structures complement the linear-branched structures formed by urea residues with methylol groups and bridges between the letter.

The structural changes which occur during the synthesis and setting of urea-formaldehyde resins (UFR)☆

Abstract A quantitative description of the structure of urea-formaldehyde resins by means of the 13 C-NMR spectra has been developed. The structure has been investigated at all stages of synthesis and during the initial stages of setting. The branched structures are shown to form at the start of resin formation. A partial fracture of the dimethylene ester bonds and a formaldehyde liberation takes place in an acid medium. Polymer chain propagation takes place during setting as a result of methylene bridges formation, but the polymers retain the linear structure with branches, although the branching intensity increases.

Investigation of isomerism in polyphenylquinoxalines by 13C NMR

Isomerism has been observed in polyphenylquinoxalines by 13C NMR spectroscopy and the properties of the isomers have been measured qualitatively. It has shown that changing the solvent in which polycondensation is carried out enables the ratio between the isomers to be varied within certain limits (from 45 : 55 to 60 : 40).

Differences in the repeated units in polyamido-acids: An investigation by 13C NMR☆

13C NMR spectroscopy has been used to observe and assess quantitatively the differences in monomer units in amido-acids obtained by reaction between the dianhydride of 3,3′,4,4′-dipehnyloxidetetracarboxylic acid and 9,9-bis-(4′-aminophenyl) fluorene (a polyamido-acid) and aniline (a model amido-acid). It has been shown that the opening of the anhydride ring occurs in such a way that the carbonyl carbon atom of the ring, located in the meta position relative to the ether oxygen bridge, preferentially undergoes nucleophilic attack and the anhydride rings of the dianhydride used may be considered to be independent. It has been established that the solvent in which the reaction is carried out has a marked effect on the selectivity of the process.

Structural characteristics of phenol-formaldehyde novolak oligomers☆

The results of the composition and structural determinations for phenol-formaldehyde Novolak oligomers have been compared by means of theoretical and experimental methods. The former is based on the probability theory in a combination with the Monte Carlo method, the latter on 13C-NMR spectroscopy. Both these methods were found to give similar results.

Analysis of the 13C and 1H NMR spectra of phenol-formaldehyde resole oligomers☆

The 1H and 13C NMR spectra have been analyzed to determine the microstructure of the para-substituted resole type phenol-formaldehyde oligomers based on diphenylolpropane and p-tert-butylphenol. Aromatic ring signals in the 13C spectra have been resolved with the aid of the results of a quantitative analysis of acetates of the studied oligomers and the values of additive increments due to ortho-substitution of the phenol ring by methylol and dimethylene ether groups in the oligomers have been refined. Particular features of the microstructure of these resins are related to the chemical shift of the signal of hydroxyl protons in the NMR spectra. This effect is attributed to the formation of intramolecular H bonds of one sort or another.

13C NMR investigation of the structure of aliphatic copolyamides

The 13C NMR method has been used to investigate the composition and unit sequence distribution in the copolymer chains of binary copolyamides based on ϵ-caprolactam and ω-dodecalactam, and similarly for copolyamides containing lactams and the salt of adipic acid and hexamethylenediamine (AG salt). Irrespective of the degree of polymerization there is a random distribution of comonomer units in the case of binary copolyamides prepared by hydrolytic copolymerization of lactams, though the product is caprolactam-enriched in the initial stage of the reaction. In ternary copolyamides prepared by copolycondensation of the monomers experimental lengths of the lactam blocks and ratios of the number of acid-lactam/acid-amine bonds are likewise consistent with a random distribution of units.

Study of oligomers and low molecular weight polycarbonate fractions by 13C and 1H NMR spectroscopy

Abstract 13 C and 1 H NMR was used to examine the type of end group in oligomers and low molecular weight polycarbonate fractions and quantitative estimates were made of the contents of hydroxyl, chloroformate, p -tert-butylphenyl and phenyl end groups, bis-and monochloroformate of diphenylolpropane, free diphenylolpropane, p -tert-butylphenol and phenol, and the number average molecular weight was determined. Results of deformations by 13 C and 1 H NMR spectra show satisfactory agreement and are in accordance with results of other methods of analysis. Structural characteristics of oligocarbonates obtained under different conditions and of low molecular weight fractions of various polycarbonates were compared.