Phase transitions in a solution of solvated oligomers

Phase diagrams of oligomer solutions have been constructed within the mean-field approximation. The free energy includes the configuration entropy and the formation free energy of both solvation shells of the oligomers and bonds between them, where this formation free energy depends upon the oligomer-solvent and solvent-solvent bonding energies. The equations for spinodal (phase separation boundary) and percolation threshold (solution-to-gel transition) in the oligomer concentration-temperature coordinates have been found. The model makes it possible to determine the position of phase boundaries by quantum-chemical calculations of these energies and to explain the change in the composition of colloid particles and gel, depending on the solvent. The phase separation region has been predicted to narrow and to be displaced toward higher oligomer concentrations in a series of solvents with progressively increasing intermolecular bond energy.     

Molecular mass distribution of oligomers in products of tetrafluoroethylene radical telomerization

The molecular mass distributions (MMD) of perfluorinated oligomers in products of tetra-fluoroethylene (TFE) radical polymerization in various solvents (telogens) were determined from an analysis of differential thermogravimetric curves and data of gel permeation chromatography and mass spectrometry. Radiolysis of the telogens generates radicals initiating polymer chain growth. The choice of the solvent and TFE concentration makes it possible to obtain oligomers with the controlled average chain length from 4 for 40 CF₂ fragments and specified terminal groups. The polymerization of TFE in THF and propylene oxide affords oligomers with cyclic terminal groups capable of further polymerization due to ring opening. The appearance of two MMD maxima (low-molecular-weight at n ₁ ～6–8 and high-molecular-weight at n ₂ > 10 shifting towards high n with an increase in the TFE concentration) is caused by the formation of colloidal solutions of oligomers.     

Radical polymerization of tetrafluoroethylene in solutions of trimethoxysilanes: Formation of colloidal solution and gel of oligomers

Radical polymerization of tetrafluoroethylene (TFE) in solutions of trimethoxysilanes leads to the formation of fluoroalkoxysilane oligomers and the products of their subsequent hydrolysis and dimerization that occur when methoxyl groups are replaced by hydroxyl groups and Si–O–Si links to bind the oligomers are subsequently formed. The chain length of the oligomers increases with the initial TFE concentration, thereby leading to the formation of colloidal solutions. Colloid particles contain oligomers and solvent molecules, the number of which per TFE unit decreases as the chain length grows to 4–6. Partial replacement of the starting solvents, which are also capable of creating a silicone skeleton during polycondensation, makes it possible to control the number of fluoroalkyl chains attached to this skeleton.     

Formation of colloid solutions and gels during the radiation-chemical telomerization of tetrafluoroethylene in acetone

The polymerization of tetrafluoroethylene (TFE) is initiated by the radicals generated by the radiolysis of acetone. The oligomer chain length n increases with an increase in the initial monomer concentration C. The transition from the true to the colloid solution at C ≈ 0.07 mol/l takes place at an average telomer chain length of $ \bar n $ \bar n ≈ 5. The transition of the colloid solution into gel is observed at C ≈ 0.30–0.35 mol/l. The acetone/TFE ratio in the centrifugation sediment decreases with an increase in the centrifugal acceleration and C. Its minimal value of ∼4 is attained at C > 2 mol/l when $ \bar n $ \bar n ≈ 12–15. The mean particle diameter in the phase separation region is 0.6–0.8 μm. A scaling model of radical polymerization is proposed to explain the molecular-mass distribution of telomers.     

Determination of rate constants for radical telomerization chain growth and transfer from the molecular-mass distribution of oligomer

The ratio of rate constant for growth and transfer X(n), as a function of the chain length n has been found from the measured molecular-mass distributions of the products of tetrafluoroethylene telomerization in acetone, ethyl acetate, chloroform, and carbon tetrachloride. For all these telogens, the function increases by a factor of 1.5–2.5 in the range of n from 2 to 5, is almost constant for n of 6 to 10, and increases by a factor of 7–10 in the range of n from 12 to 20. This behavior of the function X(n) has been explained in terms of the model of diffusion-controlled propagation and kinetic chain transfer. The model takes into account the change in the diffusion nature of oligomers in the form of rigid rods with an increase in their length. A sharp increase in X(n) occurs when the oligomers that accumulate in the environment of growing macroradical sterically restrict the withdrawal of the forming oligomer to the bulk by an effective solid angle, which decreases with the increasing oligomer length and becomes minimal in the region of formation of colloidal particles.     

