Polymerization, activation living

Zirconium bis(amides) such as (35) and (36) display moderate ethylene polymerization activities.133,134 Complex (37) containing a chelating diamide ligand has been shown to initiate the living polymerization of a-olefins such as 1-hexene (Mw/Mn= 1.05-1.08) with activities up to 750gmmol-1 h-1.135-137 The living polymerization of propylene using this system activated with... 

Based on this approach Schouten et al. [254] attached a silane-functionalized styrene derivative (4-trichlorosilylstyrene) on colloidal silica as well as on flat glass substrates and silicon wafers and added a five-fold excess BuLi to create the active surface sites for LASIP in toluene as the solvent. With THF as the reaction medium, the BuLi was found to react not only with the vinyl groups of the styrene derivative but also with the siloxane groups of the substrate. It was found that even under optimized reaction conditions, LASIP from silica and especially from flat surfaces could not be performed in a reproducible manner. Free silanol groups at the surface as well as the ever-present impurities adsorbed on silica, impaired the anionic polymerization. However, living anionic polymerization behavior was found and the polymer load increased linearly with the polymerization time. Polystyrene homopolymer brushes as well as block copolymers of poly(styrene-f)lock-MMA) and poly(styrene-block-isoprene) could be prepared. 

These observations led to the catalytic application of well-defined ruthenium alkyUdenes, some of them freely soluble and sufficiently stable in water (Scheme 7.9) although their stability was found somewhat less in aqueous solutions than in methanol [21,27,28], With these catalysts a real living ROMP of water-soluble monomers could be achieved, i.e. addition of a suitable monomer to a final solution of a quantitative reaction resulted in further polymerization activity of the catalyst [28], This is particularly important in the preparation of block copolymers. 

Temperature effects on the polymerization activity and MWD of polypropylene have been examined in the range of —78 °C to 3 °C 82 The MWD of polypropylene obtained at temperatures below —65 °C was close to a Poisson distribution, while the MWD at higher temperatures above—48 °C became broader (Slw/IWIii = 1.5-2.3). At higher temperatures the polymerization rate gradually decreased during the polymerization, indicating the existence of a termination reaction with deactivation of active centers. It has been concluded that a living polymerization of propylene takes place only at temperatures below —65 °C. 

As a complement to the stable metallacyclobutane catalysts, a series of stable alkylidene catalysts have been prepared and shown to be active living polymerization catalysts. The complex W(CHt-Bu)(NAr)(Ot-Bu)2 [47,48] (Ar = 2,6-diisopro-pylphenyl) (35) was reacted with 50-200 equivalents of norbornene in toluene at 25 °C, followed by end-capping with benzaldehyde, yielding polymers in which the major component has a molecular weight proportional to the number of equivalents of norbornene consumed, with dispersities of approximately 1.05,... 

Now one of two things can occur. A chain terminator can be added (e.g., a small amount of an alcohol) from which each chain will abstract a hydrogen ion and become a neutral polymer (a dead chain). This is the termination step shown in the preceding reaction sequence. Or, more monomer can be added and the polymerization will continue until the new sample of monomer has been consumed. In other words, these anionic polymerizations are living polymerizations, so named because the chains remain active until they are deliberately terminated (become dead ). (The terms living and dead describe relative states of chemical reactivity only and not any bio-... 

Examples of preparation of copolymers are scarce. Mun et al. [81, 82] showed that the binary system of cobaltocene/ bis(ethylacetoacetato) copper (II) effectively initiates the living radical polymerizaton of MMA at 25 °C in acetonitrile. The polymerization activity of this initiator system was markedly affected by the solvent used. The synthesis of PMMA-b-PS copolymers with molecular weights reaching 700000 was successfully attempted by adding styrene to the living PMMA. The yield of the copolymers reached 80% when the MMA polymerization was carried out for three days. The same team [91] also synthesized PS-b-PMMA copolymers from the polymerization of MMA with polystyrene obtained in the presence of reduced nickel/halide systems. The yields range from 84 to 91% depending on the halide complex used. 

