Molecular weight distribution control

Molecular Weight Distribution Control in Continuous-Flow Reactors... 

G.R. Meira. Forced oscillations in continuous polymerization reactors and molecular weight distribution control. A survey. J. Macromol. Sci.- Rev. Macro-mol. Chem., 20(2) 207-241, 1981. 

Iwasaki T, Yoshida J (2005) Free radical polymerization in microreactors. Significant improvement in molecular weight distribution control. Macromolecules 38 1159-1163... 

Echevarria, A. Leiza, J.R. Molecular-weight distribution control in emulsion polymerization. AIChE J. 1998, 44 (7), 1667-1679. 

Precipitation/ dispersion Molecular weight and molecular weight distribution control via environment, ease of isolation May require solution and subsequent precipitation for purification and/or fabrication Precipitation may limit molecular weight... 

Although GTP demonstrates the fundamental characteristics of a living polymerization, namely narrow molecular weight distribution, control of molecular weight derived from the monomer/initiator stoichiometry, and the... 

The fact that the molecular weight increased linearly with the increase of monomer/initiator ratio, as shown in Figure 9.3, suggests that the chain-transfer reaction does not play a significant role in this system. Extremely fast mixing, effective temperature control, and residence time control by virtue of the microsystem seem to be responsible for the excellent molecular-weight distribution control. 

No reaction to break the polymerization by forming dead polymer chains occurs during the polymerization. Therefore, no termination reactions and no chain-transfer reactions take place. Excellent molecular weight and molecular-weight distribution controls support these two requirements are satisfied in a practical sense. 

Table 9.2 Effect of the microflow system on molecular-weight distribution control...

Polymerization of benzyl methacrylate (BMA) is much slower than that of BA. Although the yield of the polymer increased with an increase in the residence time, the polymerization did not complete within 12 min. The value of M /Mn was much smaller than that for BA, both in the microflow system and the macrobatch system. The effect of the microflow system on molecular-weight distribution control is, however, smaller than for the BA case. Probably, temperature control for BMA polymerization is better than that for BA polymerization, even in the macrobatch system, because heat generation per unit time for BMA polymerization seems to be much less than that for BA polymerization. 

Vinyl benzoate (VBz) polymerization is slower than MMA polymerization. It is noteworthy that M /Mn for the polymer obtained in the microflow system is very similar to that for the macrobatch system, suggesting that the superior heat removal ability of the microflow system is not important for molecular-weight distribution control in VBz polymerization. This is presumably because the heat generation in VBz polymerization is smaller and controllable even in the macrobatch system on the laboratory scale. 

Studies on the relative rates of the polymerization are helpful in obtaining a deeper insight into the effect of the microflow system on molecular-weight distribution control. It can be seen from Figure 9.11, where the polymer yield obtained in the microflow system is plotted against the reaction time (residence time) for each monomer, that the rate of polymerization increases in the order St < VBz < MMA < BMA < BA. This trend is consistent with the propagation rate constants reported in the literature (Table 9.2). It is reasonable to consider that a similar order is... 

In summary, microflow systems are quite effective for molecular-weight distribution control of very fast, highly exothermic free-radical polymerizations. The superior heat transfer ability of the microflow system in comparison with conventional macrobatch systems seems to be responsible for the high molecular-weight distribution controllability. It should be noted that the controllability is much lower than is achieved by conventional living free-radical polymerization, because residence time control does not work for controlling radical intermediates. The lifetime of a radical intermediate is usually much shorter than the residence time in a microflow system. It is also noteworthy that the more rapid and exothermic the polymerization is, the more effective the microflow system is. These facts speak well for the potentiality of microflow systems in the control of highly exothermic free-radical polymerization without deceleration by reversible termination. 

The concept of flash chemistry can be applied to polymer synthesis. Cationic polymerization can be conducted in a highly controlled manner by virtue of the inherent advantage of extremely fast micromixing and fast heat transfer. An excellent level of molecular weight control and molecular-weight distribution control can be attained without deceleration caused by equilibrium between active species and dormant species. The polymerization is complete within a second or so. The microflow system-controlled cationic polymerization seems to be close to ideal living polymerization within a short residence time. 

Free-radical polymerization can also be conducted in microflow systems. A fairly good level of molecular-weight control and molecular-weight distribution control can be attained, although the level is not as high as those of conventional living-radical polymerizations. 

Shimizu, H., Tamura, S., Shioya, S. and Suga, K., 1993, Kinetic study of poly-D(-)-3-hydroxybutyric acid (PHB) production and its molecular weight distribution control in a fed-batch culture of Alcaligenes eutrophus. J. Ferment. Bioeng., 76 465-469. 

In both methods the positive charge of the carbocationic center is reduced and thereby the acidity of the p-proton is reduced to suppress the chain transfer. As a result, good molecular weight control and molecular weight distribution control are attained. On the basis of the principles, a number of initiating systems have been developed for living cationic polymerization [99]. 

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