Polymerization, free-radical addition mechanism

Proliferous Polymerization. Eady attempts to polymerize VP anionicaHy resulted in proliferous or "popcorn" polymerization (48). This was found to be a special form of free-radical addition polymerization, and not an example of anionic polymerization, as originally thought. VP contains a relatively acidic proton alpha to the pyrroHdinone carbonyl. In the presence of strong base such as sodium hydroxide, VP forms cross-linkers in situ probably by the following mechanism ... 

The specificity of the reaction mechanism to the chemistry of the initiator, co-initiator and monomer as well as to the termination mechanism means that a totally general kinetic scheme as has been possible for free-radical addition polymerization is inappropriate. However, the general principles of the steady-state approximation to the reactive intermediate may still be applied (with some limitations) to obtain the rate of polymerization and the kinetic chain length for this living polymerization. Using a simplified set of reactions (Allcock and Lampe, 1981) for a system consisting of the initiator, I, and co-initiator, RX, added to the monomer, M, the following elementary reactions and their rates may be... 

Allyl esters, unsaturated polyesters, as well as some of what are known as vinyl or acrylic esters are cured by free radical addition polymerization. In the case of allyl esters, the monomers, themselves, are cross-linked. On the other hand, unsaturated polyesters are copolymerized with monomers such as styrene or methyl methacrylate. Since the unsaturated polyesters have many main-chain double bonds and the structure of a cross-linked network is fixed after quite low conversions, only a few double bonds actually react. These unconverted double bonds can then react later with atmospheric agents, and so produce poor weathering properties of the crosslinked networks. In addition, the polymerization produces many free chain ends that contribute nothing or even disadvantageously to the mechanical properties. The newly developed vinyl or acrylic esters avoid both of these problems in that the monomers capable of cross-linking only have unsaturated double bonds at the molecular ends (see also Section 26.4. S). 

Most commercial grades are amorphous, and the lack of crystalline regions provides transparency and no clearly defined melt temperature. Polystyrene can be polymerized via many mechanisms such as free radical, anionic, cationic, and Ziegler reactions. Commercially available polystyrene is generally produced via free radical-addition polymerization. The polymerized resin is a transparent solid having very low specific gravity (1.054-1.070)... 

Grafting reactions are carried out conventionally through free radical addition copolymerization mechanism, where free radicals are generated on a polymeric backbone by direct oxidation of certain transition metal ions (e.g., Ce ", Cr +, Co ). The redox... 

The preceding discussion of free-radical addition polymerization has considered only homogeneous reactions. Considerable polymer is produced commercially by a complex heterogeneous free-radical addition process known as emulsion polymerization. This process was developed in the United States during World War II to manufacture synthetic rubber. A rational explanation of the mechanism of emulsion polymerization was proposed by Harkins [4] and quantified by Smith and Ewart [5] after the war, when information gathered at various locations could be freely exchanged. Perhaps the best way to introduce the subject is to list the feed sent to a typical reactor. 

In order to be able to predict the degree and rate of polymerization for a particular free radical addition polymerization system it is necessary to know the individual rate constants A , kp and kt. Also in order to determine Xn, the mechanism of termination must be known. This can be done using radioactively labelled initiator molecules. The rate constant for the breakdown of initiator ki can be determined from measurements upon the initiator alone. However, ki, so determined, may be different from that in the presence of monomer molecules. So far we have developed two equations that can be used to determine the three rate constants. The equations are... 

In free radical addition polymerization the distribution of molar mass depends upon the mechanism of termination. The simplest case to consider is when the mechanism is through disproportionation and is equal to the kinetic chain length v. In this case the chain grows and terminates at the length to which it had grown. At this point it is necessary to define a new... 

This reaction proceeds through a chain mechanism. Free-radical additions to 1-butene, as in the case of HBr, RSH, and H2S to other olefins (19—21), can be expected to yield terminally substituted derivatives. Some polymerization reactions are also free-radical reactions. 

Acrylamide polymerization by radiation proceeds via free radical addition mechanism [37,38,40,45,50]. This involves three major processes, namely, initiation, propagation, and termination. Apart from the many subprocesses involved in each step at the stationary state the rates of formation and destruction of radicals are equal. The overall rate of polymerization (/ p) is so expressed by Chapiro [51] as ... 

