Cationic polymerization concentration

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that form tertiary carbocations In some cases reaction conditions can be developed that favor the formation of higher molecular weight polymers Because these reactions proceed by way of carbocation intermediates the process is referred to as cationic polymerization We made special mention m Section 5 1 of the enormous volume of ethylene and propene production in the petrochemical industry The accompanying box summarizes the principal uses of these alkenes Most of the ethylene is converted to polyethylene, a high molecular weight polymer of ethylene Polyethylene cannot be prepared by cationic polymerization but is the simplest example of a polymer that is produced on a large scale by free radical polymerization... 

The potentiometry sensor (ion-selective electrode) controls application for determination of polymeric surface-active substances now gets the increasing value. Potentiometry sensor controls are actively used due to simple instmment registration, a wide range of determined concentrations, and opportunity of continuous substances contents definition. That less, the ionometry application for the cation polymeric SAS analysis in a solution is limited by complexity of polycation charge determination and ion-exchanger synthesis. 

Polybutene resins. These liquid resins are obtained by cationic polymerization of petroleum C4 streams in the presence of AICI3 at relatively low temperature. Temperature and AICI3 concentration are important factors as they influence the molecular weight and viscosity of the final resin. After reaction, the mixture is deactivated with water, methanol, ammonia or aqueous sodium hydroxide. The organic layer is separated and distilled to remove solvent and unconverted material. 

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that fonn tertiary carbocations. In some cases reaction conditions can be developed that favor the formation of higher molecular-weight polymers. Because these reactions proceed by way of carbocation intermediates, the process is refened to as cationic polymerization. 

Results of these orienting experiments compiled in Table 3 in regard to the effect of temperature, medium polarity, initiator concentration, monomer concentration, and coinitiator concentration are similar to those reported by others36"39 for cationic polymerization of a-methylstyrene. For example, decreasing temperature, the molecular weight increases and increasing medium polarity, the yield increases. 

Cationic polymerization of cyclic acetals generally involves equilibrium between monomer and polymer. The equilibrium nature of the cationic polymerization of 2 was ascertained by depolymerization experiments Methylene chloride solutions of the polymer ([P]0 = 1.76 and 1.71 base-mol/1) containing a catalytic amount of boron trifluoride etherate were allowed to stand for several days at 0 °C to give 2 which was in equilibrium with its polymer. The equilibrium concentrations ([M]e = 0.47 and 0.46 mol/1) were in excellent agreement with that found in the polymerization experiments under the same conditions. The thermodynamic parameters for the polymerization of 1 were evaluated from the temperature dependence of the equilibrium monomer concentrations between -20 and 30 °C. 

Provided that the uncomplexed species is suceptible to destruction, while the complex is protected, the lifetime of living polymer increases at higher monomer concentrations. Cationic polymerization of propylene induced by AlBr3. HBr studied by Fontana and Kidder22) exemplifies perhaps such a system. 

Ionic Polymerization. Ionic polymerizations, especially cationic polymerizations, are not as well understood as radical polymerizations because of experimental difficulties involved in their study. The nature of the reaction media is not always clear since heterogeneous initiators are often involved. Further, it is much more difficult to obtain reproducible data because ionic polymerizations proceed at very fast rates and are highly sensitive to small concentrations of impurities and adventitious materials. Butyl rubber, a polymer of isobutene and isoprene, is produced commercially by cationic polymerization. Anionic polymerization is used for various polymerizations of 1,3-butadiene and isoprene. 

A variety of initiators have been used for cationic polymerization. The most useful type of initiation involves the use of a Lewis acid in combination with small concentrations of water or some other proton source. The two components of the initiating system form a catalyst-cocatalyst complex which donates a proton to monomer... 

The kinetic expressions which describe the rate and degree of polymerization in cationic polymerizations are derived in a manner analogous to that for radical polymerization. The results are similar with the main difference being that the direct and inverse dependencies of the rate and degree of polymerization, respectively, on the initiator concentration or initiation rate are both first-order, not half-order as in radical polymerization. The difference arises from cationic termination being mono-molecular in the propagating species instead of bimolecular as in radical polymerization. 

