Propagation Steps Of Polymerization Research Articles

Effect of the loading of organomodified clays on the thermal and mechanical properties of a model dental resin

Experimental dental resins composed of triethyleneglycol dimethacrylate, urethane dimethacrylate, ethoxylated bisphenol-A-dimethacrylate and bisphenol A glycidyl methacrylate containing an organoclay as filler were prepared by photopolymerization. The addition of organoclay fillers results in slower polymerization rates (0.59 and 0.24 mol L-1 min-1, for the formulations without and with 15% of organoclay, respectively) and lower degrees of conversion (0.50 and 0.35 for the formulations without and with 15% of organoclay), as determined by photocalorimetry. The influence of the organoclay on the thermal and mechanical properties of the resins was evaluated using thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) techniques. The (tan δ) and E´ curves indicate the formation of more rigid materials and the damping curve data indicates more homogeneous materials. An increase of storage modulus, E’, was observed for composites with the higher organoclay filler contents. These effects are due to the higher viscosity of the systems with organoclay, resulting in a lower mobility of the radicals during the propagation step of polymerization, as well as to the scattering of the incident photopolymerizing radiation, which lowers the amount of initiation centres.

Cure kinetics, dynamic mechanical and morphological properties of epoxy resin–Im6NiBr2 system

The aim of this study was to examine effects of a catalyst of Nickel–imidazole, i.e. Im6NiBr2 on the cure reaction and network properties of diglycidyl ether of bisphenol A. DSC and FT-IR measurements are carried out on the epoxy resin loaded with 5, 15, and 30 phr of nickel salt to get some insights into the storage stability. It has been concluded that the storage stability has mainly depended on the epoxy composition and was the lowest for the highest level of curing agent. Kinetics of the cure has been described by applying iso-conversional method of Ozawa to scanning DSC data demonstrating that the studied cure reaction is autocatalytic in nature. The Kamal phenomenological approach has been utilized to fit the experimental isothermal DSC data. The model showed a satisfactory fitting of the experimental results at either early stages or at the final steps of the studied cure reaction. Further, the model did not provide a reasonable fitting in the propagation step of polymerization, 0.3<α<0.5, possibly due to existence of the additional reaction/s which did not consider in the used model. DMTA is used to find the optimum cure schedule. It has been shown that the sample with a postcure treatment offers the highest value of glass transition temperature (Tg) in the tanδ peak. SEM and X-ray analyses are employed to investigate the fracture surface morphology and to understand the distribution of nickel in the cured samples exhibiting that the nickel is scattered in the continuous phase homogenously. Flexural properties of the sample cured at optimum conditions are also determined.

Chelation Effect in Polymerization of Cyclic Esters by Metal Alkoxides: Structure Characterization of the Intermediate Formed by Primary Insertion of Lactide into the Al−OR Bond of an Organometallic Initiator

Well-defined dialkylaluminum alkoxides derived from 2-methoxyethanol [R2Al(μ-OCH2CH2OMe)]2 [where R = Me (1a) and tBu (1b)] and rac-ethyl lactate [Me2Al(μ-OCH(Me)CO2Et)]2 (2) were used as model complexes mimicking intermediate species in the initiation and propagation steps in the ring-opening polymerization of cyclic esters with metal alkoxides. Particularly, the effect of Lewis base termini of both the supporting ligand and the first polymer chain repeating unit on the aluminum alkoxides activity in the polymerization of ε-caprolactone (ε-CL) and rac-lactide (LA) were explored. It revealed that the initiation process is affected by the extent of chelation to a similar degree for ε-CL and LA as the character of the donor-functionalized alkoxide ligand determines the access of an incoming cyclic ester molecule to the effective coordinate site. On the other hand, a significant chelation effect on the propagation step was observed only in the polymerization of LA. The difference in the reactivity is rationalized on the basis of the first structurally authenticated intermediate [Me2Al(μ-OCH(Me)C(O))2O(CH2)2OMe]2 (1-LA) resulting from primary insertion of a lactide molecule into the aluminum−alkoxide bonds of the [Me2Al(μ-OCH2CH2OMe)]2 initiator. The direct relationship between the extent of intramolecular coordination and initiation and propagation steps of polymerization is also presented.

Electronic Structure and Ground State Properties of PMMA Polymer: I. Step-By-Step Formation and Stereo-Regularity of the Polymeric Chain—AM1-MO Treatment

ABSTRACT The equilibrium ground state geometric parameters of benzoyl peroxide (as free radical initiator), methylmethacrylate monomer (as a reference molecule), benzoyl-MMA radical (I), phenyl-MMA radical (II) and the three polymeric chain of PMMA (syndiotactic, isotactic, and atactic) are investigated theoretically using AMI-MO method. The stability of phenyl, benzoyl-MMA radicals, and the conformational properties of isotactic, syndiotactic, and atactic structures of PMMA polymers are discussed. The initiation and propagation steps of polymerization are investigated theoretically. The calculated geometric parameters are analyzed in terms of heat of formation, total energy, dipole moment, net charge, ionization potential, electron affinity, energy gap, and charge density maps (Total, HOMO, and LUMO).

“Rounding” of the sulfur living polymerization transition under spatial confinement

“Rounding” effects and other specific changes of liquid sulfur’s polymerization transition are studied using Raman scattering in the case of fluid’s spatial confinement in nanonoporous sol-gel glasses. Specifically, in this paper we demonstrate the smearing or “rounding” of the monomer↔polymer transition caused by geometrical confinement of the liquid, a phenomenon that is in marked contrast to ordinary (bulk) liquid sulfur, where the polymerization transition is quite sharp, bearing a close resemblance to a second-order-type transition. In noticeable agreement with recent theories of living polymerizations, the limited growth of polymeric species brings about a diminishing of the strength of the transition as evidenced in the temperature dependence of many physical properties. The careful determination of the extent of polymerization by analyzing Raman spectra made it possible to recalculate the magnitude of certain thermodynamic parameters changes across the polymerization transition. As a result, the temperature dependence of the equilibrium constants for the initiation and propagation step of polymerization and the number average chain length of polymeric species have been reconsidered. A comparison between the temperature dependence of these quantities for the case of bulk (unconfined) and the corresponding ones for the case of confined liquid has been advanced in the framework of recent mean-field models of living equilibrium polymerization.

Fast Monomers: Factors Affecting the Inherent Reactivity of Acrylate Monomers in Photoinitiated Acrylate Polymerization

A systematic study on the effect of molecular structure on the photoinitiated polymerization of acrylates was undertaken. Initially, the research was focused on the effect of hydrogen bonding, and it was found that preorganization via hydrogen bonding enhances the maximum rate of polymerization (Rp). This hydrogen bonding facilitated preorganization also affected the tacticity of the resultant polymer. Next, the effect of polarity as represented by the calculated dipole moment (μcalc) of a given monomer was investigated. A direct linear correlation between Rp and the calculated Boltzmann-averaged dipole moment (μcalc) was observed. The Rp−μcalc correlation holds for pure monomers, mixtures of monomers, and even mixtures of monomers with inert solvents. This correlation enables the rational design of monomers with a required reactivity. In addition, these studies suggest that the propagation step of polymerization is influenced by hydrogen bonding while the dipole moment influences the termination rate con...

Novel anionic polymerization of β‐lactones mediated by alkali metal supramolecular complexes

AbstractThe anionic polymerization of β‐lactones mediated by supramolecular complexes of alkali metal alkoxides and naphthalenides is demonstrated. The novel chemistry of the initiation and the propagation steps of polymerization is discussed.