Propagating ethylene radical

In these equations I is the initiator and I- is the radical intermediate, M is a vinyl monomer, I—M- is an initial monomer radical, I—M M- is a propagating polymer radical, and and are polymer end groups that result from termination by disproportionation. Common vinyl monomers that can be homo-or copolymeri2ed by radical initiation include ethylene, butadiene, styrene, vinyl chloride, vinyl acetate, acrylic and methacrylic acid esters, acrylonitrile, A/-vinylirnida2ole, A/-vinyl-2-pyrrohdinone, and others (2). 

Branching occurs in some. free-radical polymerization of monomers like ethylene, vinyl chloride, and vinyl acetate in which the propagating polymer radical is very reactive and can lead to branching by chain transfer to the backbone chain of another polymer molecule or onto its own chain (see Chapter 6). 

It was confirmed that the fracture of the ethylene monomers at 77 K produced no free radicals. The quintet ESR spectrum shown in Fig. 7.24 can be undoubtedly attributed to the propagating radical of polyethylene, —CpHp2 (Ca ) H 2, when both the polymers and the monomers are simultaneously fractured. The quintet is due to hyperfine splitting of two a- and two fi-hydrogen nuclei. No trace of the Pl FE radical was detected in the observed spectrum. Accordingly the polymerization of ethylene, which was proved by ESR, had been initiated not by the ethylene radicals but by the PTFE mechano radicals at as low a temperature as 77 K. This extremely high reactivity of the radicals is rather surprising because both PTFE and ethylene react in the solid state at 77 K. The mechano radicals newly created by the chain scission are surrounded by the monomer molecules because the radicals are trapped in the fresh surface formed by the mechanical destruction. 

The several chain reaction sequences propagated by methyl radicals follow a common pattern, with initiation by abstraction or addition, and propagation by radical decomposition, but there are notable differences. Thus with ethane, only abstraction is possible, but with ethylene both abstraction and addition sequences occur with acetylene the abstraction sequence is blocked by the stability of the CoH radical so that only addition is important, while with propylene, abstraction of the allylic hydrogen is apparently much faster than addition, and the former predominates. Finally, it should be noted that initially there is no radical-chain decomposition of methane further, that the chain sequences initiated by abstraction do not consume methane, and only with the addition sequences, beginning with ethylene, is methane consumed even then it is a limited chain, limited by the amount of olefin present. 

Copolymers of VF and a wide variety of other monomers have been prepared (9,81,82,94-98). The high energy of the propagating VF radical strongly influences the course of these polymerizations. VF incorporates well with other monomers that do not produce stable free radicals, such as ethylene and vinyl acetate, but is sparingly incorporated with more stable radicals such as acrylonitrile [107-13-1] and vinyl chloride. An Alfrey-Price Q value of 0.010 0.005 and an e value of 0.8 0.2 have been determined (99). The low value of Q is consistent with little resonance stability and the e value is suggestive of an electron-rich monomer. 

The cis-1,2 (6, Scheme 16.2) polymerization occurs when the 1-2 double bond is opened by reaction with a propagating free radical, leaving a pendant ethylene group and an allylic chlorine. This structure accounts for approximately 2% of the polymer units at 40 °C. 

Ethylene oxide is a coproduct, probably formed by the reaction of ethylene and HOO (124—126). Chain branching also occurs through further oxidation of ethylene hydroxyl radicals are the main chain centers of propagation (127). 

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 ... 

I Propagation Polymerization occurs when the carbon radical formed in the initiation step adds to another ethylene molecule to yield another radical. 

Thus propagating radicals were initially proposed to add reversibly to dipbeny[ethylene as shown in Scheme 9.14.101... 

The carbon atom with the unpaired eiectron is another free radical, so the stage is set for propagation. The unpaired eiectron on the carbon atom attacks the n bond of another molecule of ethylene, making a new C—C a bond and ieaving yet another carbon free radical ... 

Monomers, such as ethylene, propylene, isobutylene, and isoprene, containing the carbon-carbon double bond undergo chain polymerization. Polymerization is initiated by radical, anionic or cationic catalysts (initiators) depending on the monomer. Polymerization involves addition of the initiating species R, whether a radical, cation, or anion, to the double bond followed by its propagation by subsequent additions of monomer... 

Radical Polymerization. Radical chain polymerization involves initiation, propagation, and termination. Consider the polymerization of ethylene. Initiation typically involves thermal homolysis of an initiator such as benzoyl peroxide... 

Chain propagation in oxidized 1,2-substituted ethylenes proceeds via addition of dioxygen followed by the elimination of the hydroperoxyl radical [156] ... 

Propagation Now this radical is made to react with the ethylene in a very rapid process to yield a Polymer chain radical as given below ... 

The low tendency of 1,2-disubstituted ethylenes to polymerize is due to kinetic considerations superimposed on the thermodynamic factor. The approach of the propagating radical to a monomer molecule is sterically hindered. The propagation step is extremely slow because of steric interactions between the P-substituent of the propagating species and the two substituents of the incoming monomer molecule ... 

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