Particle formation mechanism and kinetic model of ultrasonically initiated emulsion polymerization

A general kinetic model of particle formation in an ultrasonically initiated emulsion polymerization system is presented. This model takes into account homogeneous, micelle entry, and monomer droplet nucleation mechanism. The effects of the ultrasound in producing free radical, degrading free radical and influencing the fashion of the nucleation are also considered. Moreover, chain transfer to monomer and termination in the aqueous phase, capture of oligomer radicals by particles, and coagulation of particles are also considered. An analytical solution is obtained for the initial particle stage consideration. This model predicts that, if the desorption of radical from particles can be neglected, the concentration of the total radical in the aqueous phase is directly proportional to the cavitation concentration. Model predictions are in good agreement with experimental data obtained from the literature.     

Visualization of supramolecular framework of perfluorinated-oligomer colloid particles by supercritical drying

Aerogels of the tetrafluoroethylene radical polymerization products H(C₂F₄)_nR, where R is the radical formed by the abstraction of a hydrogen atom from a solvent molecule, have been obtained by replacing the solvent with supercritical CO₂ and its subsequent rapid evaporation. According to the data of scanning electron and atomic force microscopy, the aerogel consists of loosely bound particles of 1–3 μm in diameter, which is two to three times that of colloid particles in the initial solution, where the particles consist of an oligomer framework filled with solvent molecules. The internal structure of the framework is manifested in the surface topography with a roughness coefficient of 1.6–1.8. High roughness leads to the formation of ultrahydrophobic coatings with contact angles of >160°. A model of supercritical drying in which the solvent is removed from the colloidal particles without alteration of the supramolecular structure is discussed.     

超临界二氧化碳光聚合制备可交联含氟聚合物颗粒及应用

通过自由基无规共聚和酰化反应两步法制备了两类侧链含丙烯酸酯双键的含氟低聚物,并通过调节反应物种类、配比和链转移剂用量,制备出一系列不同双键含量、氟含量的多官能度低聚物。以制备的含氟低聚物为原料,进行超临界二氧化碳光聚合制粒。聚合得到的含氟颗粒作为增强相,加入TPGDA光固化配方中,可实现颗粒与树脂的化学交联,显著提高基材的交联密度,改善疏水性,降低吸水率,并增强热稳定性。     

Photoinitiated polymerization of methacrylic monomers in a polystyrene matrix: Kinetic,mechanistic, and structural aspects

The kinetics and mechanism of the photoinitiated polymerization of tetrafunctional and difunctional methacrylic monomers [1,6‐hexanediol dimethacrylate (HDDMA) and 2‐ethylhexyl methacrylate (EHMA)] in a polystyrene (PS) matrix were studied. The aggregation state, vitreous or rubbery, of the monomer/matrix system and the intermolecular strength of attraction in the monomer/matrix and growing macroradical/matrix systems are the principal factors influencing the kinetics and mechanism. For the PS/HDDMA system, where a relatively high intermolecular force of attraction between monomer and matrix and between growing macroradical and matrix occurs, a reaction‐diffusion mechanism takes place at low monomer concentrations (<30–40%) from the beginning of the polymerization. For the PS/EHMA system, which presents low intermolecular attraction between monomer and matrix and between growing macroradical and matrix, the reaction‐diffusion termination is not clear, and a combination of reaction‐diffusion and diffusion‐controlled mechanisms explains better the polymerization for monomer concentrations below 30–40%. For both systems, for which a change from a vitreous state to a rubbery state occurs when the monomer concentration changes from 10 to 20%, the intrinsic reactivity and k_p/k_t^1/2 ratio (where k_p is the propagation kinetic constant and k_t is the termination kinetic constant) increase as a result of a greater mobility of the monomer in the matrix (a greater k_p value). The PS matrix participates in the polymerization process through the formation of benzylic radical, which is bonded to some extent by radical–radical coupling with the growing methacrylic radica, producing grafting on the PS matrix. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2049–2057, 2001     

Drying of colloidal solutions of fluoroalkyl oligomers

The change in the supramolecular structure upon drying (solvent removal) of colloidal solutions of fluoroalkyl oligomers at atmospheric pressure has been studied using atomic force microscopy. In an initial colloidal solution, micrometer-sized particles of the dense phase consist of randomly oriented oligomers in the form of rigid rods of a 3–5 nm length forming a porous framework filled with solvent molecules, which solvate the oligomer chains. The drying-induced capillary pressure, which in nanosized pores is of the same order of magnitude as the solvation energy, leads to framework deformation, collapse of the pores, and the formation of lamellar and dendritic structures on a 50–100 nm scale. The ordering of these structures (formation of blocks of parallel oriented fibers typical of a fluoroplastic) increases as the heat-treatment temperature and the drying rate are increased, increasing the roughness of the surface (ratio of real to smooth surface area) and its hydrophobicity.     

