Comonomers, reactivity rate

The Phillips and Ziegler-Natta catalyst systems are actually believed to possess more than one type of catalyst "site," with each site having distinct ratios of chain-transfer to propagation rates and different comonomer reactivity ratios. 

In general, during copolymerisation, the two reacting monomers are not incorporated into the polymer chain in the same proportion as in the initial comonomer mixture. The reason for this is that the two monomers generally have different reactivities towards the growing chain ends. In the terminal model, it is assumed that the nature of the monomer at the growing chain end determines relative comonomer reactivity. In other words, there are four types of propagating step, each defined by a different rate constant ... 

In batch polymerisation (399), the components of the emulsion are charged into a stirred reactor, which is then heated to begin polymerisation. No material is added or removed during the entire polymerisation. Since most polymerisations are highly exothermal, the rate of heat generation can easily exceed the heat removal capabilities, and a mnaway reaction is possible. The batch polymerisation method offers little or no control over the copolymer composition, but depends upon the comonomer reactivity ratios and the partitioning of the comonomers in the latex particles (288, 340). 

In copolymerisations (70, 408), the copolymer composition may be controlled by the relative rates of monomer addition. In this way, any large differences in the comonomer reactivity ratios or water solubilities can be overcome to produce a copolymer with uniform composition. In order to maintain control of the monomer concentration in the polymer particles, the polymerisation may have to be performed under monomer-starved conditions. This means that the polymer particles are not saturated with monomer, but are being polymerised at an instantaneous conversion of 90% or greater. If the monomer addition rate is greater than the polymerisation rate, the reactor will be operating under flooded conditions, and control over the copolymer composition is lost. 

Copolymerization. In free-radical copolymerization (qv), the composition of the copolymer is controlled by the comonomer reactivity ratios (23). The monomer reactivity ratio is defined as the quotient of the rate constants for chain homopropagation to the rate constant for chain cross-propagation. 

Siloxy substitution at the 3-position of the indenyl ligand (17) was found to remarkably improve the 1-olefin copolymerization ability, whereas substitution at the 2-position (15) slightly reduced the copolymerization ability as compared to the unsubstituted 5. The reason for this was suggested to be mainly the increased coordination gap aperture of the 3-siloxy-substituted complexes. Table 1 summarizes the ethylene reactivity ratio data obtained for the siloxy-substituted complexes 15, 16, and 17 The large difference in the ethylene and comonomer reactivity ratio values, the product of which is much below unity, emphasizes the prevailing tendency of the catalysts to produce copolymers with isolated comonomer units. The reason for the 15 0% lower incorporation of 1-hexadecene than 1-hexene was explained by the higher steric bulk and lower rate of diffusion of the longer a-olefin. 

Ethene high-pressure copolymerization smdies have also been carried out for the systems E-MMA, E-BMA, E-acrylic acid, and E-methacrylic acid. Whereas the comonomer reactivity ratios turn out to be slightly different, the ethene reactivity ratios for the entire set of (meth)acrylates and for (meth) acrylic acid are remarkably close to each other. The similarity of rE = feEE/ Ex is probably due to the crosstermination propagation rate coefScient rate ftEx being dominated by the highly reactive ethene-terminated radical. ... 

In the most common production method, the semibatch process, about 10% of the preemulsified monomer is added to the deionised water in the reactor. A shot of initiator is added to the reactor to create the seed. Some manufacturers use master batches of seed to avoid variation in this step. Having set the number of particles in the pot, the remaining monomer and, in some cases, additional initiator are added over time. Typical feed times ate 1—4 h. Lengthening the feeds tempers heat generation and provides for uniform comonomer sequence distributions (67). Sometimes skewed monomer feeds are used to offset differences in monomer reactivity ratios. In some cases a second monomer charge is made to produce core—shell latices. At the end of the process pH adjustments are often made. The product is then pumped to a prefilter tank, filtered, and pumped to a post-filter tank where additional processing can occur. When the feed rate of monomer during semibatch production is very low, the reactor is said to be monomer starved. Under these... 

The reactivity of macromonomers in copolymerizalion is strongly dependent on the particular comonomer-macromonomer pair. Solvent effects and the viscosity of the polymerization medium can also be important. Propagation may become diffusion controlled such that the propagation rate constant and reactivity ratios depend on the molecular weight of the macromonomer and the viscosity or, more accurately, the free volume of the medium. 

