Cationic coordination polymerization chain transfer

ROP of p-lactones is highly prone to numerous side reactions, such as transester-fication, chain-transfer or multiple hydrogen transfer reactions (proton or hydride). Specifically, the latter often causes unwanted functionalities such as crotonate and results in loss over molecular weight control. Above all, backbiting decreases chain length, yielding macrocyclic structures. All these undesired influences are dependent on the reaction conditions such as applied initiator or catalyst, temperature, solvent, or concentration. The easiest way to suppress these side reactions is the coordination of the reactive group to a Lewis acid in conjunction with mild conditions [71]. p-BL can be polymerized cationically and enzymatically but, due to the mentioned facts, the coordinative insertion mechanism is the most favorable. Whereas cationic and enzymatic mechanisms share common mechanistic characteristics, the latter method offers not only the possibility to influence... 

On the basis of the nature of the initiation step, polymerization reactions of unsaturated hydrocarbons can be classified as cationic, anionic, and free-radical polymerization. Ziegler-Natta or coordination polymerization, though, which may be considered as an anionic polymerization, usually is treated separately. The further steps of the polymerization process (propagation, chain transfer, termination) similarly are characteristic of each type of polymerization. Since most unsaturated hydrocarbons capable of polymerization are of the structure of CH2=CHR, vinyl polymerization as a general term is often used. 

The use of late transition metals as olefin polymerization catalyst requires the suppression of chain transfer while at the same time a high chain growth rate should be maintained. These new catalysts have an electron-deficient, in most cases, 14-electron and cationic metal center with a vacant coordination site. The most... 

Resulting poly(a-hydroxyacids) are important biomaterials used as resorbable sutures and prostheses [196]. The mechanism of polymerization is not well established. Polymerization may be initiated with Lewis acids (SbF3, ZnCl2, SnCl4) however, other typical cationic initiators (e.g, triethyloxonium or triphenylcarbenium salts) fail to initiate polymerization [197]. Thus, it is not clear whether polymerization proceeds by typical cationic mechanism or rather involves the coordination mechanism. The chain transfer to polymer resulting in transesterification was postulated [198,199] and confirmed later by detailed, 3C NMR studies of lactide copolymers [200]. 

This group usually leads to anionic or coordinate polymerization which are not covered by this review. Nevertheless, polymerizations of oxetanes and THF, known to proceed exclusively by a cationic mechanism, have also been induced by various organometallic initiators. In many cases, these initiators lead to higher molecular weight polymers, probably reacting fast and first with impurities that could, if not destroyed, lower the molecular weight by chain transfer. 

Chain polymerization involves three processes chain initiation, chain propagation, and chain termination. (A fourth process, chain transfer, may also be involved, but it may be regarded as a combination of chain termination and chain initiation.) Chain initiation occurs by an attack on the monomer molecule by a free radical, a cation, or an anion accordingly, the chain polymerization processes are called free-radical polymerization, cationic polymerization, or anionic polymerization. (In coordination addition or chain polymerization, described below separately, the chain initiation step is. 

Various catalytic systems both of ion [240] and ion-coordinated [174, 245] types were proposed for piperylene polymerization. Polymer with low MM is formed under cationic polymerization due to the high rate of chain transfer reaction on monomer (it rises when catalysts acidity increases) and solvent (it reduces with the increase of polarity of solvent). Some halogenides of metals of Ill-V groups were tested as catalysts of cationic polymerization. TiCU and SnCU turned to be the most useful. Application of SbCls and InCU doesn t provide acceptable polymerization rate, and in the case of AICI3 insoluble polymer was formed [246]. 

The next step in both MAO-activated and MAO-free systems is believed to involve the transfer of an alkylide group to a Lewis acid (eqs. 8 and 9) or a proton (eq. 10), which creates an electron-deficient (14 e ) metal center with a site for the potential coordination of an olefin. The nature of the counterion strongly affects the abihty of the site to polymerize olefins, and indeed may change the active-site geometry enough to drastically change the selectivity of polymerization as well as the chain-transfer rate. Cation-anion interactions have been reviewed by Bochmann (125). The reactivity of anion-free cationic metallocenes has been probed by ion-cyclotron resonance (ICR) spectroscopy (126). 

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