Reaction mechanisms, polymers chain-growth polymerization

Chain gro tvth polymerization begins when a reactive species and a monomer react to form an active site. There are four principal mechanisms of chain growth polymerization free radical, anionic, cationic, and coordination polymerization. The names of the first three refer to the chemical nature of the active group at the growing end of the monomer. The last type, coordination polymerization, encompasses reactions in which polymers are manufactured in the presence of a catalyst. Coordination polymerization may occur via a free radical, anionic, or cationic reaction. The catalyst acts to increase the speed of the reaction and to provide improved control of the process. 

Wallace Carothers, duPont s famous polymer chemist, proposed classifying polymers by reference to the stoichiometry of the polymerization reaction. If the entire monomer molecule ends up in the polymer, he called it an addition polymer, whereas if there is a byproduct, often water, the primary product is called a condensation polymer. He thus considered vinyl polymers to be addition polymers and polyesters to be condensation polymers. However, it was later learned that it is possible to make some addition polymers by reactions in which there is a byproduct and to make some condensation polymers by reactions in which there is no byproduct. Paul Flory, who started his career as the theoretician in Carother s duPont research group, later proposed that the reaction mechanism be used as the basis for classifying polymers. In this scheme, in a step polymerization, any two reactive molecules can combine, so polymerization occurs uniformly throughout the reaction mixture. In chain growth polymerization, on the other hand, monomer units are added only to species containing an active center or initiator, which can be a free radical, an ion, or an active catalyst site. Condensation polymers are usually produced by step reactions, and addition polymers are usually made by... 

In this chapter, we have considered the reaction engineering of chain-growth polymerization. In order to manufacture polymers of desired physical and mechanical properties, the performance of the reactors must be closely controlled. To do this, various transport equations governing their performance must be established, which, in principle, can be solved numerically. The usual Runge-Kutta technique takes considerable computational time and, at times, gives numerical instability. To overcome aU of these problems, a semianalytical approach can be used. 

Some 50 years ago, Paul Flory chose the terms step-growth and chain-growth polymerization to describe the processes by which many monomers are converted to polymer (Flory 1953). Although not perfect, the terms are still commonly used and can help us understand the major mechanisms of polymerizations. A mechanism for a reaction describes the processes and pathways by which that reaction proceeds. Mechanisms are important because they help us understand the details of a chemical reaction as well as help us predict the outcome of new reactions. 

The most common type of chain-growth polymerization is free-radical polymerization. An initiator or a photochemical reaction produces a free radical that attaches itself to a monomer molecule, creating a group with odd-electron configuration (reactive center) at which monomer molecules are added until two such centers react with one another or, more rarely, a center is deactivated by some other process. This is a mechanism much like that of ordinary chain reactions (see Chapter 9 the term "chain" in chain growth refers to that kind of mechanisms, not to the growing molecular chain of repeating units in the polymer.)... 

In chain-growth polymerizations the mechanisms and rates of the reactions that initiate, continue, and terminate polymer growth are different. 

Several epoxy formulations are cured by both step-growth and chain-growth polymerizations occurring sequentially or in parallel. For example, BF3 complexes or tertiary amines may be added as catalysts of an amine-epoxy reaction, leading to different reaction mechanisms taking place whose relative significance depends on the cure temperature (or thermal cycle) and the initial stoichiometry. The structure and properties of the resulting polymer networks depend on the relative contribution of both mechanisms. 

Although these definitions were perfectly adequate at the time, it soon became obvious that notable exceptions existed and that a fundamentally sounder classification should be based on a description of the chain-growth mechanism. It is preferable to replace the term condensation with step-growth or step-reaction. Reclassification as step-growth polymerization now logically includes polymers such as polyurethanes, which grow by a step-reaction mechanism without elimination of a small molecule. 

On the other hand, butyllithium-aluminum alkyl initiated polymerizations of vinyl chloride are unaffected by free-radical inhibitors. Also, the molecular weights of the resultant polymers are unaffected by additions of CCI4 that acts as a chain-transferring agent in free-radical polymerizations. This suggests an ionic mechanism of chain growth. Furthermore, the reactivity ratios in copolymerization reactions by this catalytic system differ from those in typical free-radical polymerizations An anionic mechanism was also postulated for polymerization of vinyl chloride with t-butylmag-nesium in tetrahydrofuran. ... 

As two non-petroleum chemicals readily accessible from renewable resources, both furfural and HMF are suitable starting materials for the preparation of versatile fine chemicals [14, 102-106] and can also serve as renewable monomers for preparation of sustainable polymer products [107]. Schemes 3, 4, and 5 depict the stmctures of the selected furan-based monomers [107-113]. As a typical precursor, furfural can be converted to a vast array of furan-based monomers bearing a moiety which can normally be polymerized by chain-growth polymerization mechanisms [108-113]. As shown in Scheme 3, these monomers are all readily polymerizable by chain-growth reactions. However, depending on their specific structure, the nature of the polymerization mechanism is different, ranging from free radical, cationic, anionic, to stereospecific initiation [108-113]. On the other hand, furfuryl... 

---