Kinetics of formation of the topological structure during radical polymerization accompanied by the reaction of chain transfer to the polymer

The structural-kinetic features of radical polymerization accompanied by chain transfer to the polymer are considered in this study. The kinetic equations are solved in a general form without any assumptions. The expression for the critical conversion of gel formation (the gel point), α_c, is obtained: $\alpha _c^{ - 1} = 1 + (1 - \delta )[M]_0 (k_{trp} - (k_{trp} )_c )/k_t [R]$ . The resulting α_c values are compared with the data calculated through the monoradical approach, when it was assumed that the steady-state approximation not only for total radical concentration [R] but also for macroradical concentration [R _i ] can be used. A close agreement between the results is demonstrated. This fact means that the latter approach can be used to analyze the kinetics of the process. The main topological characteristics of the polymer are obtained: the molecular-mass distribution and the distribution of macro-molecules over the degrees of branching.     

Emulsifier-free polymerization of <Emphasis Type="Italic">n</Emphasis>-butyl acrylate involving trithiocarbonates based on oligomer acrylic acid

The effect of the chain length of oligomer acrylic acid obtained in the presence of a low-molecularmass trithiocarbonate and the position of trithiocarbonate fragment (within the chain or at the chain end) on the process of emulsion polymerization of n-butyl acrylate and characteristics of the resulting dispersions has been studied for the first time. It has been found that, when using an oligomer with trithiocarbonate group located within the chain in the emulsion polymerization of n-butyl acrylate in a wide range of monomer–water phase compositions, triblock copolymers self-organizing in aqueous medium to give stable particles with the core–shell structure are formed. Oligomers with M _n ～ (5–10) × 10³ are optimal for synthesis of stable dispersions. In this case, block copolymers with the controlled length of hydrophobic block and a rather narrow MWD may be obtained. Thin films formed from these copolymers retain the structure of the initial dispersions on solvent removal. If the trithiocarbonate group in the oligomer is located at the chain end, the main polymerization product is a diblock copolymer. In this case, the formation of polymer–monomer particles occurs during a longer period of time, the control of MWD is weakened, and the dispersions of particles lose the aggregative stability after thin film formation.     

Isolation and identification of the linear and cyclic oligomers of poly(ethylene terephthalate) and the mechanism of cyclic oligomer formation

Chromatographic techniques are described which can be used to isolate and identify the linear and the cyclic oligomers of poly(ethylene terephthalate). Extraction of the oligomers from high molecular weight polymer produces at least eight different cyclic species, some of which are isolated and identified. The cyclic dimer, the cyclic trimer, and the cyclic tetramer of poly(ethylene terephthalate) have also been prepared by acid chloride esterification and transesterification. Similar materials can be isolated from the ethylene glycol distillate obtained from the polymer melt. The mechanism of cyclic oligomer formation has been studied by determining the rate of formation of the cyclic oligomers during polymerization and during melt extrusion of polyesters which did not initially contain cyclic oligomers. The rate of formation depends upon the concentration of hydroxyl groups; hence, the cyclic oligomers are formed by transesterification from the chain ends or cyclodepolymerization. Therefore oligomers are inevitably produced during polymerization.     

Action of N-hydroxyphthalimide on chain stereoregularity in the radical polymerization of methyl methacrylate

N-Hydroxyphthalimide (NHPI) has a dual function in the radical polymerization of methyl methacrylate (MMA): this compound acts as an initiator and also provides for stereochemical control of the polymer chain. Lowering the polymerization temperature leads to increased syndiotactic specificity in the polymethyl methacrylate molecule. The action of NHPI as an initiator and stereoregulator is attributed to the capacity of this compound to form a hydrogen bond with the monomer molecule and growing macroradical.     

Living radical miniemulsion polymerization by RAFT in the presence of beta-cyclodextrin

Living radical polymerization of styrene in a miniemulsion by reversible addition–fragmentation chain transfer (RAFT) was successfully realized in the presence of beta-cyclodextrin (CD), using sodium dodecyl sulfate and hexadecane as surfactant and costabilizer, respectively. The drawback of instability (red layer formation) encountered in the living radical polymerization in emulsion or miniemulsion was overcome. The linear relationship between the monomer conversion and the molecular weight, as well as lower molecular weight distribution (MWD), shows that the polymerization process was under control. The addition of CD was found to have little influence on the polymerization rate. However, MWD of the polymer synthesized is obviously decreased. The mechanism of stability and controllability improvement in the presence of CD proposed that the complex formation between CD and RAFT agent or RAFT agent-ended oligomer increased their diffusion ability from monomer droplet to polymerization locus and improved the homogeneity of the RAFT agent level among the polymerization loci.     

