Molar mass distribution radical polymerization

For a free-radical polymerization and a condensation polymerization process, explain why the molar mass distribution of the polymer product will be different depending on whether a mixed-flow or a plug-flow reactor is used. What will be the difference in the distribution of molar mass ... 

In Ref. [107] the procedure above has been employed for the measurement of the molar mass distribution of a broad molecular weight polystyrene, obtained by radical polymerization with ethylacetate as solvent. The scaling parameters for this polystyrene in this marginal solvent have been determined to be a 2.8 x 10-4 cm2/s and b 0.52 [107]. The upper curve in Figure 17 shows the resulting molar mass distribution in comparison with the one obtained by SEC. 

As a consequence of the free radical polymerization kinetics, the termination rates are extremely fast in comparison to the slow initiation rates. This results in the formation of high molar mass chains at the initial stage of the polymerization and decreasing molar masses in the latter stages due to the decrease in the monomer concentration. Under these circumstances, broad molar mass distributions are inevitable. 

A second route is termed sequential anionic polymerization. More recently, also controlled radical techniques can be applied successfully for the sequential preparation of block copolymers but still with a less narrow molar mass distribution of the segments and the final product. In both cases, one starts with the polymerization of monomer A. After it is finished, monomer B is added and after this monomer is polymerized completely again monomer A is fed into the reaction mixture. This procedure is applied for the production of styrene/buta-diene/styrene and styrene/isoprene/styrene triblock copolymers on industrial scale. It can also be used for the preparation of multiblock copolymers. 

Broad distribution A PS obtained by radical polymerization served as a model system with a broad smooth molar mass distribution. The solvent was ethylace-... 

So far we have discovered very few polymerization techniques for making macromolecules with narrow molar mass distributions and for preparing di-and triblock copolymers. These types of polymers are usually made by anionic or cationic techniques, which require special equipment, ultrapure reagents, and low temperatures. In contrast, most of the commodity polymers in the world such as LDPE, poly(methyl methacrylate), polystyrene, poly(vinyl chloride), vinyl latexes, and so on are prepared by free radical chain polymerization. Free radical polymerizations are relatively safe and easy to perform, even on very large scales, tolerate a wide variety of solvents, including water, and are suitable for a large number of monomers. However, most free radical polymerizations are unsuitable for preparing block copolymers or polymers with narrow molar mass distributions. 

The controlled free-radical miniemulsion polymerization of styrene was performed by Lansalot et al. and Butte et al. in aqueous dispersions using a degenerative transfer process with iodine exchange [91, 92]. An efficiency of 100% was reached. It has also been demonstrated that the synthesis of block copolymers consisting of polystyrene and poly(butyl acrylate) can be easily performed [93]. This allows the synthesis of well-defined polymers with predictable molar mass, narrow molar mass distribution, and complex architecture. 

Polymerization by ionic initiation is much more limited than that by free-radical initiation with vinyl monomers, but there are monomers such as carbonyl compounds that may be polymerized ionically but not through free radicals because of the high polarity. The polymerization is much more sensitive to trace impurities, especially water, and proceeds rapidly at low temperature to give polymers of narrow molar-mass distribution. The chain grows in a living way and, unlike in the case of free-radical polymerization, is generally terminated not by recombination but rather by trace impurities, solvent or, rarely, the initiator s counter-ion (Fontanille, 1989). 

Since the discovery of living polymerizations by Swarc in 1956 [1], the area of synthesis and application of well-defined polymer structures has been developed. The livingness of a polymerization is defined as the absence of termination and transfer reactions during the course of the polymerization. If there is also fast initiation and chain-end fidelity, which are prerequisites for the so-called controlled polymerization, well-defined polymers are obtained that have a narrow molar mass distribution as well as defined end groups. Such well-defined polymers can be prepared by various types of living and controlled polymerization techniques, including anionic polymerization [2], controlled radical polymerization [3-5], and cationic polymerization [6, 7]. 

Radical polymerization is a balanced reaction in which the balance is usually well in favor of the polymer. High pressure further enhances this effect. Raising the temperature accelerates the formation of the radicals. Pressure and temperature also influence the molar mass (chain length), molar mass distribution, and the degree of branching. 

