Chain-growth polymerizations viscosity

Chain-growth polymerizations are diffusion controlled in bulk polymerizations. This is expected to occur rapidly, even prior to network development in step-growth mechanisms. Traditionally, rate constants are expressed in terms of viscosity. In dilute solutions, viscosity is proportional to molecular weight to a power that lies between 0.6 and 0.8 (22). Melt viscosity is more complex (23) Below a critical value for the number of atoms per chain, viscosity correlates to the 1.75 power. Above this critical value, the power is nearly 3 4 for a number of thermoplastics at low shear rates. In thermosets, as the extent of conversion reaches gellation, the viscosity asymptotically increases. However, if network formation is restricted to tightly crosslinked, localized regions, viscosity may not be appreciably affected. In the current study, an exponential function of degree of polymerization was selected as a first estimate of the rate dependency on viscosity. 

Chain-growth polymerizations that are carried out in the absence of a diluent are characterized by high viscosity and poor heat transfer. High-molecular-weight... 

Reactions that are bimolecular can be affected by the viscosity of the medium. The translational motions of flexible polymeric chains are accompanied by concomitant segmental rearrangements. Whether this applies to a particular reaction, however, is hard to tell. For instance, two dynamic processes affect reactions, like termination rates, in chain-growth polymerizations. If the termination processes are controlled by translational motion, the rates of the reactions might be expected to vary with the translational diffusion coefhcients of the polymers. Termination reactions, however, are not controlled by diffusions of entire molecules, but only by segmental diffusions within the coiled... 

Easier than for chain-growth polymerization because viscosity only increases significantly during the last stages of the process when only a limited amount of heat is generated... 

Heterogeneous processes are of great importance for the free-radical chain-growth polymerization of CH2=CRR monomers on an industrial scale. In comparison with bulk polymerization, they allow us to overcome the rapid viscosity increase of the reaction medium with conversion, as well as its consequences, such as the difficult heat removal and the autoacceleration phenomenon. Because in many cases the continuous phase is water, they are also more environmentally friendly techniques than solution polymerization methods, where the use of organic solvents remains hazardous and expensive. Ultimately, heterophase polymerization techniques are the original routes to polymer particles ranging from a few tens of nanometers to a few millimeters in diameter. 

Rotational (torque measurements) or capillary process viscometers, as well as ultrasound and tube oscillations [58] can be used to measure viscosity online during a polymerization reaction. The first two techniques have been widely applied in the monitoring of both chain growth polymerizations and step-growth polymerizations. 

Torque measurements have been used for online monitoring of the viscosity of polymerization reactions. The advantage of the torque measurement as compared with that of the capillary is that no treatment (dilution and flow) of the reaction medium is needed. Several examples of monitoring chain-growth polymerization reactions [62, 63] and step-growth polymerization reactions (specially curing reactions) can be found in the hterature [64]. 

The term solution acrylics refers to acrylic resins prepared by chain-growth polymerization using a solutionbased polymerization process. Here, acrylic monomers and initiators are slowly added to an organic solvent and polymerization is carried out at a predetermined temperature and inert atmosphere with efficient stirring. Both monomers and the polymer formed are miscible in the selected solvent. With the progress of polymerization, the solution viscosity will Increase and heat transfer becomes difficult, limiting the solid content of the final solution. Both thermoplastic and thermosetting solution acrylics can be prepared by this technique. 

It is rare for a polymerization reaction to proceed to completion because the viscosity of a polymer increases during chain growth - this is why monomers are usually liquid, while the polymers produced are hard solids. The last step during polymer formation is a termination reaction known as radical annihilation , in which two radicals meet and then spin-pair to form a covalent bond. 

This is one of the simplest methods of polymerization. It is often used in the polymerization of step-growth polymers.28 In these types of systems the viscosity remains low for a large portion of the reaction and heat transfer is easily controlled. Chain-growth polymers are more difficult to polymerize by this method due to the rapid and highly exothermic reactions. As the viscosity increases, thermal control becomes more difficult and may result in thermal runaway or localized hot spots. Commercial use of bulk polymerization for vinyl polymers is rather limited for... 

The major problem in temperature control in bulk and solution batch chain-growth reactions is the large increase in viscosity of the reaction medium with conversion. The viscosity of styrene mixtures at I50°C will have increased about 1000-fold, for example, when 40 wt % of the monomer has polymerized. The heat transfer to a jacket in a vessel varies approximately inversely with the one-third power of the viscosity. (The exact dependence depends also on the nature of the agitator and the speed of fluid flow.) This suggests that the heat transfer efficiency in a jacketed batch reactor can be expected to decrease by about 40% for every 10% increase in polystyrene conversion between 0 and 40%. 

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