Free radical addition cationic

The electron-rich carbon—carbon double bond reacts with reagents that are deficient in electrons, eg, with electrophilic reagents in electrophilic addition (6,7), free radicals in free-radical addition (8,9), and under acidic conditions with another butylene (cation) in dimerization. 

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

Kinetics. Monomer can be converted into polymer by any chemical reaction which creates a new covalent bond. Most of this review will concern polymerization of vinyl monomers by free radical addition polymerization. However, some attention will be given to cationic polymerization of epoxy functional materials. No extensive review of polymerization processes and kinetics will be given here, but some of the fundamental notions will be described. For reviews, see (4a-d). 

Chain-growth, or addition, polymers are made by adding one monomer unit at a time to the growing polymer chain. The reaction requires initiation to produce some sort of reactive intermediate, which may be a free radical, a cation, or an anion. The intermediate adds to the monomer, giving a new intermediate, and the process continues until the chain is terminated in some way. Polystyrene is a typical free-radical chain-growth polymer. 

Free radical promoted, cationic polymerization also occurs upon irradiation of pyridinium salts in the presence of acylphosphine oxides. But phosphonyl radicals formed are not oxidized even by much stronger oxidants such as iodonium ions as was demonstrated by laser flash photolysis studies [51, 52]. The electron donor radical generating process involves either hydrogen abstraction or the addition of phosphorus centered or benzoyl radicals to vinyl ether monomers [53]. Typical reactions for the photoinitiated cationic polymerization of butyl vinyl ether by using acylphosphine oxide-pyridinium salt combination are shown in Scheme 10. 

Generalized methods of initiating the polymerization of these monomers have recently been reviewed in detail [9], and were also mentioned briefly earlier in this Chapter. As with vinyl monomers initiation can be efficient and rapid, with the production of a fixed number of active centres. Propagation appears to be much slower, however, and rates of polymerization are comparable to those in free radical addition polymerizations. Techniques such as dilatometry, spectrophotometry etc. are therefore convenient for kinetic investigation of this type of cationic reaction. 

Ionic addition yields isopropyl bromide because a secondary cation is formed faster than a primary. Free-radical addition yields w-propyl bromide because a secondary free radical is formed faster than a primary. Examination of many cases of anti-Markovnikov addition shows that orientation is governed by the ease of formation of free radicals, which follows the sequence 3° > 2 > T. 

Initiation reactions are usually started by an active free radical such as peroxide (-0-0-), e.g. benzoyl peroxide is a good inititator for the free radical addition polymerisation of styrene to produce polystyrene AICI3 is an initiator for the cationic addition polymerisation of isobutylene to form isobutyl synthetic rubber or azobisiso-butyronitrile compounds (-N=N-) (abbreviated to AIBN). Propagation reactions are the continuing process and, eventually, lead to the termination stage that occurs by combination or disproportionation. This usually occurs when the free radicals combine with themselves and signals the end of the polymerisation process. All polymers formed by this process are thermoplastics. Table 4.1 is a list of common polymers prepared by the addition process. 

Crivello s group followed either or both of two strategies that described for the additives in acceleration of photoinitiated cationic polymerization of epoxide monomers. These are stabilization of free radicals and cations by resonance and inductive effect, and the activated monomer mechanism. Comparative studies of novel monomers with conventional monomers show that newly designed monomers given in... 

Olefins are also oxidized by metal salts. Whereas Pb(IV) and Mn(III) acetates lead to 7-lactones by free-radical addition routes (Heiba et at., 1968b, c), Co(III) in aqueous sulfuric acid at room temperature causes oxidative cleavage of a glycol which is formed by a cation-radical route [(35)- (38)] (Bawn and Sharp, 1957). The final products are ketones and carboxylic acids. 

Since the introduction of photocuring of coatings as a viable industrial process well over a decade ago, the UV curing industry has followed a line of steady growth. This paper deals with the mechanisms and components which characterize photocurable coatings. Free radical and cationic photoinitiated polymerization are discussed in light of key review references from the recent literature. In addition, a section is dedicated to discussion of the lamp sources available for photocuring operations. 

