Radical chain polymerization initiation

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

Addition polymerization involves three steps initiation, propagation, and termination. During initiation, either radicals (Figure 5.9) or ionic species are generated from the controlled decomposition of an initiator molecule. The reactive intermediates are then sequentially added to the C—C bonds of monomers to propagate the growing polymer chain. Free-radical polymerization is the most common method currently used to synthesize polymers from vinyl-based monomers. 

Side reactions such as termination and transfer were investigated in the polymerizations of styrene,291 acrylates,292 and methacrylates.293 The occurrence of thermally initiated radical polymerizations was observed in the copper-catalyzed styrene polymerization, while the resulting polymer chain can be converted into the dormant polymer terminal via abstraction of halogens from the persistent metal radical in higher oxidation states.294... 

Another example is the copper-catalyzed surface-initiated radical polymerization of MMA from S-7 at room temperature without addition of free initiator. The molecular weights and MWDs of the polymers were directly measured after removing the brushes from the surface. For example, the surface with 40 nm thickness had Mn of 68900 and MWDs of 1.45. A high graft density (180 A2/chain) and decreased surface roughens (0.54 nm) were observed. This method is free from solution and thermal polymerizations due to the absence of free initiators and a low polymerization temperature, which permits a simple washing step without Soxhlet extraction. 

The most common water-soluble initiator systems are based on persulfate or hydrogen peroxides. The former is most easily handled, the latter group is, without special precautions, often subject to unwanted side reactions with formation of oxygen which acts as inhibitor, probably through formation of poly(vinyl chloride) peroxides. The inhibition period is proportional to the square root of the amount of oxygen [32]. In the presence of bases, such as a phosphate buffra, the polymeric peroxides are subject to a base-catalysed decomposition to chain-initiating radicals [33]. 

The dissociation of BPO during the polymerization reactions yields both benzoyloxy and phenyl radicals, both of which initiate radical polymerization. Furthermore, chain transfer from polymeric radicals to solvent (toluene) can... 

Iniferter polymerizations were also combined with anionic polymerization. The representative example involves the synthesis of PCL-l7-(PMMA-co-PSt)-l7-PCL. ° A polymeric thermal iniferter, PCL-substituted tetraphenylethane, was prepared by anionic polymerization of CL in the presence of aluminum triisopropoxide and benzopinacol. The benzopinacolate groups incorporated into the polymer chain initiated the polymerization of St and MMA via a controlled radical mechanism at 95 °C to yield the desired block copolymers (Scheme 47)... 

The alkoxy radicals are very reactive and initiate radical polymerizations readily On the other hand, the borinate radicals are stabilized by the empty p-orbitals of boron through back-donating electron density and are too stable to initiate polymerizations. During the polymerization, the borinate radicals may form weak and reversible bonds with the growing chains. 

To be more quantitative with the rates of these processes, the rate expressions for a thermally initiated radical polymerization (assuming all the radicals generated during initiator decomposition form growing chains) are given in eqns [5]-[7] ... 

Polymer brushes can be grafted onto the pore wall of porous AAO membranes to improve the binding capacity, stability, and selectivity of the membranes. To initiate radical polymerization on the wall, surfaces need to be modified with an appropriate initiator or chain-transfer agent. Generally, a chlorosilane- or alkoxysilane-modifled initiator/chain-transfer agent is used however, the use of silanes requires anhydrous reaction conditions, which can be restrictive in some cases. Therefore, Klok and coworkers utilized catechol as an alternative anchoring moiety for surface-initiated polymerization [153]. The advantage of catechol-based initiators is that they can be immobilized in aqueous solutimi, without the need for... 

Cu(II)-polymer complexes initiate radical polymerization and often show higher activity than the corresponding monomeric analogues. The systems of Cu(II) ion with nylon [97], a-aminocaproic acid [96], nylon oligomers [96], polyethylene-polyamine [98,99], pt)ly(vinylamine-co-vinyl alcohol) [100], and cellulose have been made by Imoto and Takemoto as initiation systems for free radical polymerization of vinyl monomers, such as methyl methacrylate (MMA). Inaki et al. reported that Cu(II)-polyvinylamine complexes in the presence of CCI4 in an aqueous solution showed higher activity as an initiator than Cu(II)-diaminopropane complex for the radical polymerization of methyl methacrylate, acrylonitrile, and styrene [101]. The pH dependence for activity indicates that the free amine groups on the poly(vinylamine) chain are involved in the catalysis. The initiation mechanism is proposed as follows [Eqs. (78-80)] ... 

The mechanism of free radical polymerization of ethylene is outlined m Figure 6 17 Dissociation of a peroxide initiates the process m step 1 The resulting per oxy radical adds to the carbon-carbon double bond m step 2 giving a new radical which then adds to a second molecule of ethylene m step 3 The carbon-carbon bond forming process m step 3 can be repeated thousands of times to give long carbon chains... 

The molecular weight distribution for a polymer like that described above is remarkably narrow compared to free-radical polymerization or even to ionic polymerization in which transfer or termination occurs. The sharpness arises from the nearly simultaneous initiation of all chains and the fact that all active centers grow as long as monomer is present. The following steps outline a quantitative treatment of this effect ... 

Despite numerous efforts, there is no generally accepted theory explaining the causes of stereoregulation in acryflc and methacryflc anionic polymerizations. Complex formation with the cation of the initiator (146) and enoflzation of the active chain end are among the more popular hypotheses (147). Unlike free-radical polymerizations, copolymerizations between acrylates and methacrylates are not observed in anionic polymerizations however, good copolymerizations within each class are reported (148). 

Fig. 3. Polymerization initiation and propagation by radiation-generated free radicals. A is the initiating radical produced by irradiating the Hquid coating. (1) represents the Hquid monomer—unsaturated polymer reactive coating system. R is functional. (2) is the growing polymer chain (free radical). The cured...

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