Reactivity ratios, homogeneous

Vinyhdene chloride copolymerizes randomly with methyl acrylate and nearly so with other acrylates. Very severe composition drift occurs, however, in copolymerizations with vinyl chloride or methacrylates. Several methods have been developed to produce homogeneous copolymers regardless of the reactivity ratio (43). These methods are appHcable mainly to emulsion and suspension processes where adequate stirring can be maintained. Copolymerization rates of VDC with small amounts of a second monomer are normally lower than its rate of homopolymerization. The kinetics of the copolymerization of VDC and VC have been studied (45—48). 

Barrett and Thomas (10)proposed that these effects of differential monomer adsorption could be modeled by correcting homogeneous solution copolymerization reactivity ratios with the monomer s partition coefficient between the particles and the diluent. The partition coefficient is measured by static equilibrium experiments. Barrett s suggested equations are ... 

Bajoras and Makuska investigated the effect of hydrogen bonding complexes on the reactivities of (meth)acrylic and isotonic acids in a binary mixture of dimethyl sulfoxide and water using IR spectroscopy (Bajoras and Makuska, 1986). They demonstrated that by altering the solvent composition it was possible to carry out copolymerization in the azeotropic which resulted in the production of homogeneous copolymers of definite compositions at high conversions. Furthermore, it was shown that water solvent fraction determines the rate of copolymerization and the reactivity ratios of the comonomers. This in turn determines the copolymer composition. 

Compositionally uniform copolymers of tributyltin methacrylate (TBTM) and methyl methacrylate (MMA) are produced in a free running batch process by virtue of the monomer reactivity ratios for this combination of monomers (r (TBTM) = 0.96, r (MMA) = 1.0 at 80°C). Compositional ly homogeneous terpolymers were synthesised by keeping constant the instantaneous ratio of the three monomers in the reactor through the addition of the more reactive monomer (or monomers) at an appropriate rate. This procedure has been used by Guyot et al 6 in the preparation of butadiene-acrylonitrile emulsion copolymers and by Johnson et al (7) in the solution copolymerisation of styrene with methyl acrylate. 

In homogeneous copolymerization, the instantaneous composition of copolymer is decided only by monomer reactivity ratio. On the contrary, in emulsion copolymerization, the copolymer composition depends not only on the monomer reactivity ratio but also on the distribution of monomers between oil (polymer-monomer particles) and aqueous phases (18). 

Differences in monomer reactivity ratios often result in non-homogeneous copolymers. 

Dramatic shape selectivities in competitive olefin epoxidation was observed with picnic basket metalloporphyrins312 313 designed to exclude bulky axial ligands on one sterically protected porphyrin face. When oxidized with PhIO in acetonitrile in the presence of the rigid p-xylyl-strapped porphyrin, cis-2-octene reacted selectively versus ds-cyclooctene or 2-methyl-2-pentene, giving >1000 reactivity ratios.313,314 Some immobilized manganese(III) porphyrins proved to be as efficient as their homogeneous equivalents in epoxidation with PhIO.151,315... 

Some polymer-composition vs. conversion curves were obtained for the copolymerizations with different f s (Figure 2), and all of them seem to intersect the ordinate at 1.0. From the initial slope of the curves and the monomer ratio in the aqueous phase the monomer reactivity ratio was calculated, but the calculation resulted in a negative r2. Therefore, it was concluded that the copolymerization could not be regarded as a homogeneous one even just after the beginning of the reaction. The first stage was considered to be a transitional stage to establish the particle formation. 

Typical values of comonomer relative reactivity ratios in ethylene/propylene copolymerisations run with various heterogeneous and homogeneous Ziegler-Natta catalysts are listed in Table 3.5 [30, 72, 454]. 

All the mentioned types of the nontrivial dynamic behavior are excluded for the systems where the reactivity ratios ry can be described by the expressions of the well-known Alfrey-Price Q-e scheme [20], and as a result they are to follow the simplified terminal model (see Sect. 4.6). In these systems, due to the relations Bj(X)/Bj(x) = ajj/ajj which holds for all i and j, the functions 7e,-(2) according to relations (4.10) are the ratios of the homogeneous polynomials of degree 2. Besides, for the calculations of the coefficients ak of Eq. (5.11) one can use the simple formulae presented in terms of determinants Dj and D [6, p. 265]. The theoretical analysis [202] leads to the conclusion that in such systems even the limited cycles are not possible and all azeotropes are certainly unstable. Hence any trajectory H(p) and X(p) when p -> 1 inevitably approaches the SP corresponding to the homopolymer the number of which can be from 1 to m. The set of systems obtained due to the classification within the framework of the simplified model essentially impoverishes in comparison with the general case of the terminal copolymerization model since some types of systems cannot be principally realized under the restrictions which the Q-e scheme puts on the reactivity ratios r. ... 

Schuller [150] and Guillot [98] both observed that the copolymer compositions obtained from emulsion polymerization reactions did not agree with the Mayo Lewis equation, where the reactivity ratios were obtained from homogeneous polymerization experiments. They concluded that this is due to the fact that the copolymerization equation can be used only for the exact monomer concentrations at the site of polymerization. Therefore, Schuller defined new reactivity ratios, TI and T2, to account for the fact that the monomer concentrations in a latex particle are dependent on the monomer partition coefficients (fCj and K2) and the monomer-to-water ratio (xp) ... 