Chain polymerization involves three processes chain initiation, chain propagation, and chain termination. (A fourth process, chain transfer, may also be involved, but it may be regarded as a combination of chain termination and chain initiation.) Chain initiation occurs by an attack on the monomer molecule by a free radical, a cation, or an anion accordingly, the chain polymerization processes are called free-radical polymerization, cationic polymerization, or anionic polymerization. (In coordination addition or chain polymerization, described below separately, the chain initiation step is. 

The case of chain (co) polymerization is far more complicated [16]. An initiation step produces an active centre (free radical, anion, cation, etc,...), that leads to a fast growth through successive monomer additions (propagation step). The propagating chain ends through transfer and/or termination steps. 

Lipids present an excellent opportunity for the production of bioplastics. They can be directly polymerized using free-radical or cationic processes with the addition of some additives. The reactivity present in many triglycerides (hydroxyl groups, double bonds) allows for the facile manipulation to whatever the desired functional group for polymerization may be. Triglycerides are an excellent, sustainable platform chemical for polymer production the explosion in their use since the early 2000s will only continue as concerns over petrochemical feedstocks and prices grow. 

Figure 1. Polymerization of a methacrylate-substituted spiro orthoester (1) by free radical addition to the methacrylate group followed by cationic ring opening.

Certain polymerizations can be stopped by additives. Diphenyl picryl-hydrazyl, for example, is a free radical scavenger, and stops free radical polymerizations. Ionic mechanisms are not affected. Benzoquinone, on the other hand, is also an inhibitor for free radical polymerizations. Because it is strongly basic, it reacts with cations, however, so that it is impossible to employ this additive to distinguish between free radical and cationic polymerizations. 

The free-radical addition of TFE to pentafluoroethyl iodide yields a mixture of perfluoroalkyl iodides with even-numbered fluorinated carbon chains. This is the process used to commercially manufacture the initial raw material for the fluorotelomer -based family of fluorinated substances (Fig. 3) [2, 17]. Telomeri-zation may also be used to make terminal iso- or methyl branched and/or odd number fluorinated carbon perfluoroalkyl iodides as well [2]. The process of TFE telomerization can be manipulated by controlling the process variables, reactant ratios, catalysts, etc. to obtain the desired mixture of perfluoroalkyl iodides, which can be further purified by distillation. While perfluoroalkyl iodides can be directly hydrolyzed to perfluoroalkyl carboxylate salts [29, 30], the addition of ethylene gives a more versatile synthesis intermediate, fluorotelomer iodides. These primary alkyl iodides can be transformed to alcohols, sulfonyl chlorides, olefins, thiols, (meth)acrylates, and from these into many types of fluorinated surfactants [3] (Fig. 3). The fluorotelomer-based fluorinated surfactants range includes noiuonics, anionics, cationics, amphoterics, and polymeric amphophiles. 

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)... 

In addition polymerization, simple molecules or monomers are added to each other to form long-chain molecules (polymers) without by-products, thus yielding a polymeric product in which the molecular formula of the repeating unit is identical to that of the monomer. The molecular weight of the polymer so formed is thus the total of the molecular weights of all of the combined monomer units. There are three commonly used types of addition polymerization free-radical polymerization, cationic polymerization, and anionic polymerization, which are described below. An example of addition polymerization is given by ... 

The reactive species R may be a free radical, a cation, or an anion, which adds to the unsaturated monomer molecule by opening the n bond and simultaneonsly regenerating a reactive center of the same type. The new reactive center then adds to another monomer molecule, M, and the process is repeated in quick succession leading to addition of many more monomer molecules to the same chain, the reactive center being always shifted to the end of the growing chain with each addition. The chain propagation process can thns be represented by... 

In conclusion, intramolecular free radical addition may be a useful method of synthesizing bridged cyclic compounds. From the examples of the Cy5/Cy6 case noted in this section, it may be concluded that a large preference for the (Cy 5) radical formation again exists and that, in some cases, a very high stereoselectivity in the last transfer step may be observed. The same stereoselectivity is often observed in cationic-induced cyclizations. 

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