Alkene polymerization chain transfer

The corresponding reactions of transient Co(OEP)H with alkyl halides and epoxides in DMF has been proposed to proceed by an ionic rather than a radical mechanism, with loss of from Co(OEP)H to give [Co(TAP), and products arising from nucleophilic attack on the substrates. " " Overall, a general kinetic model for the reaction of cobalt porphyrins with alkenes under free radical conditions has been developed." Cobalt porphyrin hydride complexes are also important as intermediates in the cobalt porphyrin-catalyzed chain transfer polymerization of alkenes (see below). 

This is clo.sely related to the Tertiary radical synthesis" scheme for the preparation of organocobalt porphyrins, in which alkenes insert into the Co—H bond of Co(Por)H instead of creating a new radical as in Eq. (13). If the alkene would form a tertiary cobalt alkyl then polymerization rather than cobalt-alkyl formation is observed. " " " The kinetics for this process have been investigated in detail, in part by competition studies involving two different alkenes. This mimics the chain transfer catalysis process, where two alkenes (monomer and oligomers or... 

Mixtures of alkyl halides and Lewis acids are well-known initiating systems for the polymerizations of alkenes, and the mechanism suggested for these reactions by Kennedy [55] appears to be generally accepted (Scheme 9), although the importance of the chain transfer step from initiator has been questioned [56]. 

The predominant chain termination mechanisms are (i) /3-hydride elimination see Polymerization), giving a chain with a terminal alkene (ii) reaction with hydrogen, frequently used to control chain length (iii) chain transfer to monomer by <7-bond metathesis see a-Bond Metathesis)-, and (iv) chain transfer to cocatalyst (equation 4). 

In an extreme case—if the alkene is of low reactivity and the transfer agent of high reactivity—chain transfer is so effective that there is no polymerization. Then... 

Scheme 11.18 shows a reasonable mechanism for polymerization that consists of first complexation of alkene, followed by 1,2-insertion. Ideally, the two steps will continue until synthesis of high-molecular-weight polymer occurs. Chain growth will cease if chain transfer occurs. Scheme 11.19 indicates two possible mechanisms for chain transfer, both of which are reasonable and indistinguishable by experiment. In both cases, the steric bulk of the diimine ligand seems to hinder the transfer process.102... 

Mixtures of Ni and Co salts with organoaluminum reagents catalyze the polymerization of 1,3-butadiene, whereas these catalysts are not effective for alkene polymerization due to facile chain transfer by /1-hydrogen elimination of the growing polymer. The polymerization of 1,3-butadiene usually proceeds via the intermediate n-allyl complexes, which are more stable than the alkyl metal complexes and hardly cause any /1-hydrogen elimination [56]. The late transition metal compounds polymerize 1,3-butadiene even in... 

Several chain transfer to polymer reactions are possible in cationic polymerization. Transfer to cationic propagating center can occur either by electrophilic aromatic substitution (as in the polymerization of styrene as well as other aromatic monomers) or hydride transfer. Short chain branching found in the polymerizations of 1-alkenes such as propylene may be attributed to intermolecular hydride transfer to polynier. The propagating carbocations are reactive secondary carbocations that can abstract tertiary hydrogens from the polymer ... 

Table 9.4 Chain Transfer Constants (ktr/kp) for Alkene Polymerization with Various Heterogeneous Ziegler-Natta Catalysts...

As previously mentioned, the molecular masses of the polymers obtained from these aqueous reactions are generally higher than desired due to the small number of active species. Although the propagating species in these polymerizations do not typically react with acyclic alkenes, modest control over molecular mass is possible when certain acyclic chain-transfer agents are employed [35, 44]. For example, Feast and Harrison have used very high concentrations of as-2-butene-l,4-diol or its dimethyl ether as chain-transfer agents [35]. The chain-transfer constants in these reactions were small, and inclusion of these alkenes in the reaction... 

The cationic polymerization of propylene, 1-butene, and higher 1-alkenes yields only very low molecular weight polymers DP < 10 - 20) with highly complicated strucmres that arise due to various combinations of 1,2-hydride and 1,2-methide shifts, proton transfer, and elimination, besides chain transfer during polymerization. In the polymerization of ethylene, initiation involving protonation and ethylation is quickly followed by energetically favorable isomerization ... 

Chain Transfer and Termination There are a variety of reactions by which a propagating cationic chain may terminate by transferring its activity. Some of these reactions are analogous to those observed in cationic polymerization of alkenes (Chapter 8). Chain transfer to polymer is a common method of chain termination. Such a reaction in cationic polymerization of tetrahydrofuran is shown as an example in Fig. 10.1. Note that the chain transfer occurs by the same type of reaction that is involved in propagation described above and it leads to regeneration of the propagating species. Therefore, the kinetic chain is not affected and the overall effect is only the broadening of MWD. 

The most important side reactions are disproportionation between the eobalt(ll) complex and the propagating species and/or -elimination of an alkene from the cobalt(TII) intennediate. Both pathways appear unimportant in the case of acrylate ester polymerizations mediated by Co"TMP but are of major importance with methacrylate esters and S. This chemistry, while precluding living polymerization, has led to the development of cobalt complexes for use in catalytic chain transfer (Section 6.2.5),... 

Many examples of such eliminations have now been seen for the f-block and for d° metals. This type of jS-alkyl elimination is recognized as an important chain transfer step in Ziegler-Natta and metallocene polymerization catalysis. When it occurs the polymer chain terminates in a C=C bond (equation 2) and in certain cases the alkene chain end can undergo reinsertion and get back into the polymer growth... 

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