S. A. Chesnokov

Formation of heterogeneous polymer structures during photoinduced crosslinking of oligo(ester acrylates) in the presence of a nonpolymerizable component

The morphology of polymers prepared through the photoinduced polymerization of oligo(carbonate dimethacrylate) in the presence of different nonpolymerizable additives (methanol, dinonyl phthalate, hexane, toluene, benzene, and carbon tetrachloride) is studied via the method of atomic force microscopy. Depending on the nature and concentration of an additive, the photoinduced polymerization of the above composite systems is shown to be accompanied by microphase separation and formation of a porous polymeric material. In the case of methanol, homogeneous porous structures with characteristic pore sizes of several hundred nanometers are formed. In the case of dinonyl phthalate, the characteristic pore sizes lie below 100 nm. The synthesized porous polymers can sorb both polar and nonpolar solvents. The photoinduced polymerization of an oligomer in the medium of toluene, benzene, or carbon tetrachloride leads to the formation of polymer nanoparticles whose dimensions are controlled by the nature of a solvent.

The catalytic system tri-n-butyl boron-p-quinone in the free-radical polymerization of styrene

The AIBN-initiated polymerization of styrene is conducted at 60 and 80°C in the presence of tri-n-butyl boron and several p-quinones. The rate of polymerization and the molecular-mass characteristics of the polymers depend on the structure of the used p-quinone and the temperature of the process.

General conditions and experimental design of sustained frontal photopolymerization in photopolymerizable liquid compositions

It has been shown that, in order to accomplish frontal photopolymerization in the mode of a self-sustained travelling wave, it is necessary to use compositions that are optically transparent behind the front and experience the onset of gelling at a minimal degree of polymerization, e.g., systems based on oligoether (meth)acrylates. With the use of a composition containing oligocarbonate methacrylate and the o-benzoquinone-amine photoinitiator system, frontal photopolymerization in a layer of more than 100 mm in thickness was experimentally revealed. It was shown that the photoinitiator and monomer conversion fronts synchronously propagate into the layer. It was found that the coordinate of the leading edge of the monomer conversion front h is related to the irradiated time τ by the equations h = A1 log τ − B and h = A2τ − B at the first and the second step of frontal polymerization, respectively. It was found that the thickness of the area of the formulation layer in which the condition h = A1 logτ − B is fulfilled is equal to the width of the photoinitiator and monomer conversion front. A general equation of motion of the coordinate of the photoinitiator conversion label in the layer of a liquid photopolymerizable composition during sustained frontal photopolymerization was proposed for describing experimental data.

Influence of o-benzoquinone nature on initiation of radical polymerization by the o-benzoquinone—tert-amine system

Abstracto-Benzoquinones initiate radical polymerization of methacrylates under visible light irradiation in the presence of tertiary amines. Spectral sensitivity of the initiating system coincides with absorption bands of o-benzoquinone attributed to the S(π→π*) (λmax ≈ 400 nm) and S(n→π*) (λmax ≈ 600 nm) transitions. The amine radicals (Am·) initiating polymerization are generated by the photoreduction of Q in the presence of AmH from the triplet radical pair 3(QH·, Am·). The yield of Am· depends on the difference between the volumes of substituents in the 3 and 6 positions of the quinoid ring and is maximal for symmetrically substituted o-benzoquinones. For a series of derivatives of symmetrical 3,6-di-tert-butyl-o-benzoquinone, the rate of photopolymerization of α,ω-bis(methacryloyloxyethyleneoxycarbonyloxy)ethyleneoxyethylene (OCM-2) in the presence of N,N-dimethylaniline is determined by the free energy (ΔGe) of electron transfer from the amine to photoexcited o-benzoquinone. The ΔGe value includes the energies of oxidation of the amines and reduction of the o-quinones and the energy of the 0→0 transition of the triplet excited state of o-benzoquinones, which are equal to their redox potentials. The photopolymerization rate is maximal for ΔGe ≈ 0.

Photoreduction ofortho-benzoquinones in the presence ofpara-substitutedN,N-dimethylanilines

Photoreduction ofo-benzoquinones irradiated at the wavelengths λmax ≈ 400 and 600 nm corresponding to the S(π → π*) and S(n → π*) electron transitions in the >C=0 groups, respectively, in the presence ofN,N-dimethylaniline and its derivatives was studied. The apparent rate constants of the photoreduction (kH) ofo-quinones are determined by the free energy of electron transfer from the amine molecule to a photoexcitedo-quinone molecule (ΔGe.t). The ΔGe.t. values are calculated as the sums of the energies of the 0→0 transitions of the lowest triplet excited state ofo-quinones, the reduction energies ofo-quinones, and the oxidation energies of amines (the last two terms are numerically equal to the corresponding redox potentials). The maximum rate of photoreduction was found for ΔGe.t≈0. The reaction mechanism is proposed, in which the reversible formation of a triplet exiplex is the rate-determining stage and hydrogen transfer proceeds in parallel with electron transfer within the exiplex.

