Copolymerization monomer feed

In the Type II case, the copolymerization tends toward an alternating arrangement of monomer units. Curve II of Figure 1 shows an example of an alternating copolymer that has an azeotropic copolymer composition, ie, a copolymer composition equal to the monomer feed at a single monomer feed composition. This case is analogous to a constant Foiling mixture ia vapor—Hquid equihbria.T) III... 

Table 2 shows characteristic reactivity ratios for selected free-radical, ionic, and coordination copolymerizations. The reactivity ratios predict only tendencies some copolymerization, and hence some modification of physical properties, can occur even if and/or T2 are somewhat unfavorable. For example, despite their dissimilar reactivity ratios, ethylene and propylene can be copolymerized to a useful elastomeric product by adjusting the monomer feed or by usiag a catalyst that iacreases the reactivity of propylene relative to ethylene. 

Copolymerization occurred when the olefin had a basicity lower than that of the aldehyde (with respect to the initiator used), but sufficiently high occasionally to displace a molecule of initiator and give rise to an active species this situation produced copolymers with varying proportions of ether units in the chain, depending on the monomers feed ratio and on the olefin used. Isopropenylbenzene gave the best results with alternate copolymerization over a fairly wide range of feed ratios rt = 0.03 0.03, r2 = 0.4 0.1 (2-furaldehyde = Mj) indene produced copolymers with lower 2-furaldehyde contents. 

One final point should be made. The observation of significant solvent effects on kp in homopolymerization and on reactivity ratios in copolymerization (Section 8.3.1) calls into question the methods for reactivity ratio measurement which rely on evaluation of the polymer composition for various monomer feed ratios (Section 7.3.2). If solvent effects arc significant, it would seem to follow that reactivity ratios in bulk copolymerization should be a function of the feed composition.138 Moreover, since the reaction medium alters with conversion, the reactivity ratios may also vary with conversion. Thus the two most common sources of data used in reactivity ratio determination (i.e. low conversion composition measurements and composition conversion measurements) are potentially flawed. A corollary of this statement also provides one explanation for any failure of reactivity ratios to predict copolymer composition at high conversion. The effect of solvents on radical copolymerization remains an area in need of further research. 

The rate of copolymerization often shows a strong dependence on the monomer feed composition. Many theories have been developed to predict the rate of copolymerization based on the terminal model for chain propagation (Section 7.3.1.1), This usually requires an overall rate constant for termination in copolymerization that is substantially different from that observed in homopolymerization of any of the component monomers. 

Values of 0 required to fit the rate of copolymerization by the chemical control model were typically in the range 5-50 though values <1 are also known. In the case of S-MMA copolymerization, the model requires 0 to be in the range 5-14 depending on the monomer feed ratio. This "chemical control" model generally fell from favor wfith the recognition that chain diffusion should be the rate determining step in termination. 

The solvent in a bulk copolymerization comprises the monomers. The nature of the solvent will necessarily change with conversion from monomers to a mixture of monomers and polymers, and, in most cases, the ratio of monomers in the feed will also vary with conversion. For S-AN copolymerization, since the reactivity ratios are different in toluene and in acetonitrile, we should anticipate that the reactivity ratios are different in bulk copolymerizations when the monomer mix is either mostly AN or mostly S. This calls into question the usual method of measuring reactivity ratios by examining the copolymer composition for various monomer feed compositions at very low monomer conversion. We can note that reactivity ratios can be estimated for a single monomer feed composition by analyzing the monomer sequence distribution. Analysis of the dependence of reactivity ratios determined in this manner of monomer feed ratio should therefore provide evidence for solvent effects. These considerations should not be ignored in solution polymerization either. 

RAFT of MMA with benzyl dithiobenzoate provides very poor control394 yet copolymerization of S with MMA with this RAFT agent provides low dispersities with as little as 5% S in the monomer feed. 

Fig. 4. Dependence of copolymer composition on monomer feed in copolymerization of BCMO with THF at 30 and 120 °C...

The influence of changes in these other variables on MWD in a homopolymerization has not yet been tested, but whatever perturbations are introduced to the feed in a radical polymerization in a laboratory-scale CSTR, they are unlikely to introduce dramatic changes in the MWD of the product because of the extremely short life-time of the active propagating chains in relation to the hold-up time of the reactor. This small change in MWD could be advantageous in a radically initiated copolymerization where perturbations in monomer feeds could give control over polymer compositions independent of the MWD. This postulate is being explored currently. 

