Adrian A. Caraculacu

Isocyanates in polyaddition processes. Structure and reaction mechanisms

Abstract The multiple possible addition processes of isocyanates, in accordance with a new proposed general classification based on the interatomic bond type changes is presented. Starting from the latest achievement of the physico-chemical methods, new aspects regarding the isocyanates structure and different hydroxy compound association forms as part of the addition processes were revealed. Some basic reaction mechanisms presented in the current research were re-evaluated in accord with recent quantum chemical and kinetic data. The intramolecular hydrogen bonding in the symmetrical diols was found to determine a significant difference between their two –OH group reactivities. Recent kinetic and other reactivity measurements, extended to polyfunctional systems reported in some macromolecular synthesis, were presented. The perspective of these findings, in order to obtain in the near future an acceptable mathematical model of the polyaddition process by involving the multiple new synergetic effects observed in different real complex reaction mixtures, is analysed.

A new method for a kinetic study of reactions between di-isocyanates and alcohols. Part 1. Symmetrical di-isocyanates

A new method of following the kinetics of the uncatalysed reaction between di-isocyanates and alcohols, which allows the determination of the concentration of all the species occurring during the reaction, by means of the high performance liquid chromatography (HPLC), is described. It has been found that the relationship between the unchanged fraction and the fraction that reacts, theoretically inferred for ideal systems (null catalytic effects), is validated over the whole reaction for real systems. This fact is a verification that the reduced time, τ, is a result of the total catalytic effects, and is a surprising simplification of the kinetics of this type of complex process. The following value ratios of the rate constants (R) for the first and the second NCO groups for various di-isocyanates were determined: 4,4′-BDI R= 1.36, 4,4′-BBDI R= 1.30, p-PDI R= 2.57, m-PDI R= 3.07.

The heterocyclization of several reactive polymers Poly(parabanic acids)

Abstract Some poly(parabanic acids) were synthesized by intermolecular reaction between the urea group from polyureas and oxalyl chloride in the presence of pyridine as catalyst. All the synthesized polyureas and poly(parabanic acids) were characterized by thermogravimetric, i.r. and elemental analysis.

Dibenzyl structure on the macromolecular chain—IV. Polyureas and poly(parabanic acid)s☆

Abstract Polyureas with the dibenzylic system on the macromolecular chain are considered. The behaviour of these polyureas during cyclization with oxalyl chloride, leading to polyparabanic derivatives, was studied. The influences of polymer structures and reaction conditions on the heterocyclization is discussed. All the polymers were characterized by elemental analyses, inherent viscosities, i.r. spectra and thermogravimetric analyses.

New Heterocyclic Polyurethane-ureas based on 4,4′-dibenzyl diisocyanate, Part 1: Influence of Oxadiazole Structure on Mechanical Properties

A series of polyurethane-ureas was synthesized by solution polyaddition reaction of the following components: a polyester-diol, such as poly(ethylene adipate) (PEA), a polyetherdiol, such as poly(tetramethylene glycol) (PTMG) and a diisocyanate, such as 4,4′-dibenzyl diisocyanate (DBDI) or 4,4′-methylene-bis(phenyl-isocyanate) (MDI). In addition, 2,5-bis-(4-amino-phenylene)-1,3,4-oxadiazole (DAPO) was used as a chain extender. The hard segment content ranges between 1.384 and 1.819 mol NHCOO/1000 g polymer. For comparative studies, similar polyurethane-ureas were prepared by the same way but using 4,4′-methylene dianiline (MDA) and 4,4′-diamino-dibenzyl (DAB) as chain extenders. The resulting linear polymers were characterized by IR spectra, elemental and thermogravimetric analyses, differential scanning calorimetry, gel permeation chromatography, inherent viscosity measurements, wideangle X-ray scattering and mechanical properties. The study of the relationships between structure and properties reveals the influence of the nature of starting materials, macrodiols, the heterocyclic diamine and diisocyanates on the mechanical properties of the polymers. The great number of hydrogen bonding that formed between urea and urethane groups and the presence of oxadiazole rings in the polymer chain induced an improved thermal stability and good mechanical properties. The mechanical behavior of these polymers was discussed and compared with that of earlier reported linear polyurethane-ureas based on MDI or with that of related polyurethanes chain-extended with diols.

