Monomer feed rate effects

Other strategies for controlling copolymer composition Although the use of monomer-starved conditions for control of copolymer composition is widespread, the low monomer feed rates which need to be used give rise to low rates of copolymerization and have significant effects upon the molar mass and molar mass distribution of the copolymers formed (see Section 7.4.4.4). Hence, alternative procedures have been developed which facilitate higher feed rates, but nevertheless allow for control of copolymer composition. These procedures are briefly described in this section. 

One should note that this ratio should be controlled at the desired level regardless of the magnitude of the total radical concentration, and hence impurities which affect the radical concentration will have no effect on copolymer composition control. The on-line measurement of heat generation rate can thus be used to set the appropriate monomer feed rate. If polymerization is too slow because of radical scavengers, one can increase the radical generation rate to compensate, and, in parallel, increase the monomer inflow rate to maintain the ratio constant (or, vice versa to compensate for a possible auto-acceleration operating point). Additional discussion is provided in Dub6 etal. [56]. 

A series of SBR copolymers were prepared by emulsion polymerisation and an SAN copolymer was polymerised by a semicontinuous process in the presence of SBR to form a core/shell morphology. The effects of initiator concentration, monomer feeding rate, core/shell ratio, and gel fraction of the core on the core/shell particles morphology were studied. Morphology and Tg were characterised by TEM, DSC, and dynamic mechanical spectroscopy. 28 refs. 

In this context, the use of ACOMP as a versatile and absolute method to quantify in a novel manner different aspects of the reaction kinetics in semicontinuous mode offers a significant advance because it not only provides means to immediately assess the regime the reaction occurs in but also allows verification of the effects of any change in the monomer feed rates on key parameters during the synthesis. 

FIGURE 12.9 The effects of the monomer feed rate on for reactions in semibatch mode versus batch mode (left panel) mass distributions... 

The Effects of Monomer Feed Rate on the Particle Size and Number... 

Britton D, Heatley F, Lovell PA. Effect of monomer feed-rate on chain transfer to polymer in emulsion polymerization of vinyl acetate studied by NMR spectroscopy. Macromolecules 2000 33 5048-5052. 

To a lesser extent, the initiator concentration also affects the total particle number in the reactor. At higher concentrations, smaller polymer particles are produced. Similarly, the temperature of the reaction has a positive effect on the total particle number in the reactor as it increases. Smaller particles are produced at higher temperatures. And finally, the monomer feed rate in a semibatch reactor influences the reaction rate (under starved feed) and the particle growth. In addition, it can indirectly induce secondary nucleation at high conversions, thus reducing the particle size and increasing the total particle number. The PSPI is also affected by these four decision variables it increases with the surfactant and initiator flow rates as well as with reactor temperature, and it decreases with the monomer feed rate. 

Chern [42] developed a mechanistic model based on diffusion-controlled reaction mechanisms to predict the kinetics of the semibatch emulsion polymerization of styrene. Reasonable agreement between the model predictions and experimental data available in the literature was achieved. Computer simulation results showed that the polymerization system approaches Smith-Ewart Case 2 kinetics (n = 0.5) when the concentration of monomer in the latex particles is close to the saturation value. By contrast, the polymerization system under the monomer-starved condition is characterized by the diffusion-con-trolled reaction mechanisms (n > 0.5). The author also developed a model to predict the effect of desorption of free radicals out of the latex particles on the kinetics of the semibatch emulsion polymerization of methyl acrylate [43]. The validity of the kinetic model was confirmed by the experimental data for a wide range of monomer feed rates. The desorption rate constant for methyl acrylate at 50°C was determined to be 4 x 10 cm s ... 

In the co-polymerization, the productivity also reduces with increasing concentration of the co-monomer in the feed. With some co-monomers a rate-enhancement effect can be observed upon the addition of small quantities of co-monomer. The effects of the co-monomer are shown in Fig. 9.5-8, in which the productivity of the same mc-Me2Si[Ind]2ZrCl2 catalyst is plotted versus the concentration of 1-hexene-co-monomer in the feed. 

In the semibatch experiments, the particle size distributions of the final latexes were affected by the residual surfactant in the seed latex, which tended to facilitate homogeneous nucleation during the entire feed period. The monomer feedrate determined the polymerization rate and had little effect on copolymer composition. The polymer compositions for the runs with different monomer feeding modes tended to be identical at very low feedrate. 

The rates of radical-monomer reactions are also dependent on considerations of steric effects. It is observed that most common 1,1-disubstituted monomers — for example, isobutylene, methyl methacrylate and methacrylo-nitrile—react quite readily in both homo- and copolymerizations. On the other hand, 1,2-disubstituted vinyl monomers exhibit a reluctance to ho-mopolymerize, but they do, however, add quite readily to monosubstituted, and perhaps 1,1-disubstituted monomers. A well-known example is styrene (Ml) and maleic anhydride (M2), which copolymerize with r — 0.01 and T2 = 0 at 60°C, forming a 50/50 alternating copolymer over a wide range of monomer feed compositions. This behavior seems to be a consequence of steric hindrance. Calculation of A i2 values for the reactions of various chloroethylenes with radicals of monosubstituted monomers such as styrene, acrylonitrile, and vinyl acetate shows that the effect of a second substituent on monomer reactivity is approximately additive when both substituents are in the 1- or cr-position, but a second substituent when in the 2- or /3-position of the monomer results in a decrease in reactivity due to steric hindrance between it and the polymer radical to which it is adding. 

In practice, (f) can be calculated by inserting experimental copolymerization rates into Eq. (7.64). The values of (j> thus obtained are frequently greater than unity, and these deviations are ascribed to polar effects that favor cross-termination over homotermination. However, this is not always unambiguous, since the apparent cross-termination factor may vary with monomer feed composition in a given system [25,26]. It is clear also that termination reactions are at least partially diffusion controlled [27,28]. A dependence of segmental diffusivity on the structure of macroradicals is to be expected and dependence of diffusion controlled termination on copolymer composition seems reasonable. It is therefore plausible that the value of the overall termination rate constant ku in copolymerizations should be functions of fractions F and Fi) of the comonomers incorporated in the copolymer. An empirical expression for ku has thus been proposed [27] ... 

---