Photopolymerization of methyl methacrylate in the presence of the tri-n-butyl boron—p-quinone system

The photopolymerization of methyl methacrylate in the presence of tri-n-butyl boron and a number of p-quinones is studied in a wide concentration range. It is shown that the rate of polymerization and the molecular-mass characteristics of the polymers depend on the structure and concentration of p-quinone. PMMA isolated at various conversions initiates the secondary polymerization of methyl methacrylate. The activity of the macroinitator depends on the structure of p-quinone.

Photoinitiation of methacrylate polymerization with an o-benzoquinone-amine system

The kinetic features of the photopolymerization of mono- and dimethacrylates in the presence of the binary initiating system comprising a substituted o-benzoquinone and a tertiary amine under the action of visible light are studied. In the case of methyl methacrylate, the limiting conversion of the monomer does not exceed 10%. The photopolymerization of oligo(ester methacrylates) yields a polymer glass with a monomer conversion of 60–90%. As is shown for a series of seven o-benzoquinones, the rate of photopolymerization increases with an increase in the volume of substituents in positions 3 and 6 of the quinoid ring of o-benzoquinone. It found that trialkylamines (dimethylethanolamine and dimethylcyclohexylamine) are more efficient as coinitiators of photopolymerization than N,N-dimethylaniline. For compositions based on 3,6-di-tert-butylbenzoquinone-1,2, the spectral-sensitivity range in the visible region is 400 to 650 nm with a maximum at 600 nm. This value coincides with the maximum of the absorption band of quinone, which corresponds to the S(n → π*) electronic transition of carbonyl groups.

Synthesis and structure of Schiff bases based on 4,6-di-tert-butyl-2,3-dihydroxybenzaldehyde. New sterically hindered bis-catecholaldimines

A reaction of 4,6-di-tert-butyl-2,3-dihydroxybenzaldehyde with aliphatic (ethylenediamine, propylene-1,3-diamine, 1,4-diaminobutane) and aromatic (o-, m-, p-phenylenediamines, 1,8-diaminonaphthalene, benzidine) diamines leads to the Schiff bases containing fragments of sterically hindered catechols in high yields (up to 97%). The products exist in two tautomeric forms in solutions and in the crystalline state. In the case of aliphatic diamines, catecholaldimines exist in solutions in the quinomethide form, whereas in the case of aromatic, in the catechol form. In the crystalline state, the position of the hydrogen atom in the O-H…N fragment is also affected by the intermolecular hydrogen bonds, which promote stabilization of the quinomethide form of catecholaldimines.

Products and mechanisms of photochemical transformations of o-quinones

The photochemical transformations of quinones by the action of light at λ > 500 nm, namely, the photodecarbonylation and photoreduction reactions were studied with the use of a series of o-benzoquinones and 9,10-phenanthrenequinone as examples. The two-stage mechanism of the decarbonylation reaction of o-benzoquinones was established. At the first stage, rearrangement of a photoexcited quinone molecule into a bicyclic compound that spontaneously decomposes in the dark reaction into cyclopentadienone and CO takes place. It has been found that the formation of the photoreduction products of both o-benzoquinones and 9,10-phenanthrenequinone in the presence of various H donors (N,N-dimethylanilines and polymethylbenzenes) follows the same mechanism. In the first step, a phenol ether is produced, which subsequently undergoes quantitative transformation into pyrocatechol or ketol via the heterolytic mechanism. The stability of phenol ethers is determined by the structure and redox properties of the reactants.

Molecular and crystalline structure of pyrocatechol and hydroquinone dimethacrylates and their reactivity in melts

X-ray diffraction analysis of pyrocatechol and hydroquinone dimethacrylates (Tm = 18 and 86–88°C, respectively) shows that the oligomer molecules within crystals are packed in stacks where the methacrylate fragments of neighboring molecules are parallel to each other. The minimum distances between the centers of double bonds of adjacent methacrylate fragments in crystals of pyrocatechol and hydroquinone dimethacrylates are 4.621(3) and 4.269(4) A. The curves showing the reduced rate of photopolymerization of oligomer melts versus conversion (9,10-phenanthrenequinone used as the initiator) display a maximum at conversions of 1.5–3.0%. The limiting conversion in photopolymerization of molten pyrocatechol dimethacrylate at 25 and 40°C is 20%; for hydroquinone dimethacrylate at 95°C, it is approximately 10%. As the temperature rises from 25 to 40°C, the maximum reduced rate of photopolymerization of pyrocatechol dimethacrylate increases by a factor of 1.4.