Termination anions

Basic zirconium sulfate is also considered to be strands of [Zr(OH)2] joined by bridging sulfates (208). The resulting formula is Zr(OH) - (OH) (SO 3 -nH,0, wheie h are bridging anions and t are terminal anions. 

The addition of living poly(styrene) to AIBN leads finally, especially for high coupling efficiencies, to the elimination of one nitrile group [72]. More recently, Ren et al. [73] have used bis(2-chloroethyl)2,2 -azodiisobu-tyrate (see scheme 19) to terminate anionically initiated poly(butadiene) chains. Since the azo transfer agent possesses two functional groups (Cl) that are able to termi-... 

Tung et al21> have reported on the use of a polymeric thiol transfer agent for use in block copolymer production. Various methods have been used for the anion thiol conversion. Near quantitative yields of thiol arc reported to have been obtained by terminating anionic polymerization with ethylene sulfide and derivatives (Scheme 7.27). Transfer constants for the polymeric thiols are reported to be similar to those of analogous low molecular weight compounds.273... 

Richards et at. carried out extensive studies on the use of mercury,2 6 277 lead278 279 and silver compounds to terminate anionic polymerization and form polymeric organometallic species which can be used to initiate polymerization. 

Lastly, another type of reaction which also belongs to the general type al is the synthesis of polymers with azo groups along their main chain by terminating anionic polymerizations with AIBN 68>69> ... 

Many more substances are capable to terminate anionic polymerization, although they do not act as proton donors. We will postpone, however, the discussion of their action until we had a chance to consider polymerization systems which do not involve any termination. 

C. C. Cummins, Terminal, Anionic Carbide, Nitride, and Phosphide Transition-Metal Complexes as Synthetic Entries to Low-Coordinate Phosphorus Derivatives, Angew. Chem. Int. Ed. 45, 862-870 (2006). 

Fig. 2.24. Tc dependence in the j6-type (ET)2X salts with hnear, triatomic anions vs anion length (sum of bond length and twice the van der Wrteils radius of the terminal anion). Prom [126]...

Termination. Anionic polymerization has no termination associated with it in the time scale of the polymerization reaction. For this reason, anionic polymerization is sometimes called living polymerization. As a result, if the starting reagents are pure and if the polymerization is moisture- and oxygen-free, propagation can proceed until all monomer is consumed. In this case, termination occurs only by the deliberate introduction of oxygen, carbon dioxide, methanol or water as follows ... 

A novel transformation reaction of living poly(tetrahydrofuran) from cationic into anionic propagation species has been published. This species was formed by end-capping of living poly(THF) with potassium iodide followed by the reduction with bis(pentamethylcyclopentadienyl)samarium (Cp 2Sm). The formed terminal anionic carbanion is... 

It should be noted that such reactions can only occur if the anion formed in the first stage is sufficiently nucleophilhc to initiate polymerization of the second monomer for example, subsequent addition of styrene to a methacrylate terminated anion can not be achieved. For instance, an equimolar mixture of styrene and methyl methacrylate produces almost pure poly(methyl methacrylate). In fact it is generally the case that methyl methacrylate and related systems cannot copolymerize with non-polar monomers. ... 

Analogous reactions with the monodentate maleato dianion, however, do not occur and this is not surprising organic chemistry since the terminal anion should deactivate the olefin to nucleophilic attack. It became interesting, therefore, to examine the chelated maleato complex where both carboxylate ions are bound to the metal center and where both should have some ester-like quality. 

Fig. R. Sqjaration of the model mixture containing formate (Fo), phosphonoformate (PFo), phosphono-aoetate (PAc), phosphate (P) and acetate (Ac) in the amounts 12, 8, 8, 10 and 18 pmol, respectively. Leading electrolyte 0.01 M HCl + 0.019 M urotro-pine -1- 0.003 M CaCl terminator 0.015 M glutamic add, leading anion (L) Cl , and terminating anion (T) glutamate...

Fig. 22. Diagram of migration of MY" complex. The leading and the terminating anions are denoted by L and T", respectively. For simplification, the common counter-ion from the leading electrolyte is not indicated in the zones. and Y denote free cation and free ligand anion, respectively, formed by dissociation of complex MY M -b Y".

Grigg and coworkers also pioneered the work on cyclization-anion capture reactions using hydride-, [45] carbonyl- [46] or cyanide- [47] capture (Scheme 6.10). The terminating anion capture is, however, not limited to these three examples in fact, the organopalladium(n) intermediate can be captured by a wide variety of anionic, neutral and organometallic agents [47]. 

A living polymerization would require 100 gm MMA and 1 mmole DPHL to obtain a 100,000 molecular weight PMMA. Methacrylate monomers usually contain impurity levels above 1% and below 5%. The major impurity in MMA is methanol which rapidly terminates anionic polymerizations. Assuming 1% methanol content, every gram MMA added would contain 0.01 gm methanol, or on a molar basis, 0.3 mmoles. Thus, after only 3 g MMA added (out of 100 g total) 90% of the chains would be terminated. After one more gram MMA is added, the... 

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