Quantum-chemical simulation of the nanostructure of colloid particles and gels of tetrafluoroethylene telomers in acetone

The telomers H(C₂F₄) _n CH₂COCH₃, where n = 3,7; their complexes with acetone molecules; and the entities composed of two complexes were calculated by the quantum-chemical method of the density functional theory using the software program Priroda. The existence of two configurations of the complexes with the bond energies of 2 to 6 kcal/mol per acetone molecule was established. The bonding of acetone molecules with the macroradical during the chain transfer reaction ensures the spatially correlated arrangement of telomers and leads to the formation of a network, which determines the internal structure of both colloid particles and the gel into which the colloid particles transform with an increase in the network density.     

Effects of external donors and hydrogen concentration on oligomer formation and chain end distribution in propylene polymerization with Ziegler‐Natta catalysts

The effect of type and concentration of external donor and hydrogen concentration on oligomer formation and chain end distribution were studied. Bulk polymerization of propylene was carried out with two different Ziegler‐Natta catalysts at 70 °C, one a novel self‐supported catalyst (A) and the other a conventional MgCl₂‐supported catalyst (B) with triethyl aluminum as cocatalyst. The external donors used were dicyclopentyl dimethoxy silane (DCP) and cyclohexylmethyl dimethoxy silane (CHM). The oligomer amount was shown to be strongly dependent on the molecular weight of the polymer. Catalyst A gave approximately 50 % lower oligomer content than catalyst B due to narrower molecular weight distribution in case of catalyst A. More n‐Bu‐terminated chain ends were found for catalyst A indicating more frequent 2,1 insertions. Catalyst A also gave more vinylidene‐terminated oligomers, suggesting that chain transfer to monomer, responsible for the vinylidene chain ends, was a more important chain termination mechanism for this catalyst, especially at low hydrogen concentration. Low site selectivity, due to low external donor concentration or use of a weak external donor (CHM), was also found to increase formation of vinylidene‐terminated oligomers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 351–358, 2010     

Particle nucleation in emulsion polymerization. II. Nucleation in emulsifier-free systems investigated by seed polymerization

Experiments with seeded polymerization in emulsifier-free systems were carried out with styrene in order to test the theory of particle nucleation presented in the first article in this series. The effect of amount, size, and surface charge density of the seed particles on the formation of new particles was investigated. An expression for the capture rate of oligomeric radicals from the water phase was evaluated in which the rate of capture was considered to be governed by the absorption of oligomers with chain length one less than the critical chain length for precipitation of the oligomer. Coagulation of primary particles was also included in the expression for the number of new particles obtained in the system. Limited coagulation of primary particles with already formed particles and with seed particles was found to play an important role in determining the final number of new particles found at the end of the runs.     

NMR identification of the terminal groups of the telomers of tetrafluoroethylene with tetrahydrofuran

¹H, ¹³C, and ¹⁹F high-resolution NMR spectra with heteronuclear spin-spin decoupling and without it were recorded for identification of the terminal groups of oligomers obtained by radical polymerization of tetrafluroethylene (TFE) in tetrahydrofuran (THF) solutions. The analysis of the spectra and their comparison with the quantum-chemically calculated spectra of possible polymerization products led to the conclusion that the terminal groups of oligomers are the α radical of THF and the hydrogen atom. The structure of oligomers found in this study opens up an opportunity of synthesizing from them polymers consisting of a flexible main chain with substituents in the form of rigid perfluorinated rods.     

Polymerization of vinyl chloride at reduced monomer accessibility. III. Polymerization kinetics and molecular structure using PVC latex as seed

Vinyl chloride was polymerized at 53–97% of the saturation pressure in a water-suspended system at 55°C with an emulsion PVC latex as seed. A water-soluble initiator was used in various concentrations. The monomer was continuously charged as vapor from a storage vessel kept at lower temperature. Characterization included determination of molecular weight distribution and degree of long-chain branching by gel chromatography and viscometry and by thermal dehydrochlorination. To avoid diffusion control intense agitation was necessary. At a certain conversion, aggregation of primary particles resulted in restricted polymerization rate. Before aggregation, formation of new particles did not occur as the number of particles was high enough to ensure capture of all oligoradicals. The kinetic equation accepted for ordinary emulsion polymerization of vinyl chloride was qualitatively found to be valid after the pressure drop as well. Decreased termination rate may result in increased polymerization rate at reduced monomer concentration, i.e., a gel effect, especially at low particle numbers and high polymer contents. The molecular weight decreased with decreasing monomer concentration. This is in accordance with the new mechanism suggested for chain transfer to monomer starting with occasional head-to-head additions.