Poly(parabanic acid)s. New aspects of structure–properties relationships

Abstract Polymers with parabanic units in the macromolecular chain were prepared by intermolecular reaction between various polyureas and oxalyl chloride. The influence of polymer structure on the heterocyclization process and on the properties of the resulting polymers are discussed. All polymers were characterized by elemental and thermogravimetric analyses, inherent viscosities, IR spectra, differential scanning calorimetry, X-ray diffraction, and electrical conductivity measurements.

Polyhydrazides and poly(1,3,4-oxadiazole)s with parabanic structures

Abstract Polyhydrazides with parabanic structures were prepared by polycondensation between a new monomer containing a preformed parabanic ring, viz. 1,3 p (bis-chloroformylphenyl) parabanic acid and dihydrazides. By thermal cyclization with polyphosphoric acid, the polyhydrazides were converted to poly(1,3,4-oxadiazole)s with parabanic structures. The films obtained from these polymers exhibit electrical insulating properties and good thermal stability.

Dibenzylic structures on macromolecular chain: IX. The interaction of urethane-isocyanate groups in polyurethane formation

Abstract The interaction of isocyanate-urethane and isocyanate-amide groups in urethane group formation is studied. Bibenzil-2,2′-di-yl diisocyanate (2,2′ BBDI) presents specific behaviour due to the steric specificity of its structure. Rotation of the two aromatic nuclei around ethylenic bridges favours strong intramolecular anchimeric assistance in the second step of the 2,2′ BBDI with n -butanol (BuOH) reaction. This assistance also enhances secondary reactions leading to unexpected cyclic allophanate structure. Mechanisms for these reactions are proposed in concordance with kinetic measurements.

Study of the Mechanical Properties of Dibenzyl-Based Polyurethane Containing a Molecularly Dispersed UV Absorber

Blends of certain polyurethanes based on 4,4′ -dibenzyl diisocyanate (DBDI) with 1,2-bis-[2-(2-hydroxy-5-methylphenyl)-5-benzotriazolyl]-ethane (BHMBE) as the UV absorber were prepared and their mechanical properties were studied during environmental exposure. Two methods were used to prepare the blends: the introduction of the UV absorber into the polymer solution, or the addition of the UV absorber to the melted prepolymer during the polyurethane synthesis. The latter method allows temporary chemical binding of the UV absorber to polyurethane chain as a result of the additional reaction of OH phenolic groups to the isocyanate end-capped prepolymer. A process was performed and the new urethane groups were broken releasing the absorber as a molecular-level dispersion in the polymer matrix. The improved photo-oxidative stability of dibenzyl diisocyanate-based polyurethane treated with the UV absorber, 1,2-bis-[2-(2-hydroxy-5-methylphenyl)-5-benzotriazolyl]-ethane (BHMBE) is discussed in relation to the chemical structure and the content of UV absorber.

Parabanic polymers obtained by the cyclocondensation reaction of polyureas containing 2,6-pyridyl structure

Abstract The polymers containing 1,3-imidazolidine-2,4,5-trione rings on the macromolecular backbone are known for a few years and named poly(parabanic acid)s. The paper presents some polymers and model compounds with urea and/or parabanic structures, as a result of the cyclocondensation reaction of the urea derivatives containing 2,6-pyridyl rest, with the oxalyl chloride. The 2,6-pyridyl radical has an acid acceptor character that determines an important distinct influence on the cyclocondensation reaction. The influence of the reaction conditions (temperature, time, beside of the presence or absence of pyridine (Py)) on the progress of the cyclocondensation was studied. The advance of the reaction was followed by IR spectra. The products were characterized by the elemental analysis, 1H NMR and IR spectra, solubilities and viscosity measurements. Thermal properties were determined by thermogravimetric analyses and differential scanning calorimetry. The analytical and spectral data demonstrated that, in absence of Py, the cyclocondensation performed almost completely only in the case of the aliphatic polyureas, while, in the same conditions, the aromatic polymers were only partly transformed. By using the Py as catalyst, both aromatic and aliphatic polyureas can be transformed into parabanic polymers at above 90% transformation degree (TD). A calculation method of the TD based on the IR spectroscopy was discussed.