Propagation reactions group transfer

Different methods for determination of the number of active centers during catalytic olefin polymerization are proposed. There are two basic groups of method applied to determine the kinetic characteristics of propagation reactions and transfer reactions of polymer chains (values of Cp, kp, and K ) for catalytic olefin polymerization. 

Formation of block polymers is not limited to hydrocarbon monomers only. For example, living polystyrene initiates polymerization of methyl methacrylate and a block polymer of polystyrene and of polymethyl methacrylate results.34 However, methyl methacrylate represents a class of monomers which may be named a suicide monomer. Its polymerization can be initiated by carbanions or by an electron transfer process, the propagation reaction is rapid but eventually termination takes place. Presumably, the reactive carbanion interacts with the methyl group of the ester according to the following reaction... 

This means that the isopropyl group stabilises the secondary ion sufficiently (compared to but-l-ene) for hydride transfer reactions to be suppressed, at least at low temperature. At about -120 °C a high polymer of structure (IV) is formed. This is a true phantom polymer, since there exists no corresponding monomer. Evidently, at the very low temperature the propagation reaction which would lead to structure (III) becomes much slower than the isomerization reaction ... 

Most of the methods for synthesizing block copolymers were described previously. Block copolymers are obtained by step copolymerization of polymers with functional end groups capable of reacting with each other (Sec. 2-13c-2). Sequential polymerization methods by living radical, anionic, cationic, and group transfer propagation were described in Secs. 3-15b-4, 5-4a, and 7-12e. The use of telechelic polymers, coupling and transformations reactions were described in Secs. 5-4b, 5-4c, and 5-4d. A few methods not previously described are considered here. 

Hydroperoxyalkyl radicals can react in several ways. First, of course, alkylperoxy radical isomerization is reversible (Reaction —4). Secondly, several modes of decomposition (Reaction 5) occur, giving O-hetero-cycles, alkenes, and saturated and unsaturated aldehydes and ketones. (Alcohols can also be formed by the decomposition of the alkylperoxy-alkyl radicals which result from isomerization by group transfer in alkylperoxy radicals.) These modes of decomposition have been enumerated (24, 25) only examples need be given here. Each mode of decomposition gives one or more product molecules plus one free radical, acting, therefore, as a propagation step. Moreover, the radical produced... 

Propagation steps are the heart of any chain and generally fall into two classes atom or group transfer reactions and addition reactions to tr-bonds (or the reverse elimination). The rate of the chain transfer step is especially important in synthetic planning because, by fixing the maximum lifetime that radicals can exist, it determines what reactions will (or will not) be permitted. Termination steps are generally undesirable but are naturally minimized during chain reactions because initiation events are relatively uncommon. 

Alternatively, covalent propagation may proceed by a pericyclic reaction involving a multicenter rearrangement such as a group transfer polymerization no examples of these type of reactions have been reported so far. 

The research group of Shoichet [57, 59] also has demonstrated that using CO2 as a polymerization medium can improve the effectiveness of the propagation reaction over chain transfer reactions in fluoroolefin synthesis. Shoichet s... 

The fast initiation step is followed by an equally fast propagation reaction. While the rate constant of the former has been measured by Kunitake and Takarabe the dimerisation kinetics have not been measured for this particular system. The following slow reactions consist of the proton transfer between the dimeric cation and the monomer to gjve alternatively the unsaturated dimer or the indanylic one. Finally, the protonated dimer can isomerise to a more stable configuration due to the direct interaction of the two phenyl groups through space polarisation effects Thus ... 

The poly(methyl methacrylate)s prepared in this experiment, as well as polymers formed in model reactions with silanes that contain bulky substituents (such as phenyldimethyl and diphenylmethyl groups), have predominantly syndiotactic structures identical to polymers prepared by the conventional group-transfer process. This result supports a two-step dissociative mechanism for a group-transfer process, because steric hindrance from bulky silyl groups should increase the proportion of isotactic triads in the hypothetical associative concerted propagation step. 

Because of the radical mechanism for SET reactions, introduction of both a perfluoroalkyl group and a heteroatom moiety to the carbon-carbon double [17-20] and even triple [21] bonds is possible. The initially generated perfluoroalkyl radicals add first to olefins to form a new radical intermediate (23), which then couples with anions (22) to form new anion radicals (24). The formation of the product (25) and the chain propagation via electron transfer from anion radicals (24) to perfluoroalkyl halides constitutes a chain reaction as shown in Scheme 2.38. Sulfur [19], selenium [20], tellurium [21], and phosphorus [22] anions (22) have been employed for these reactions [23]. 

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