Hydrogen chain transfer reaction

In a recent development, a new process of preparing borane-terminated isotactic polypropylene (z -PPs) via an in situ chain-transfer reaction was achieved by a styrene/hydrogen consecutive chain-transfer reagent, which avoids the use of a B—H containing chain-transfer agent.74 This has resulted in the utilization of milder polymerization conditions due to the use of the alkylaluminoxane cocatalyst (MAO) (50) (Fig. 33), which cannot normally be used in the presence of a B—H chain-transferring... 

Radiochemical experiments show that the number of polymer chains terminated by allyl groups are a minor fraction of the total, and the majority of chains derive from the realkylated hydrides. In the presence of hydrogen it is evident from Table III that chain transfer reactions dominate and some saturated polymer is formed ... 

Intramolecular hydrogen abstraction by primary alkyl radicals from the Si-H moiety has been reported as a key step in several unimolecular chain transfer reactions.59,60 In particular, the 1,5-hydrogen transfer of radicals 14-17 [Eq. (5)], generated from the corresponding iodides, was studied in... 

Grafting of styrene (ST) onto polybutadiene (PB) can occur in two ways Via a chain-transfer reaction with an allylic hydrogen of the 1,4- and the 1,2-units (Case 1) via copolymerization with C=C-double bounds of polybutadiene, in particular with the vinyl groups of the 1,2-units (Case 2) ... 

If hydrogen abstraction from the zinc salt is fast enough to compete with that from the hydrocarbon substrate, then Scheme 2 amounts to a chain-transfer reaction similar to that illustrated in Reaction 9 for phenolic inhibition. Also, an alternative termination via the peroxyalkyl-ester radical (X) is conceivable since this radical might be expected to cyclize... 

The chain transfer reaction played an important role, particularly because of abstraction of the active hydrogen at a-carbon from the allyl group. Moreover, unreacted double bonds were present in the copolymer obtained. The influence of chain transfer reaction could be diminished by applying multimonomers which do not contain allyl groups. This was shown in the example of copolymerization of multimethacrylate prepared by esterification of poly(2-hydroxyethyl methacrylate) with methacryloyl chloride. Copolymerization of the multimethacrylate with vinyl monomers such as styrene or acrylonitrile can be represented by the reaction ... 

It has been known for some time [see Ref. (176) for earlier work] that if poly(vinyl alcohol), produced by hydrolysis of poly(vinyl acetate) is reacetylated, the PVAc so obtained has a lower MW than the original PVAc prior to hydrolysis, though the MW of the material is not lowered any further by subsequent cycles of hydrolysis and reacetylation. Various explanations had been advanced for this phenomenon Wheeler explained it as a consequence of the presence of branches joined to the main chain through ester linkages which would be broken on hydrolysis and not re-formed on reacetylation. These branches were ascribed to chain transfer reactions with acetate groups, either in the polymer, or in monomer molecule subsequently polymerized at their double bonds. Transfer reactions by attack on hydrogen atoms other than those in... 

Fluorenyl Carbanions. Salts of fiuorene (pAa = 22.6) (6) are more hindered and less reactive than many other organometallic initiators. These carbanions can be readily formed by reaction with alkali metal derivatives as shown in equation 19 for 9-methylfiuorene (99). Carbanion salts of 9-methylfiuorene are preferable to fiuorene, since the latter generate chain ends which retain reactive, acidic fluorenyl hydrogens which can participate in chain-transfer reactions (100,101). Fluorenyl salts are useful initiators for the polymerization of alkyl methacrylates, epoxide, and thiirane monomers. 

Watson et al.124-1261 studied the polymerization of ethylene and propylene with Lu(n5-C5Me5)2(CH3) ether in toluene or cyclohexane at 30-80 °C. The Lu complex produced polymers of Mn = 10M04 for ethylene, and oligomers for propylene. In the oligomerization of propylene an unusual chain transfer reaction due to 0-alkyl elimination was found together with P-hydrogen elimination from Lu-alkyls as chain-terminating processes 125). 

A termination frequently encountered in many polymerizations results from a chain transfer process. In a radical polymerization such a reaction involves usually a transfer of a hydrogen atom and yields a radical which may or may not initiate further polymerization. The first alternative may be referred to as a proper chain transfer reaction, and such a transferring agent is known as a polymerization modifier. The second alternative is known as an inhibition or retardation of polymerization, the inhibitor or retarder being a substance which forms a stable radical, not sufficiently reactive in respect to the monomer, and therefore unable to initiate further polymerization. 

Another reaction type associated with such reactions is chain-transfer (or hydrogen abstraction) (14.2). 

Where no specific chain transfer agent has been added to the polymerisation system, three chain transfer reactions are usually considered transfer with the monomer [scheme (36)], transfer with the trialkylaluminium activator [scheme (37)] and spontaneous transfer [scheme (38)]. The transfer with the monomer and the spontaneous transfer involve a reaction of /1-hydrogen elimination from the growing polymer chain, whereas the transfer with the activator is the exchange reaction of substituents at two metals [240,241]. 

The chain transfer reaction with hydrogen is as follows ... 

Occasional regioerrors appear significantly to inhibit the polymerisation of a-olefins by methylaluminoxane-activated metallocene catalysts [114, 138, 253— 261], In order to reduce the number of secondary Zr-CH(R)-CH2 species, and therefore to accelerate the polymerisation, advantage has been taken of the chain transfer reaction with hydrogen ... 

Hydrogen chain transfer reaction, which may occur as intermolecular or intramolecular processes, leads to the formation of oleflnic species and polymeric fragments. Moreover, secondary radicals can also be formed from hydrogen abstraction through an intermolecular transfer reaction between a primary radical and a polymeric fragment. 

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