Terminal double bond polymerization

Radicals are employed widely in the polymer industry, where their chain-propagaling behavior transforms vinyl monomers into polymers and copolymers. The mechanism of addition polymerization involves all three types of reactions discussed above, i.e., initiation, propagation by addition to carbon-carhon double bonds, and termination. 

Fig. 14 Accumulated weight fraction distribution development with and without terminal double bond polymerization...

Nonlinear polymer formation in emulsion polymerization is a challenging topic. Reaction mechanisms that form long-chain branching in free-radical polymerizations include chain transfer to the polymer and terminal double bond polymerization. Polymerization reactions that involve multifunctional monomers such as vinyl/divinyl copolymerization reactions are discussed separately in Sect. 4.2.2. For simplicity, in this section we assume that both the radicals and the polymer molecules that formed are distributed homogeneously inside the polymer particle. 

The development takes into account transfer to monomer, transfer to polymer, and terminal double bond polymerization. For the vinyl acetate system where transfer to monomer is high, the generation of radicals by transfer to monomer is much greater than the generation of radicals by initiation, so that essentially all radicals present have terminal double bonds hence, effectively all dead polymer molecules also have a terminal double bond. Thus, for vinyl acetate polymerization, the terminal double bond polymerization can be significant, and has been built into the development. The equations for the moments of the molecular weight distribution and the average number of branches per polymer molecule are as follows ... 

Tip 8 Terminal double bond polymerization. Transfer to monomer and termination by disproportionation lead to dead polymer molecules with a TDB. This TDB may react with a polymer radical, thus forming a radical center somewhere along the chain of the combined molecules. This radical center, on propagation with monomer, will evenmally form a trifunctionally branched chain. 

CATIONIC POLYMERIZATION OF THE CARBON-CARBON DOUBLE BOND 387 Termination occurs by either alkylation... 

The bismaleimides can be reacted with a variety of bifimctional compounds to form polymers by rearrangement reactions. These include amines, mercaptans, and aldoximes (Figure 4.22). If the reaction is carried out with a deficiency of the bifunctional compound, the polymer will have terminal double bonds to serve as a cure site for the formation of a cross-linked polymer via a double bond polymerization mechanism during molding. The cross-linked in this case occurs without the formation of any volatile by-products. 

Free-radical polymerization, like coordination polymerization discussed in Chapter 2, involves the sequential addition of vinyl monomer(s) to an active center. For FRP the active centers are free radicals. The increase in chain length is very rapid an individual chain is initiated, grows to high MW and is terminated in a few seconds or less. After termination, the high-MW polymer chain does not react further (barring side reactions such as chain transfer to polymer or terminal/internal double bond polymerization) and is considered dead . Dead chains have a residence time of minutes or hours in the reactor, such that the final polymer product is an intimate mixture of chains formed under time and/or spatially varying conditions. 

Poly((3-malic acid) is the most simple carboxylated polyester [47-49]. It is prepared by ring opening polymerization of the mono-benzyl ester (3-lactone of malic acid and subsequent debenzylation. It has been explored as drug carrier. Reaction of itaconic anhydride with PCL with hydroxy terminals results in polyesters with carboxylic and C=C double bond functional terminals, suitable for further reactions to form networks and gels, (5) and (6) [50,51]. 

In the first case (reaction (A)) this could form the initiation stage of a polymerization reaction through the double bonds of the diene polymer. In turn this reaction could involve those other reaction mechanisms associated with double bond polymerization such as chain transfer to solvent, to monomer and to polymer chain termination and inhibition by free radicals including those from antioxidants. 

Vinyl polymerization terminal double-bond polymerization, monomer concentration varies, transfer to monomer and to... 

The free-radical polymerization of methacrylic monomers follows a classical chain mechanism in which the chain-propagation step entails the head-to-taH growth of the polymeric free radical by attack on the double bond of the monomer. Chain termination can occur by either combination or disproportionation, depending on the conditions of the process (36). 

Additive Polyimides. Rhc ne-Poulenc s Kin el molding compound and Kerimid impregnating resin (115), Mitsubishi s BT Resins (116), and Toshiba s Imidaloy Resin (117) are based on bismaleimide (4) technology. Maleic anhydride reacts with a diamine to produce a diimide oligomer (7). Eurther reaction with additional diamine (Michael addition) yields polyaminohismaleimide prepolymer with terminal maleic anhydride double bonds. Cure is achieved by free-radical polymerization through the terminal double bonds. 

In anionic polymerization, as in carbonium ion polymerization, termination does not involve bimolecular reaction between two growing chains. Neither can recombination of ions lead to termination, since a carbon-metal bond is highly polar, in the case of alkali metals frequently completely ionized, and in every case very reactive. The termination step leading to the formation of a terminal C=C double bond is not too probable. This reaction involves the formation of a metal hydride, and this does not contribute greatly to the driving force. Consequently, such a termination is observed at higher temperatures only and it is probably more common in coordination polymerization where the metals involved are less electropositive. 

In the period 1910-1950 many contributed to the development of free-radical polymerization.1 The basic mechanism as we know it today (Scheme 1.1), was laid out in the 1940s and 50s.7 9 The essential features of this mechanism are initiation and propagation steps, which involve radicals adding to the less substituted end of the double bond ("tail addition"), and a termination step, which involves disproportionation or combination between two growing chains. 

Addition polymers, which are also known as chain growth polymers, make up the bulk of polymers that we encounter in everyday life. This class includes polyethylene, polypropylene, polystyrene, and polyvinyl chloride. Addition polymers are created by the sequential addition of monomers to an active site, as shown schematically in Fig. 1.7 for polyethylene. In this example, an unpaired electron, which forms the active site at the growing end of the chain, attacks the double bond of an adjacent ethylene monomer. The ethylene unit is added to the end of the chain and a free radical is regenerated. Under the right conditions, chain extension will proceed via hundreds of such steps until the supply of monomers is exhausted, the free radical is transferred to another chain, or the active site is quenched. The products of addition polymerization can have a wide range of molecular weights, the distribution of which depends on the relative rates of chain grcnvth, chain transfer, and chain termination. 

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