Under certain stringent conditions, all ion reactivity is concentrated on propagation. The resulting polymerizations are living. In other cases, the instability of the ions is manifested by a complex of poorly defined reactions leading to transfers, retardation, and decay of the polymerizing activity of the centres, i.e. termination. Termination is either an inherent feature of the respective polymerizing system or it may be caused by accidental impurities or, finally, it may be a consequence of deliberately added compounds. 

Although ATRP behaves differently from conventional free radical polymerization, the fundamental reactions involved are very similar and include initiation, propagation, transfer and termination (see Scheme 6). Since chain termination does not occur in a truly living polymerization, the living character of the chains in ATRP derives from the fact that chain propagation is first order with respect to radical concentration and irreversible bi-molecular termination is second order. As such, the concentration of the radicals is kept very low, the rate of bi-molecular termination is greatly reduced, and typically less than 10% of all of the chains will terminate. Unlike conventional free radical polymerization, where the rate is dictated by a steady state between the initiation and termination rates, the rate and concentration of propagating radicals in ATRP is controlled entirely by the equilibrium between activation and deactivation [255]. 

This procedure is carried out in exactly the same way as described in Protocol 2. However, in this approach, once complete conversion ofthe initial monomer feed has been attained (as determined by GPC analysis) a second monomer is added to the reactor flask. Since the polymerization exhibits living characteristics, active polymer chain ends still exist in the monomer starved flask and further addition of another monomer enables the polymerization to continue, therefore producing an AB block copolymer. 

First example of living pol3Tnerization was discovered by Michael Szwarc (f909-2000) in 1956 in the anionic polymerization of styrene in special cat-al3dic system. In this type of polymerization active chain end is negatively charged which prevents most of termination processes. 

A new technique was developed recently, by introducing cationic to anionic transformation. A living carbocationic polymerization of isobutylene is carried out first. After it is complete, the ends of the chains are transformed quantitatively to polymerization-active anions. The additional blocks are then built by an anionic polymerization. A triblock polymer of poly(methyl methacrylate)-polyisobutylene-poly(methyl methacrylate) can thus be formed. The transformation involves several steps. In the first, a compound like toluene is Friedel-Craft alkylated by a,6t>"di-rerr-chloro-polyisobutylene. The ditolylpolyisobutylene which forms is lithiated in step two to form a,cu-dibenzyllithium polyisobutylene. It is then reacted with 1,1-diphenylethylene to give the corresponding dianion. After cooling to -78 °C and dilution, methyl methacrylate monomer is introduced for the second polymerization in step three. 

The end of the new block in such block polymerizations is still living, that is, further monomer can be added on. Living polymers and block polymers can, however, be killed by isomerization or by the purposeful addition of suitable reagents. If the new species thereby formed cannot start further polymerization, these are called termination reactions. If, however, the new species can initiate polymerization, then transfer reactions are involved. Thus, differentiation between transfer and termination reactions is on the basis of polymerization activity, not, however, on the basis of the mechanism, since something is transferred in many termination reactions. 

Since anionic polymerization provides for the initiation of all chains to occur at the same time, there can be no new chains started during the course of polymerization, as would be the case in free-radical polymerization. This means that the precipitated system is set and that chain growth must occur by Incorporation of monomer at the particle. If polymerization is to occur at any reasonable rate, then the active living" ends of the chains must be accessible to the monomer. The data in Table 5 demonstrates that the chain ends are readily accessible to the monomer. In this experiment, trimethylchlorosilane was used to terminate the reaction. The amount of silicon measured was compared to that expected if all the carbanion chain ends were converted to the silane derivative. Based upon the close agreement between the measured and theoretical silicon values, it appears that the carbanions in the particle are readily accessible up to 85% conversion. 

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