Based on the structure of the monomers in the monomer feed, the systems under study can be classified as acceptor (MA) — acceptor (TASM) systems and the assumption that alternating copolymerization occurs in these systems seems at first sight... 

The values of K and (3(K > 0 and 0 < P < 1) were calculated for each monomer pair from the logarithmic plot of the ratio of the monomers in the monomer feed [MJ/[M2] to the comonomer units in the copolymer using a modified equation of binary copolymerization ... 

Copolymer composition vs. monomer feed data were then obtained for 1-hexene (M ) and 5-methyl-1,4-hexadiene copolymerizations (Table VI). The data show that the copolymer compositions measured by 300 MHz %-NMR spectroscopy are essentially identical to the monomer feed. The calculated reactivity ratios were 1.1 + 0.2 for each of the two monomers. 

COPOLYMER COMPOSITION vs MONOMER FEED COMPOSITION FOR 1-HEXENE/5-CH3-l,4-HEXADIENE COPOLYMERIZATIONS... 

Styrene-SQ., Copolymers. I would now like to discuss two systems which illustrate the power of C-13 nmr in structural studies. The first is the styrene-SO system. As already indicated, this is of the type in which the chain composition varies with monomer feed ratio and also with temperature at a constant feed ratio (and probably with pressure as well.) The deviation of the system from simple, first-order Markov statistics, —i.e. the Lewis-Mayo copolymerization equation—, was first noted by Barb in 1952 ( ) who proposed that the mechanism involved conplex formation between the monomers. This proposal was reiterated about a decade later by Matsuda and his coworkers. Such charge transfer com-... 

We have prepared a series of copolymers under the conditions shown in Table 2. The monomer feed was always a 50 50 ratio of chloroprene to sulfur dioxide. Copolymerizations were carried out in bulk at temperatures from -78 to 100°. Initiators were tertiary butyl hydroperoxide at low temperatures, where it forms a redox system with the SO2 and is more effective than one might otherwise expect. Silver nitrate was used at 0° and 25°, azoiso-butyronitrile at 40° and 60°, and azodicyclohexanecarbonitrile... 

Multiplication by [M2] gives what are generally referred to as the copolymerization equations, Equations 7.17 and 7.19, which gives the copolymer composition without the need to know any free radical concentration, and which gives the composition of the growing polymer as a function of monomer feed (Equation 7.19). 

As noted earlier, copolymerization can be controlled by regulating the monomer feed in accordance with, for example, Equations 7.17 and 7.19. 

The copolymerization between two different monomers can be described using only four reactions, two homo-polymerizations and two cross-polymerization additions. Through appropriate arrangements, equations that allow copolymer composition to be determined from the monomer feed ratio are developed. 

The high electron richness of vinylferrocene as a monomer is illustrated in its copolymerization with maleic anhydride, where 1 1 alternation copolymers are formed over a wide range of monomer feed ratios and ri -2 = 0.003. Subsequently, a large number of detailed copolymerization studies have been undertaken using metal-containing vinyl monomers. 

Fig. 17 Top row. conversion (ln([M]o/[M]t) against time plots for 50mol% copolymerizations (a, c, e). Bottom row. relationship between the monomer feed (/i) and the actual monomer incorporation (Fi) at the initial ( 20% conversion) and final (>50% conversion) polymerization stages (b, d, f). Both conversion and monomer incorporation are shown for EtOx NonOx (a, b), MeOx NonOx (c, d), and MeOx EtOx (e, f) copolymerizations. (Reprinted with permission from [88]. Copyright (2006) American Chemical Society)...

It is important to note that the tendency of a monomer towards polymerization and therefore also towards copolymerization is strongly dependent on the nature of the growing chain end. In radical copolymerization the composition of the copolymer obtained from its given monomer feed is independent of the initiating system for a particular monomer pair, but for anionic or cationic initiation this is normally not the case. One sometimes observes quite different compositions of copolymer depending on the nature of the initiator and especially on the type of counterion. A dependence of the behavior of the copolymerization on the used catalyst is often observed with Ziegler-Natta or metallocene catalysts. 

In order to determine the reactivity ratios, the relation between monomer feed and copolymers composition was examined. However, in this case the calculation of reactivity ratios is more complex than simple copolymerization of two vinyl monomers. The question arises - how can we calculate the molar ratio of comonomers Should we use a mole concentration of hoth comonomers, or, for multimethacrylate concentration, should we apply a mole concentration of reactive groups (double bonds) Both methods are used and discussed in the literature. The results of reactivity ratio computations are presented in Table 5.2... 

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