Random copolymers reaction constants

Presently, the quantitative theory of irreversible polymeranalogous reactions proceeding in a kinetically-controlled regime is well along in development [ 16,17]. Particularly simple results are achieved in the framework of the ideal model, the only kinetic parameter of which is constant k of the rate of elementary reaction A + Z -> B. In this model the sequence distribution in macromolecules will be just the same as that in a random copolymer with parameters P(Mi ) = X =p and P(M2) = X2 = 1 - p where p is the conversion of functional group A that exponentially depends on time t and initial concen-... 

The inhibition effect of poly (vinyl alcohol) on the amylose hydrolysis was investigated. Figure 7 shows Lineweaver-Burk plots of the amylose hydrolysis rates catalyzed by the random copolymer in the presence of poly (vinyl alcohol). The reaction rate is found to decrease with increasing the concentration of poly (vinyl alcohol), and all of the straight lines obtained in the plots cross with each other at a point on the ordinate. This is a feature of the competitive inhibition in the enzymatic reactions. In the present reaction system, however, it is inferred to suggest that the copolymer and poly (vinyl alcohol) molecules competitively absorb the substrate molecules. The elementary reaction can be described in the most simplified form as in Equation 3 where Z, SI, and Kj[ are inhibitor, nonproductive complex, and inhibitor constant, respectively. Then the reaction rate is expressed with Equation 4. 

Model studies discussed in previous chapters show that the reactivity of cations and alkenes are very strongly affected by inductive and resonance effects in the substituents. Correlation of the rate constants of addition of benzhydryl cation to various styrenes with Hammett substituted benzhydryl cations to a standard alkene (2-methyl-2-pentene) gave also good correlation and p+ = 5.1 [28]. The large p value signals difficult copolymerizations between alkenes, even of similar structures. Thus, in contrast to radical copolymerization which easily provides random copolymers, cationic systems have a tendency to form either mixtures of two homopolymers or block copolymer (if the cross-over reaction is possible). 

It is assumed here that in every monomer unit there is one point such as P, and that the distance of a single jump is a. However, in certain polymers there may be two identical points such as P in one monomer unit, while in random copolymers PP may not be constant and will depend upon the order of chain growth and the presence of cis- and trans-configurations. A paper published recently [K. Kozlowski, Acta Polymerica, 30, 547 (1979)] deals with ESP studies of uncured carbon black/natural rubber mixes, their solvent extracts, and the residues therefrom. The author found it possible to identify a very narrow spectral line with rubber radicals stabilized by interaction with active sites on the carbon black surface. He concludes that his findings support Meissner s theory (B. Meissner, Rubber Chem. Technol., 48, 810 (1975)] that each structural unit (of the rubber molecule, Z. R.) has the same probability of reaction with the active site of a carbon black particle and can form with it only one bond . The relation between the evidence adduced and Meissner s theory is not, however, clear to the writer... 

The quantities k , k,p, kpp, and kp, are the rate constants of the four basic propagation reactions of copolymerization. The Stockmayer distribution function takes into account only a chemical polydispersity resulting fi om the statistical nature of copolymerization reactions. This means that all units of all chains are formed under identical conditions. If a monomer is removed from the reacting mixture at a rate which changes the monomer concentration ratio, the monomer concentration will drift, forming a copolymer which varies in the average composition and is broader in the chemical distribution. No such chemical polydispersity can be described by the Stockmayer distribution. Therefore, Eq. (84) has to be restricted in its application to random copolymers synthesized at very low conversions or under azeotropic conditions. For azeotropic copolymers, the feed monomer concentrations [a ] and are chosen in such a way that the second factor on the right-hand side of the basic relation of copolymerization kinetics... 

The processes of reaction and diffusion occur at the same time in a variety of systems. These issues are particularly important in the formation of blend systems and are central issues in the performance property enhancement of such systems. A study of the competitive effects of the rates of the two processes can be easily carried out using FTIR microspectroscopy. The rate of diffusion can be monitored by the time evolution of the absorbance (concentration) profiles while the rate of reaction can be monitored as a time evolution of the reactant (or product) absorbance (concentration). Reaction of a random copolymer of styrene and maleic anhydride (SMA) with bis(amine)-terminated poly(tetrahydrofuran) (PTHF) is one such studied system [73]. Temperature was varied while studying the effects of two different PTHF molecular weights. The reaction rate constants were obtained from the initial slope of conversion-time plots. In addition, it was shown that the rate of diffusion was faster as diffusion of PTHF into the SMA phase occurred prior to the imide formation. The imide was formed in the SMA phase and quantitatively estimated. A corresponding decrease in the carbonyl stretching vibration of the maleic anhydride peak was seen. 

It is noteworthy that a basic assumption made in the derivation of the free radical desorption rate constant is that the adsorbed layer of surfactant or stabilizer surrounding the particle does not act as a barrier against the molecular diffusion of free radicals out of the particle. Nevertheless, a significant reduction (one order of magnitude) in the free radical desorption rate constant can happen in the emulsion polymerization of styrene stabilized by a polymeric surfactant [42]. This can be attributed to the steric barrier established by the adsorbed polymeric surfactant molecules on the particle surface, which retards the desorption of free radicals out of the particle. Coen et al. [70] studied the reaction kinetics of the seeded emulsion polymerization of styrene. The polystyrene seed latex particles were stabilized by the anionic random copolymer of styrene and acrylic acid. For reference, the polystyrene seed latex particles stabilized by a conventional anionic surfactant were also included in this study. The electrosteric effect of the latex particle surface layer containing the polyelectrolyte is the greatly reduced rate of desorption of free radicals out of the particle as compared to the counterpart associated with a simple... 

To demonstrate the livingness of styrene-acrylonitrile random copolymerizations, TEMPO (0.084 g) and BPO (0.101 g) were dissolved in 30 mL of styrene and 10 mL of acrylonitrile. The reaction mixture was stirred and purged with argon. The flask was sealed, lowered into a oil bath at 125 C and the mixture allowed to reflux. Periodically the flask was removed from the bath, cooled and a sample withdrawn for GPC analysis. To measure the composition of the copolymers, a series of polymerizations taken to low conversion were done in a Parr pressure reactor. The total moles of monomer were kept constant at 0.55, and the relative amounts of the two monomers were adjusted to vary the mole fraction of acrylonitrile from 0.1-0.9. 

This reaction occurs randomly on the homopolymer chain to create a random amide(carboxylic acid) copolymer. Hydrolyzed amide polymer is prepared by adding the amount of sodium hydroxide needed to hyrolyze the desired fraction of amide units to a 1 weight percent solution of amide polymer and maintaining that solution at 60°C for 2 hr. The reaction mixture is precipitated into 5 times its volume of 2-propanone, filtered, the precipitate washed with 2-propanone, ground in a blender for 2 minutes, and dried under vacuum to constant weight. This procedure is that of Meister, et al. (31). 

Let us consider a random copolymerization, where the ratio p > 1 (see section 13.1.3). The copolymer of A and B should have a build-in ratio of p. This can be obtained by filling the reactor with both monomers in a ratio of p/p, and introducing a mixture of monomers in the ratio p with the same rate as the polymerization takes place. In practice, the copolymerization rate cannot be measured directly. One way to determine the rate indirectly is by analysing the monomer concentrations in the reaction mixture continuously, and fe the monomers with such rates that these concentrations remain constant. Another way is to make an exact kinetic model of the copolymerization, and feed the monomers with the rates that the model predicts. But even then, periodic analyses of the reaction mixture may be desirable. 

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