Features of Free-Radical Polymerization

The main features of a radical polymerization can now be summarized and contrasted with the corresponding step-growth reactions (Section 2.9). 

A high-molar-mass polymer is formed immediately when the reaction begins, and the average chain length shows little variation throughout the course of the polymerization. 

The monomer concentration decreases steadily throughout the reaction. Only the active center can react with the monomer and add units onto the chain one after the other. 

Long reaction times increase the polymer yield, but not the molar mass of the polymer. 

An increase in temperature increases the rate of the reaction but decreases the molar mass. 

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The mai n feature of free-radical polymerization is the very short lifetime of the growing radicals. Consequently, functionalization can arise only from the use of functional initiators or from transfer processes93). 

Readily Observable Features of Free-Radical Polymerizations... 

READILY OBSERVABLE FEATURES OF FREE-RADICAL POLYMERIZATIONS... 

One of the nice features of free-radical polymerization is that values of the preexponential coefficients and activation energies (or alternately half-life values at various temperatures) can be obtained in the literature (such as in Odian (1991)) or from their manufacturers (such as Wako Chemical Corp.) for a variety of initiators, and these numbers do not normally change no matter what the fluid environment the initiator molecules are in. Thus, if we want to decompose more than 99% of the starting initiator material in the reactor, we just have to wait for the reaction to proceed up to five times the initiator half-life. The other attractive feature of free-radical polymerization is that free-radical reactions are well known and radical concentrations can be directly measured. Thus, we know, for example, that if we want to preserve radicals in solution, we should not allow oxygen gas (O2) in our system, because reactive radicals will combine with oxygen gas to form a stable peroxy radical. That is why reaction fluids were bubbled with N2, CO2, Ar, or any inert gas, in order to displace O2 gas that comes from the air. Finally, Iree-radical polymerization is not sensitive to atmospheric or process water, compared to other polymerization kinetic mechanisms. 

Summary of the Main Features of Free-radical Polymerization... 

In the period 1910-1950 many contributed to the development of free-radical polymerization.1 The basic mechanism as we know it today (Scheme 1.1), was laid out in the 1940s and 50s.7 9 The essential features of this mechanism are initiation and propagation steps, which involve radicals adding to the less substituted end of the double bond ("tail addition"), and a termination step, which involves disproportionation or combination between two growing chains. 

The chain carriers in chain-growth polymerization may be anions or cations rather than free radicals. Such ionic polymerization shares many features with free-radical polymerization, but differs in one important respect Since ions of the same charge sign repel one another, spontaneous binary termination by reaction of two chain carriers with one another cannot occur. In fact, the reaction may run out of monomer with chain carriers still intact. 

Though ionic polymerization resembles free-radical polymerization in terms of initiation, propagation, transfer, and termination reactions, the kinetics of ionic polymerizations are significantly diflFerent from free-radical polymerizations. In sharp contrast to free-radical polymerizations, the initiation reactions in ionic polymerizations have very low activation energies, chain termination by mutual destruction of growing species is nonexistent, and solvent effects are much more pronounced, as the nature of solvent determines whether the chain centers are ion pairs, free ions, or both. No such solvent role is encountered in free-radical polymerization. The overall result of these features is to make the kinetics of ionic polymerization much more complex than the kinetics of free-radical polymerization. 

The mechanisms of ionic and coordination polymerizations are more complex and are not as clearly understood as those of free radical polymerization. Here, we will briefly highlight the essential features of these mechanisms, and more details will be given in Chapter 7. Initiation of ionic polymerization usually involves the transfer of an ion or an electron to or from the monomer. Many monomers can polymerize by more than one mechanism, but the most appropriate polymerization mechanism for each monomer is related to the polarity of the monomers and the Lewis acid-base strength of the ion formed. 

Thus, in summary, the kinetics of free radical polymerization are characterized by the following features ... 

Of course, quantum chemistry is not without limitations. Since the multielectron Schrodinger equation has no analytical solution, numerical approximations must instead be made. In principle, these approximations can be extremely accurate, but in practice the most accurate methods require inordinate amounts of computing power. Furthermore, the amount of computer power required scales exponentially with the size of the system. The challenge for quantum chemists is thus to design small model reactions that are able to capture the main chemical features of the polymerization systems. It is also necessary to perform careful assessment studies, in order to identify suitable procedures that offer a reasonable compromise between accuracy and computational expense. Nonetheless, with recent advances in computational power, and the development of improved algorithms, accurate studies using reasonable chemical models of free-radical polymerization are now feasible. 

We begin our discussion of copolymers by considering the free-radical polymerization of a mixture of two monomers. Mi and M2. This is already a narrow view of the entire field of copolymers, since more than two repeat units can be present in copolymers and, in addition, mechanisms other than free-radical chain growth can be responsible for copolymer formation. The essential features of the problem are introduced by this simpler special case, so we shall restrict our attention to this system. 

Bhawe (14) has simulated the periodic operation of a photo-chemically induced free-radical polymerization which has both monomer and solvent transfer steps and a recombination termination reaction. An increase of 50% in the value of Dp was observed over and above the expected value of 2.0. An interesting feature of this work is that when very short period oscillations were employed, virtually time-invariant products were predicted. 

Here, gt takes into account one interesting feature of the free radical polymerization kinetics the auto acceleration in the rate of reaction (better known as the Trommsdorff or gel effect). The gel effect is taken into account in the model (42) by including the experimental correlation developed in [18] for the termination rate ... 

The key feature of Inisurfs is their surfactant behavior. They form micelles and are adsorbed at interfaces, and as such they are characterized by a critical micelle concentration (CMC) and an area/molecule in the adsorbed state. This influences both the decomposition behavior and the radical efficiency, which are much lower than those for conventional, low molecular weight initiators. Tauer and Kosmella [4] have observed that in the emulsion polymerization of styrene, using an Inisurf concentration above the CMC resulted in an increase in the rate constant of the production of free radicals. This was attributed to micellar catalysis effects as described, for example, by Rieger [5]. Conversely, if the Inisurf concentration was below the CMC the rate constant of the production of free radicals decreased with an increase in the Inisurf concentration, which was attributed to enhanced radical recombination. Also note that a similar effect of the dependence of initiator efficiency on concentration was reported by Van Hook and Tobolsky for azobisisobutyronitrile (AIBN) [6]. 

The bimolecular termination reaction in free-radical polymerization is a typical example of a diffusion controlled reaction, and is chain-length-depen-dent [282-288]. When pseudobulk kinetics appUes, the MWD formed can be approximated by that resulting from bulk polymerization, and it can be solved numerically [289-291]. As in the other extreme case where no polymer particle contains more than one radical, the so-caUed zero-one system, the bimolecular termination reactions occur immediately after the entrance of second radical, so unique features of chain-length-dependence cannot be found. Assuming that the average time interval between radical entries is the same for all particles and that the weight contribution from ohgomeric chains formed... 

The key feature distinguishing anionic (and cationic) from free-radical polymerization is the absence of spontaneous binary termination and has already been mentioned. Unless chain transfer occurs, polymer chains keep growing until all monomer is used up. At that stage, the polymer still carries reactive centers [65] —it is said to be a "living polymer" [66,67]—, and polymerization can be started anew by addition of further monomer. Block copolymers can be synthesized from a living polymer by addition of a different monomer [68,69]. 

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