The Mayo Lewis equation, using reactivity ratios computed from Eq. 18, will give very different results from the homogenous Mayo Lewis equation for mini-or macroemulsion polymerization when one of the comonomers is substantially water-soluble. Guillot [151] observed this behavior experimentally for the common comonomer pairs of styrene/acrylonitrile and butyl acrylate/vinyl acetate. Both acrylonitrile and vinyl acetate are relatively water-soluble (8.5 and 2.5%wt, respectively) whereas styrene and butyl acrylate are relatively water-insoluble (0.1 and 0.14%wt, respectively). However, in spite of the fact that styrene and butyl acrylate are relatively water-insoluble, monomer transport across the aqueous phase is normally fast enough to maintain equilibrium swelling in the growing polymer particle, and so we can use the monomer partition coefficient. 

However, this does not preclude mini emulsion copolymerization in a CSTR for extremely water-insoluble comonomers. In spite of the fact that the copolymer composition in the continuous miniemulsion is less than that predicted using the homogeneous copolymerization reactivity ratios, the miniemulsion copolymer might be more uniform than the macroemulsion copolymer, where the possibility of significant droplet nucleation could lead to two separate homopolymers or, at the very best, copolymers of various composition. Therefore, it is very important to use CSTR data to scale up a continuous miniemulsion copolymerization product to take into account the different particle growth kinetics for batch and continuous reactors. 

Copolymers. The copolsmers were prepared by emulsion polymerization in a "pop bottle" polymerizer at 80° C using sodium aryl alkyl sulphonate (Ultrawet K) as emulsifier and ammonium persulphate as catalyst. The polymerizations were carried to greater than 95% conversion (4-8 hr) and the feed concentrations of the vinyl ketones were taken as their concentrations in the polymer. Copolymers containing 1% and 5% by weight of vinyl ketones were prepared. The cqsolymers were not homogeneous in ketone concentration since the reactivity ratios indicated that the vinyl ketones would be used ip before the end of the polymerizations. Thin films were prepared for irradiation by compression molding in a Carver Press at 150° C at 20,000 psi. The films were usually 0.22 mm thick. 

This discrepancy is partly due to the fact that random copolymers produced by a batch free-radical polymerization synthetic method can have a significant composition drift if the respective reactivity ratios of the monomers are different.2 This means that the value of the parameter/is not homogeneous in the copolymer layer at the interface. In the PS-PVP case discussed above, the random copolymers directly in contact with the PS or the PVP side of the interface at equilibrium would be PS-rich or PVP-rich, respectively. This segregation of copolymer fractions to their preferred interfaces gives rise to a broadening of these interfaces relative to the case of a random copolymer with a narrow distribution of/values. 

Experimental System The copolymerisation of styrene with methyl acrylate in toluene using azo-bis-iso- butyronitrile (AIBN) was selected as the model experimental system because the overall rate of reaction is relatively fast, copolymer analysis is relatively simple using a variety of techniques and the appropriate kinetic and physical constants are available in the literature. This monomer combination also has suitable reactivity ratios (i = 0.76 and r4 =0.175 at 80 C),(18) making control action essential for many different values if compositionally homogeneous polymers are to be prepared at higher conversions in a semi-batch reactor. 

All IR investigations of sequence distribution so far published rely on the terminal copolymerization model, which assumes that the kinetics of copolymerization are governed only by the probability that monomer units from the feed will be added to the last unit of the growing chain, and that there is only one active site present in the catalyst system, whether homogeneous or heterogeneous. As will be shown later (Section 3.4), this is only an approximation multiple active species are formed by many soluble Ziegler-Natta catalysts, so that the product of reactivity ratios determined from the normal copolymerization equation does not always exactly predict the actual sequence distribution in the copolymer. 

A measure of the relative reactivities of the monomers involved in copolymerization is reflected in their reactivity ratios and r2, the subscripts referring to monomers 1 and 213. Thus, when monomer 2 was sodium ethylenesulfonate, r2 was found to be close to zero while r (acrylamide) = 14.9 and r1 (sodium acrylate) = 5.8. A similar sluggishness in copolymerization with less-polar monomers was also found for other sulfonates. Izumi and coworkers studied the copolymerization of sodium allylsulfonate (MJ with acrylonitrile (M2) in dimethyl sulfoxide (DMSO) and DMSO-water mixtures. They found considerably lower values for r2 as compared to in aqueous DMSO and attributed it to lack of homogeneity , although no phase separation was observed in this medium14. 

A detailed structure characterization of isobutylene and 0-pinene copolymers has been carried out including homogeneity studies (by GPC), quantitative composition and sequence analysis (by PMR and reactivity ratios) and molecular weight determinations (by osmometry and viscometry). Analysis of our data leads us to conclude that isobutylene and 0-pinene can be readily copolymerized to reasonably high molecular weight materials and that the products are perfectly random, statistical copolymers showing no detectable tendency for blockiness . 

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