Polymerization reaction rate

In ionic polymerizations, reaction rates are faster in solvents with high dielectric constants, which promote the separation of the ion pair. 

Experimental Materials. All the data to be presented for these illustrations was obtained from a series of polyurethane foam samples. It is not relevant for this presentation to go into too much detail regarding the exact nature of the samples. It is merely sufficient to state they were from six different formulations, prepared and physically tested for us at an industrial laboratory. After which, our laboratory compiled extensive morphological datu on these materials. The major variable in the composition of this series of foam saaqples is the aaK>unt of water added to the stoichiometric mixture. The reaction of the isocyanate with water is critical in determining the final physical properties of the bulk sample) properties that correlate with the characteristic cellular morphology. The concentration of the tin catalyst was an additional variable in the formulation, the effect of which was to influence the polymerization reaction rate. Representative data from portions of this study will illustrate our experiences of incorporating a computer with the operation of the optical microscope. 

In traditional liquid solvents, the polymerization reaction rates are often limited by the local increase in viscosity during the process, as this lowers the mass transfer rate of the monomer to the reaction site. A lower viscosity and a higher diffusion coefficient in SCFs each contribute to overcome this limitation, however, allowing the polymerization rate to be significant up to high value of monomer conversion. 

High molecular weight polyoxymethylenes prepared under the right conditions have more than 80% of available end groups as judged by acetylation at 60°C with acetic anhydride, whereas 3-polyoxymethylene has almost no detectable available end groups. It has been indicated that acids and bases increase the polymerization reaction rate acids, however, cause side reactions, predominantly transacetalization. In metha-nolic media acetal formation is the most undesirable side reaction. The... 

Because of the differences in polymerization reaction rates the copolymer first formed is greatly crosslinked entangled), but as the reaction proceeds, and the crosslinking agent (divinylbenzene) is consumed, the structure becomes less crosslinked and consequently... 

Radius of latex particle (dm) Polymerization reaction rate (mol/L/sec) Swollen radius of latex particle (dm) Volume of monomer in droplets (L) Volume of monomer in system (L) Volume of polymer in system (L)... 

The above processes are also influenced by the change in the system temperature. Raising the temperature results in increase of the polymerization reaction rate and at the same time in decrease of the tendency of siufactant molecules to aggregate because of the increase of thermal motion that follows from the Debye equation ... 

Let us note, that for two lower monomer concentrations, adduced in Figs. 22 and 23 (c = 0.646 and 0.826 kg/1) the contributions of singular and nonsingular parts of function In (1-Q)(t) are practically equal, whereas for c = 0.888 kg/1 the contribution of the second one predominates and makes up approx. 94 %. As it follows from the Figs. 22 and 23 data, this results to significantly more rapid (1-Q) decay and, hence, to polymerization reaction rate enhancement. 

CP nanomaterials can be synthesized by chemical or electrochemical methods. In chemical reactions generally powdery nanomaterials are produced and can be scaled up. Nanostructures of CP deposited on the electrode surface as films by electropolymerization have limited surfaces compared to the materials obtained by chemical synthesis. Nanostructures generally grow along the direction of the electric field to form oriented structures. The electrochemical polymerization reaction rate can be controlled through the applied potential or current density, and controlled amount of product can also be obtained. The morphology of the nanomaterials may also be modulated by the conditions of electropolymerization. Electrochemical polymerization is an... 

Fe2(S04)3 as an oxidant and anionic surfactants such as sodium dodecylbenzenesulfone, sodium alkylnaphthalenesulfonate, or sodium alkylsulfonate, the resulting PPy exhibited enhanced conductivity and polymerization yield with an accelerated polymerization reaction rate [30]. 

Pressure exerts a marked effect on the polymerization reaction rate constant and can be used to control the reaction rate and molecular weight in addition to the more usual variables of initiator concentration and temperature. Since the number of short branches and the molecular weight are determined by chain transfer reactions which are more influenced by temperature and less by pressure than the polymerization reaction, it follows that the molecular weight decreases and the degree of short branching increases with increasing temperature (and vice versa with pressure). 

Two strategic parameters for semicrystalline polymers are the polymerization reaction rate and degree of crystallization [10]. 

Natural rubber s processing by two roll-mixing, in the presence of styrene, at temperatures ranging within the 75-130 C interval permits a rapid polymerization of the monomer. The reaction is influenced by the elastomer/styrene weight ratio, being observed that its decrease causes increase of the polymerization reaction rate, which comes to exceed the one corresponding to styrene s thermal polymerization. 

The initiator was also found to make an unusual contribution to the polymerization reaction rate. In dispersion (emulsion) polymerization, initiator concenfra-tion is controlled to control the molecular weight of the polymer. Sometimes the reaction is conducted at a lower... 

The preferred polymerization medium was a saturated fluorocarbon or chlorofluorocarbon solvent, although an aqueous medium could also be used. The solvent system was used to control the reaction conditions and increase the polymerization reaction rate. The reaction medium could also improve melt processability and increase the thermal stability and chemical resistance of the polymer. The reaction could be carried out by bulk, solution, suspension, emulsion, or vapor phase polymerization regimes. Azo compounds, peroxy compounds, ultraviolet radiation, or high-energy ionizing... 

On the other hand, the decay of the polymerization reaction rate is too fast to explain it as a hrst-order reaction. The time-conversion curves are htted as a second-order reaction, but it does not explain the effects of the catalyst concentration. These results indicate that this polymerization proceeds by a single site catalyst under different morphological conditions and/or under variable monomer concentrations, for example, a polymerization in the crystalline polymer and in the amorphous polymer state. 

Beside its impact on the solubility the temperature directly affects the polymerization reaction rate [1], This might be expected because polymerization involves a kinetic phenomenon. However, the influence of temperature cannot be discussed without considering the two different pH regions [4],... 

In the course of polymerization initiation of benzoyl peroxide = 125.4 kJ/mol. Since for most of monomers E =29.1 kJ/mol, thus E = 12-20 kJ/mol then it follows that the polymerization activation energy E > 0. For this reason the polymerization reaction rate clearly increases with temperature. Thus, the degree of polymerization and the same the average molecular mass of the resulting polymer decrease. 

Another important parameter influencing the course of polymerization is the pressure. A moderate increase of pressure shows no apparent effect on the polymerization reaction rate. Pressures above 100 MPa increase the rate constant of chain growth and by the same the speed of polymerization reaction. The increase of pressure results also in the increase of average molecular weight of the formed polymer and improves the regularity of its spatial structure. 

The total activation energy for radiation polymerization varies within the limits of 0-41.8 kJ/mol. A small value of this energy causes that the radiation polymerization reaction rate is temperature independent within a few tens of Kelvins. A characteristic feature of the radiation polymerization is the presence of the retrospective effect (post-effect). It consists on the fact that after the cessation of irradiation the polymerization continues over many hours. This phenomenon concerns primarily the polymerization in solid phase or in solution precipitation. The reason for this phenomenon is the reduced mobility of macroradicals in solid phase and difficulties ending the chains by recombination. 

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