Addition polymerization kinetics

New Techniques for the Study of Electrodes and their Reactions Electron Tunneling in Chemistry. Chemical Reactions over Large Distances Mechanism and Kinetics of Addition Polymerizations Kinetic Models of Catalytic Reactions Catastrophe Theory... 

The process of synthesizing high-molecular-weight copolymers by the polymerization of mixed cyclics is well established and widely used in the silicone industry. However, the microstructure which depends on several reaction parameters is not easily predictable. The way in which the sequences of the siloxane units are built up is directed by the relative reactivities of the monomers and the active chain-ends. In this process the different cyclics are mixed together and copolymerized. The reaction is initiated by basic or acidic catalysts and a stepwise addition polymerization kinetic scheme is followed. Cyclotrisiloxanes are most frequently used in these copolymerizations since the chain growth mechanism dominates the kinetics and redistribution reactions involving the polymer chain are of negligible importance. Several different copolymers may be obtained by this process. They will be monodisperse and free from cyclics and their microstructure can be varied from pure block to pure random copolymers. 

Example 1.4-6 Free Radical Addition Polymerization Kinetics... 

The mechanism of these reactions places addition polymerizations in the kinetic category of chain reactions, with either free radicals or ionic groups responsible for propagating the chain reaction. 

Photoinitiation is not as important as thermal initiation in the overall picture of free-radical chain-growth polymerization. The foregoing discussion reveals, however, that the contrast between the two modes of initiation does provide insight into and confirmation of various aspects of addition polymerization. The most important application of photoinitiated polymerization is in providing a third experimental relationship among the kinetic parameters of the chain mechanism. We shall consider this in the next section. 

Figure 1. Typical reactor temperature profile for continuous addition polymerization a plug-flow tubular reactor. Kinetic parameters for the initiator 1 = 10 ppm Ea = 32.921 kcal/mol In = 26.492 In sec f = 0.5. Reactor parameter [(4hT r)/ (DpCp)] = 5148.2. [(Cp) = heat capacity of the reaction mixture (p) = density of the reaction mixture (h) = overall heat-transfer coefficient (Tf) = reactor jacket temperature (r) = reactor residence time (D) = reactor diameter].

Figure 4. Operation of a plug-flow tubular addition polymerization reactor of fixed size using a specified free-radical initiator (initiator kinetic parameters Ea = 32,921 Kcal/mol In k/ = 26,492 In sec f = 0,5 10 ppm initiation, 1,0 mol %...

Figure 6, Ejfect of solvent concentration on the molecular weight-conversion rehtionships of a tubular-addition polymerization reactor of fix size using a specified initiator type. Each point along the curves represents an optimum initiator feed concentrationr-reactor jacket temperature combination, (kinetic parameters of the initiator Ea = 24,948 Kcal/mol In k/ = 26,494 In sec f = 0.5)...

The free amino group of the amino ester may then react analogously with another molecule of the monomer, etc. The kinetics of the polymerization are in harmony with a mechanism of this sort. The final polypeptide may contain up to 300 or more structural units. While the polymerization of N-carboxyanhydrides is closely analogous to the addition polymerizations of ethylene oxide and of other cyclic substances, definition unfortunately classifies it as a condensation polymerization inasmuch as carbon dioxide is eliminated in the process. 

The study of polymerization kinetics allows us to understand how quickly a reaction progresses and the role of temperature on the rate of a reaction. It also provides tools for elucidating the mechanisms by which polymerization occurs. In addition, we are able to study the effect of catalysts on the rates of polymerization reactions, allowing us to develop new and better catalysts based on the measured performance. 

The objective of the present work was to determine the influence of the light intensity on the polymerization kinetics and on the temperature profile of acrylate and vinyl ether monomers exposed to UV radiation as thin films, as well as the effect of the sample initial temperature on the polymerization rate and final degree of cure. For this purpose, a new method has been developed, based on real-time infrared (RTIR) spectroscopy 14, which permits to monitor in-situ the temperature of thin films undergoing high-speed photopolymerization, without introducing any additive in the UV-curable formulation 15. This technique proved particularly well suited to addressing the issue of thermal runaway which was recently considered to occur in laser-induced polymerization of divinyl ethers 13>16. 

Once calibrated, the NIR analyzer was used to investigate a number of factors expected to affect the polymerization kinetics, including reaction temperature, initiator type, and initiator concentration (relative to monomer concentration). These experiments, in addition to improving process understanding, also mimicked the effects of inadequate process control during a reaction. Figure 15.1 shows the effect of reaction temperature on kinetics. The reaction rate nearly doubles when the temperature is raised from 65 to 75 °C, and the concentration of unreacted monomer after 85 minutes is reduced from 1.1 to 0.5%. In-hne NIR monitoring allows unusual behavior in either reaction rates or residual monomer levels to be detected and corrected immediately. 

Polymerization reactions can proceed by various mechanisms, as mentioned earlier, and can be catalyzed by initiators of different kinds. For chain growth (addition) polymerization of single compounds, initiation of chains may occur via radical, cationic, anionic, or so-called coordinative-acting initiators, but some monomers will not polymerize by more than one mechanism. Both thermodynamic and kinetic factors can be important, depending on the structure of the monomer and its electronic and steric situation. The initial step generates... 

Although the above derivations involve certain simplifications, they nevertheless represent correctly the kinetics of many addition polymerization reactions. However, the behavior is different when the polymerization is conducted under heterogeneous conditions, e.g., in suspension or in emulsion (see literature cited in Sect. 2.2.4). 

When a cychc monomer such a tetrahydrofuran or caprolactam is used as the monomer, the polymerization can be made to occur primarily by monomer reacting with the polymer rather than aU polymers reacting with each other. These kinetics are more tike addition polymerization, where only the monomer can react with the polymer. However, we stiU call this condensation polymerization because it produces this type of polymer. 

The kinetics of template polymerization depends, in the first place, on the type of polyreaction involved in polymer formation. The polycondensation process description is based on the Flory s assumptions which lead to a simple (in most cases of the second order), classic equation. The kinetics of addition polymerization is based on a well known scheme, in which classical rate equations are applied to the elementary processes (initiation, propagation, and termination), according to the general concept of chain reactions. 

Kinetics of Addition Polymerization. As the name suggests, addition polymerizations proceed by the addition of many monomer units to a single active center on the growing polymer chain. Though there are many types of active centers, and thus many types of addition polymerizations, such as anionic, cationic, and coordination polymerizations, the most common active center is a radical, usually formed at... 

Kucera M (1992) Mechanism and kinetics of addition polymerizations. Academia Press, Prague... 

The polymerization kinetics of the bisbenzocyclobutene diketone monomer 14 (Fig. 10) were studied in the melt at various temperatures by infrared spectroscopy [48]. This technique has the advantage that it is relatively insensitive to the physical state of the system as it proceeds from monomer melt through the gel point and into the vitreous state. In addition, quantitative... 

Kinetics. Monomer can be converted into polymer by any chemical reaction which creates a new covalent bond. Most of this review will concern polymerization of vinyl monomers by free radical addition polymerization. However, some attention will be given to cationic polymerization of epoxy functional materials. No extensive review of polymerization processes and kinetics will be given here, but some of the fundamental notions will be described. For reviews, see (4a-d). 

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