Polymerization, free-radical addition ionic

The mechanism of these reactions places addition polymerizations in the kinetic category of chain reactions, with either free radicals or ionic groups responsible for propagating the chain reaction. 

A factor in addition to the RTD and temperature distribution that affects the molecular weight distribution (MWD) is the nature of the chemical reaciion. If the period during which the molecule is growing is short compared with the residence time in the reactor, the MWD in a batch reactor is broader than in a CSTR. This situation holds for many free radical and ionic polymerization processes where the reaction intermediates are very short hved. In cases where the growth period is the same as the residence time in the reactor, the MWD is narrower in batch than in CSTR. Polymerizations that have no termination step—for instance, polycondensations—are of this type. This topic is treated by Denbigh (J. Applied Chem., 1, 227 [1951]). 

The first use of ionic liquids in free radical addition polymerization was as an extension to the doping of polymers with simple electrolytes for the preparation of ion-conducting polymers. Several groups have prepared polymers suitable for doping with ambient-temperature ionic liquids, with the aim of producing polymer electrolytes of high ionic conductance. Many of the prepared polymers are related to the ionic liquids employed for example, poly(l-butyl-4-vinylpyridinium bromide) and poly(l-ethyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide [38 1]. 

The five-membered cyclic amide pyrrolidone has achieved widespread attention in the area of heterocyclic polymers since the first preparation and polymerization reactions of l-vinylpyrrolidin-2-one (1) were reported in the early 1940s. Poly(vinylpyrrolidone) (2) and its copolymers are among the most thoroughly studied heterocyclic addition polymers (B-74MI11100). Monomer (1) is readily polymerized (B-77MI11100) both free radically and ionically (Scheme 1). The former method is by far the most important, and allows the preparation of a wide variety of copolymers. Interestingly, in the homopolymerization of vinylpyrrolidone (1), the molecular weight of the polymer obtained does not appear to be influenced by the initiator concentration or the reaction temperature. 

Bulk polymerizations, such as addition (free-radical- and ionic-based) and step-growth types. Grafting polymerization by small molecules. Interchain copolymer formation, based on chain cleavage, graft copolymerization, and end-group block copolymerization. 

Fig. 15.2 Schematic representation for covalent attachment of hydroxyl groups to MWCNTs by free radical addition of 4,4 -azobis(4-cyanopentanol) in water and subsequent surface-initiated ring-opening polymerization of -caprolactone in room-temperature ionic liquids (BmimBF )...

In considering chemical explanations for corona polymerization, both free radical and ionic intermediates are possibilities. Experiments were run with various additives to styrene that might be expected to inhibit each kind of reaction through combination with the active intermediate but no clear-cut reduction in yield was observed. Benzoquinone at 1 and 2 mole % gave normal yields. Water and butylamine were extensively... 

The addition polymerizations, for oleflnic monomers, are chain reactions that convert the monomers into polymers by stimulating the opening of the double bond with a free radical or ionic initiator. The product then has the same chemical composition as the starting material, e.g., acrylonitrile produces polyacrylonitrile without the elimination of a small molecule. 

Olefins (from the French olefiant, oil-forming ), or alkenes, are hydrocarbon molecules with at least one double carbon-carbon bond. Alpha (a-)olefins are alkenes with a double bond at the first (alpha-) carbon. Polyolefins are polymer molecules made using free radical or ionic initiators to open these reactive double bonds in an addition (chain-growth) polymerization, producing essentially linear high molecular weight thermoplastic polymers. 

The chain addition polymerizations require monomers with double bonds. They require free radical or ionic initiators to open the double bond and form the polymerization path in the manufacture of polymers such as polyethylene, polypropylene, polystyrene, and polyvinyl chloride which together constitute the majority of polymers, about 70 % of all polymers produced. A wide range of copolymers or terpolymers are produced by chain addition polymerization of two or three different monomers with double bonds. 

Unlike epoxies, which cme by an ionic polymerization mechanism, modified acrylics cure by free-radical addition. Therefore, careful proportioning of components is not required. In two-component systems, no mixing is required because the adhesive is applied to one substrate, the activator to the other, and the substrates are joined. Handling strength is rapidly achieved with this fast-curing system. 

Even when polymers have identical repeat units the molecules can differ in architecture leading to further types oHsomerism. Branching can have a profound effect upon the properties of the polymer. It is normally caused during addition polymerization by transfer reactions with polymer molecules (Section 2.2.9). The tendency for the transfer to take place depends principally upon the reactivity of the active centre. Free radicals are more reactive than ions and branching is more common in polymers produced by free radical addition than by ionic polymerization. In general it is found that the tendency for branching depends upon the type of polymerization system as follows ... 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

The active centers that characterize addition polymerization are of two types free radicals and ions. Throughout most of this chapter we shall focus attention on the free-radical species, since these lend themselves most readily to generalization. Ionic polymerizations not only proceed through different kinds of intermediates but, as a consequence, yield quite different polymers. Depending on the charge of the intermediate, ionic polymerizations are classified as anionic or cationic. These two types of polymerization are discussed in Secs. 6.10 and 6.11, respectively. 

---