In this chapter, we discuss free-radical substitution reactions. Free-radical additions to unsaturated compounds and rearrangements are discussed in Chapters 15 and 18, respectively. In addition, many of the oxidation-reduction reactions considered in Chapter 19 involve free-radical mechanisms. Several important types of free-radical reactions do not usually lead to reasonable yields of pure products and are not generally treated in this book. Among these are polymerizations and high-temperature pyrolyses. 

Still another, and chains, long or short, may be built up. This is the mechanism of free-radical polymerization. Short polymeric molecules (called telomers), formed in this manner, are often troublesome side products in free-radical addition reactions. 

Chain-reaction mechanisms differ according to the nature of the reactive intermediate in the propagation steps, such as free radicals, ions, or coordination compounds. These give rise to radical-addition polymerization, ionic-addition (cationic or anionic) polymerization, etc. In Example 7-4 below, we use a simple model for radical-addition polymerization. 

Common thermosets are cured by a free radical addition mechanism. These types of composites are cured by heat initiators, such as peroxides, or by photo initiators, such as a-diketones. A characteristic of cured acrylates is large shrinkage in the course of polymerization, which is undesirable for many uses. Another undesirable characteristic of acrylates is the formation of an oxygen-inhibited layer on the surface upon curing. 

Essentially, TFE in gaseous state is polymerized via a free radical addition mechanism in aqueous medium with water-soluble free radical initiators, such as peroxy-disulfates, organic peroxides, or reduction-activation systems.15 The additives have to be selected very carefully since they may interfere with the polymerization. They may either inhibit the process or cause chain transfer that leads to inferior products. When producing aqueous dispersions, highly halogenated emulsifiers, such as fully fluorinated acids,16 are used. If the process requires normal emulsifiers, these have to be injected only after the polymerization has started.17 TFE polymerizes readily at moderate temperatures (40 to 80°C) (104 to 176°F) and moderate pressures (0.7 to 2.8 MPa) (102 to 406 psi). The reaction is extremely exothermic (the heat of polymerization is 41 kcal/mol). 

The simplest way to catalyze the polymerization reaction that leads to an addition polymer is to add a source of a free radical to the monomer. The term free radical is used to describe a family of very reactive, short-lived components of a reaction that contain one or more unpaired electrons. In the presence of a free radical, addition polymers form by a chain-reaction mechanism that contains chain-initiation, chain-propagation, and chain- termination steps. 

The basis of modern organic resin production incorporates the same principles described for their predecessors but depends upon an entirely different polymerization mechanism first applied by D Alelio in 1944, called addition or vinyl polymerization. The mechanism is one of free radical induced polymerization between reactants monomers) carrying ethenyl (or vinyl) double bonds (—GH=CH2). One of the reactants must contain at least two ethenyl double bonds to effect crosslinking. Again, an understanding of the resin synthesis is afforded by showing separately in Scheme 2.2 what are in fact simultaneously occurring complex polymerization reactions between all reactant permutations. 

In this type of polymerization, an undiluted monomer or mixture of monomers is converted to a high molecular weight homopolymer or copolymer. The progress of the reaction is characterized by a large increase in the viscosity of the reacting mixture (from less than 50 Pa-s to greater than 1,000 Pa-s). Two reaction mechanisms of bulk polymerization have been implemented in REX free-radical addition and condensation reaction chemistries. 

There are three different mechanisms by which the cyclic olefin norbornene can be polymerized to reasonably high molecular weights ring-opening metathesis polymerization (or ROMP), vinyl addition copolymerization with acyclic olefins such as ethylene, and vinyl addition homopolymerization (see Fig. 4.2). Carbocationic and free-radical initiated polymerizations are ignored since they yield only low molecular weight oligomers [8]. 

The growing polymer in chain-reaction polymerization is a free radical, and polymerization proceeds via chain mechanism. Chain-reaction polymerization is induced by the addition of free-radical-forming reagents or by ionic initiators. Like all chain reactions, it involves three fundamental steps initiation, propagation, and termination. In addition, a fourth step called chain transfer may be involved. 

Poly(vinyl acetate) (PVA) and ethylene-vinyl acetate (EVA) copolymer adhesives have much in common, yet represent extremes in the degree of sophistication of their production processes. Both products are stable suspensions in water of a film-forming polymer, the particles of which are generally spherical. They are made by emulsion polymerization, which uses a free-radical addition mechanism to polymerize the monomer in the presence of water and stabilizers. Vinyl acetate is the sole or major monomeric raw material. 

---