Since the determination of absolute rate constants is one of the most urgent problems in cationic polymerization, and the styrene-perchloric acid system seemed to be so clean and simple, Gandini and Plesch set out first to check Pepper and Reilly s results by determining spectroscopically the concentration of carbonium ions during polymerization, and they intended then to extend the method to other monomers. However, their findings were not as expected. A comparison of spectroscopic and conductivity measurements with rate measurements in an adiabatic calorimeter showed [4] that in methylene dichloride solution ... 

Cx is a charge transfer complex the position of the equilibria, and, hence, the importance of Cx, and the concentration of the radical ions, may differ greatly from one system to another. The radical cation then probably reacts in most systems in such a way that the radical function is rapidly inactivated and the cationic function then propagates a quite normal cationic polymerization. 

The magnitude of c is more difficult to determine. In most studies of cationic polymerizations the nominal catalyst concentration lies in the range 10 2-10 4 mole/l. However, in syncatalytic systems, such as SnCl4-H20, the total concentration of cations, c, at any instant may be equal to the concentration of the scarcer of the two components (usually 10 3 to 10"5 mole/l) or it may be very much less than this. 

Consider now a pseudo-cationic polymerization in which the non-ionic chain-carrier can form a complex with the compound R. The algebra and resulting equations are the same as above, with the one difference that q now signifies the concentration of uncomplexed chain-carriers and w the concentration of those which are complexed with R. 

It is the author s hope that the foregoing detailed discussion has helped to clarify some features of cationic polymerizations. Many other aspects, such as co-polymerization and radiation polymerization, which I have not been able to discuss here, deserve equal attention. But perhaps the most urgent task, and one which is much more widely relevant, is the elucidation of details of reaction mechanism, and in particular the identification of the chain-carriers in many widely differing systems. The next problem then is to measure their concentration, its variation throughout the reaction, and, hence, the absolute rate constants. It is essential that the factors which decide whether a polymerization is ionic or pseudo-ionic be determined as soon as possible. 

Butyl rubber (HR) is widely used for inner tubes and as a sealant. It is produced using the cationic polymerization with the copolymerization of isobutylene in the presence of a small amount (10%) of isoprene. Thus, the random copolymer chain contains a low concentration of widely spaced isolated double bonds, from the isoprene, that are later cross-linked when the butyl rubber is cured. A representation is shown in structure 5.20 where the number of units derived from isobutylene units greatly outnumbers the number of units derived from the isoprene monomer. The steric requirements of the isobutylene-derived units cause the chains to remain apart giving it a low stress to strain value and a low Tg. 

As in the case with cationic polymerizations, the number of growing chains is constant so that a steady state exists such as the Ri = R. This is useful because it is difficult to determine the concentration of [M ] so that it can be eliminated as follows ... 

Protonic (Brpnsted) acids initiate cationic polymerization by protonation of the olefin. The method depends on the use of an acid that is strong enough to produce a resonable concentration of the protonated species... 

For polymerizations carried out to high conversions where the concentrations of propagating centers, monomer, and transfer agent as well as rate constants change, the polydispersity index increases considerably. Relatively broad molecular-weight distributions are generally encountered in cationic polymerizations. 

It is generally accepted that there is little effect of counterion on reactivity of ion pairs since the ion pairs in cationic polymerization are loose ion pairs. However, there is essentially no experimental data to unequivocally prove this point. There is no study where polymerizations of a monomer using different counterions have been performed under reaction conditions in which the identities and concentrations of propagating species are well established. (Contrary to the situation in cationic polymerization, such experiments have been performed in anionic polymerization and an effect of counterion on propagation is observed see Sec. 5-3e-2.)... 

The number-average degree of polymerization for a living cationic polymerization is defined as the concentration of monomer consumed divided by the total concentration of all propagating chains (dormant and active)... 

The anionic polymerization of lactams proceeds by a mechanism analogous to the activated monomer mechanism for anionic polymerization of acrylamide (Sec. 5-7b) and some cationic polymerizations of epoxides (Sec. 7-2b-3-b). The propagating center is the cyclic amide linkage of the IV-acyllactam. Monomer does not add to the propagating chain it is the monomer anion (lactam anion), often referred to as activated monomer, which adds to the propagating chain [Szwarc, 1965, 1966]. The propagation rate depends on the concentrations of lactam anion and W-acy I lactam, both of which are determined by the concentrations of lactam and base. 

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