In a well-controlled radical system, the monomer conversion is first order, molar mass increases linearly with monomer conversion, and the molar mass distribution MJM is below 1.5. In addition, chain end functionalization and subsequent monomer addition allow the preparation of well-controlled polymer architectures, for example, block copolymers and star polymers by a radical mechanism, which had been up to now reserved for ionic chain growth polymerization techniques. 

A.20. Appendix Molar Mass Distribution in Free Radical Polymerizations... 

The kinetics of chain-reaction polymerization is illustrated in Fig. 3.28 for a free radical process. Analogous equations, except for termination, can be written for ionic polymerizations. Coordination reactions are more difficult to describe since they may involve solid surfaces, adsorption, and desorption. Even the crystallization of the macromolecule after polymerization may be able to influence the reaction kinetics. The rate expressions, as given in Appendix 7, Fig. A7.1, are easily written under the assumption that the chemical equations represent the actual reaction path. Most important is to derive an equation for the kinetic chain length, v, which is equal to the ratio of propagation to termination-reaction rates. This equation permits computation of the molar mass distribution (see also Sect. 1.3). The concentration of the active species is very small and usually not known. First one must, thus, ehminate [M ] from the rate expression, as shown in the figure. The boxed equation is the important equation for v. 

Figure 3.33 lists a recipe for emulsion polymerization of polystyrene in a water dispersion of monomer droplets and soap micelles [20]. The reaction is started by light-sensitive, water-soluble initiators, such as benzoyl peroxide. If one compares the sizes of the dispersed droplets, one notices that the small soap micelles that contain also styrene in their interior are most likely to occasionally initiate a polymerization of the monomer on absorption of a free radical. Once initiated, the reaction continues until a second free radical molecule enters the micelle. Then the reaction is terminated, until a third radical starts another molecule. Monomers continuously add to the micelles, so that the polymerization continues. Keeping the free radical generation constant, a relatively narrow molar mass distribution can be obtained. 

In practice, all polymers contain many usually homologous components, and hence polymers can be characterized by a molar-mass distribution. Depending on the polymerization mechanism, different analytical functions are available from the literature [1. 2]. In the case of radically pwlymerized products, the Schulz-Floiy distribution W(r) can be derived from the reaction mechanism (1) ... 

The final system that is worth mentioning in this chapter on LRP in emulsion is the use of 1,1-diphenylethylene (DPE) (127). DPE adds to a growing radical and forms a radical with a reactivity too low for propagation. The exact mechanism is not elucidated, but the incorporation of DPE leads to a chain that allows chain extension or block copolymer formation. More or less similar to what was described about the xanthates in RAFT polymerization, here also the molar mass distributions are relatively broad. The greatest advantage of DPE-mediated polymerization is the fact that it results in minimal disturbance of the polymerization process. The product latex does not contain any unusual extractable material (like the catalyst in ATRP), or polymer-bound colored moieties (like the thiocar-bonylthio compound in RAFT). The obvious drawback is the limited control over chain architecture, and the limited understanding of the mechanistic details. 

The above two examples of the polymerization of styrene contrast the prototypical controlled and uncontrolled polymerizations. Controlled polymerizations offer simple molar mass control, the ability to define the polymer end groups, and give polymer samples with narrow molar mass distributions. Molar mass definition in uncontrolled polymerizations is more difficult, polymer end groups are determined by inherent termination (and transfer) reactions, and the molar mass distributions are typically broader. There is much contemporary interest in developing polymerization reactions that are controlled because of the precision with which macromolecules can be designed. Many chapters are dedicated to these endeavors with controlled radical polymerization receiving the most attention recently. 

Dispersions of molar mass distributions depend on polymerization reactions (see Table 4.1.6). Values of Dm close to 1 can be achieved by cmitrolled or living techniques whereas free radical polymerization leads to broad distributions. 

In copolymers, additional to the molar mass distribution, a distribution of chemical composition is present The term chemical heterogeneity refers to the differences in the relative percentage of monomers among copolymeric chains of different molar masses. A major source for chemical heterogeneity is the free radical polymerization of monomers with unequal reactivities resulting in different rates of incorpora-... 

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