For SCVCP in general, DB strongly depends on the comonomer ratio (y=[monomer]o/[inimer]o) [73,74]. In the ideal case,when all rate constants are equal, for y>>l, the final value of DB decreases with y as DB=2/(y+l) which is four times higher than the value expected from dilution of inimer molecules by monomers. For low values of (yreactivity ratios, the structure of polymer obtained can change from macroinimers when the monomer M is much more reactive than the vinyl groups of inimer or polymer molecules to hyperstars in the opposite limiting case. 

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. 

The instantaneous copolymer composition—the composition of the copolymer formed at very low conversions (about <5%)—is usually different from the composition of the comonomer feed from which the copolymer is produced, because different monomers have differing tendencies to undergo copolymerization. It was observed early that the relative copolymerization tendencies of monomers often bore little resemblance to their relative rates of homopolymerization [Staudinger and Schneiders, 1939]. Some monomers are more reactive in copolymerization than indicated by their rates of homopolymerization other monomers are less reactive. Further, and most dramatically, a few monomers, such as maleic... 

Various attempts have been made to place the radical-monomer reaction on a quantitative basis in terms of correlating structure with reactivity. Success in this area would give a better understanding of copolymerization behavior and allow the prediction of the monomer reactivity ratios for comonomer pairs that have not yet been copolymerized. A useful correlation is the Q-e scheme of Alfrey and Price [1947], who proposed that the rate constant for a radical-monomer reaction, for example, for the reaction of Mp radical with M2 monomer, be written as... 

The presence of a comonomer has, in certain cases, 9 marked influence on polymerization rate. For example, the mastication of natural rubber in the presence of maleic anhydride, even with small concentrations of the latter, about 5%, leads to accelerated polymerization of styrene monomer (11) either because of its high reactivity in the propagation step of heterochain copolymerization and/or because of a hardening effect. This reaction is discussed later. 

The relative reactivity of the macromonomer in copolymerization with a common comonomer, A, can be assessed by l/rA=kAB/kAA> i-e-> the rate constant of propagation of macromonomer B relative to that of the monomer A toward a common poly-A radical. In summarizing a number of monomer reactivity ratios in solution copolymerization systems reported so far [3,31,40], it appears reasonable to say that the reactivities of macromonomers are similar to those of the corresponding small monomers, i.e., they are largely determined by the nature of their polymerizing end-group, i.e., essentially by their chemical reactivity. 

The first three types of copolymers can be prepared by polymerizing the two monomers simultaneously. In this case, the distribution of comonomers is determined by their relative concentrations and reactivity ratios. The reactivity ratios (ri and r2) are the ratios of the rate constants of homopropagation and cross-propagation [Eq. (21)]. 

Assuming that classical chemical kinetics are valid and that the crosslinking reaction rate is proportional to the concentrations of polymer radicals and pendant double bonds, it was shown theoretically that the crosslinked polymer formation in emulsion polymerization differs significantly from that in corresponding bulk systems [270,316]. To simplify the discussion, it is assumed here that the comonomer composition in the polymer particles is the same as the overall composition in the reactor, and that the weight fraction of polymer in the polymer particle is constant as long as the monomer droplets exist. These conditions may be considered a reasonable approximation to many systems, as shown both theoretically [316] and experimentally [271, 317]. First, consider Flory s simplifying assumptions for vinyl/divinyl copolymerization [318] that (1) the reactivities of all types of double bonds are equal, (2) all double bonds... 

The simple copolymer model, with two reactivity ratios for a binary comonomer reaction, explains copolymer composition data for many systems. It appears to be inadequate, however, for prediction of copolymerization rates. (The details of various models that have been advanced for this purpose are omitted here, in view of their limited success.) Copolymerization rates have been rationalized as a function of feed composition by invoking more complicated models in which the reactivity of a macroradical is assumed to depend not Just on the terminal monmomer unit but on the two last monomers in the radical-ended chain. This is the penultimate model, which is mentioned in the next Section. 

The classical Mayo-Lewis scheme relating comonomer feeds to relative reactivity ratios (5i) is often applied to copolymerization of cyclosiloxanes. This scheme presumes that no depropagation of the copolymer occurs, that the copolymerization rate constants depend only on the ultimate comonomer units, and that instantaneous comonomer feed ratios and copolymer compositions are used in the analysis of data. When these assumptions hold, the Mayo-Lewis method is very useful for the analysis of copolymerization data. 

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