Living chain ends

Surprisingly, after this very first example, there was a 20 year delay in the literature in the appearance of the second report on siloxane macromonomers. However, during this period there have been numerous studies and developments in the vinyl and diene based macromonomers91 -94). The recent approach to the synthesis of siloxane macromonomers involves the lithiumtrimethylsilanolate initiated anionic polymerization of hexamethyltrisiloxane in THF 95,123). The living chain ends were then terminated by using styrene or methacrylate functional chlorosilanes as shown in Reaction Scheme X. 

Well developed is the anionic polymerization for the preparation of olefin/di-olefin - block copolymers using the techniques of living polymerization (see Sect. 3.2.1.2). One route makes use of the different reactivities of the two monomers in anionic polymerization with butyllithium as initiator. Thus, when butyl-lithium is added to a mixture of butadiene and styrene, the butadiene is first polymerized almost completely. After its consumption stryrene adds on to the living chain ends, which can be recognized by a color change from almost colorless to yellow to brown (depending on the initiator concentration). Thus, after the styrene has been used up and the chains are finally terminated, one obtains a two-block copolymer of butadiene and styrene ... 

The reaction sequences described above are simplified, since "living" chain end self-association and cross-association may further complicate the reaction. 

Figure 15. Scheme of enthalpies for the transition between the three forms of living chain ends for polystyrylsodium in THP and THF... 

Similar results, i.e., efficient formation of the hydroperoxide, were observed by oxidation of the living chain ends in the solid state. As noted by Fetters and Firer 3511 from air termination reactions, more coupling product is observed in the presence of Lewis bases such as THF and N,N,N, N -tetramethylethylenediamine. These results are consistent with oxidation being the primary reaction responsible for dimer formation in air termination 352. ... 

Unpublished work of Halasa and co-workers 31) on the study of live chain ends using 13C-NMR as a probe into their structures has led to some interesting new findings. These workers studied the lithium live ends of polybutadiene in the presence of a new polar modifier, dipiperi-dylethane (DPE). This modifier forms a complex with n-BuLi which initiates the polymerization of 1,3-butadiene to give polymer having 100% 1,2 microstructure. 

Subsequently, several nonkinetic approaches (32) were directed toward determining the structure of the live chain ends (e.g., H-NMR and 13C-NMR). For example, Bywater and co-workers (58) studied the addition of r-butyllithium to 1,3-butadiene and obtained a complex PMR spectrum for the addition product. They examined the effect of catalyst concentration on the microstructure of the polybutadiene and found that at high catalyst levels, the vinyl content increased as shown in Table II. 

There is an equilibrium between living chain ends which are attached to Nd and dormant chain ends which can be attached to Al, Mg and Zn. Very likely these chain ends exhibit differences in reactivity towards modification agents and polar monomers. This assumption will possibly result in the interpretation of inconsistent results observed in end-group functionalization and block copolymer formation, e.g. with polar comonomers such as e-caprolactone. 

Enolates, which are the actual propagating species, exist in the E (11) and Z (12) configurations. The E/Z ratio of the living chain-ends can be indirectly determined by reacting the propagating enolates 11 and 12 with chlorotrimethylsilane and converting them into the corresponding ketene silylacetals 13 and 14, which are characterized by NMR spectroscopy (equations 23 and 24). ... 

In order to shed light on the polymerization mechanism, the attention of several research groups was focused on the structure of the living chain-ends. A major problem is that enolates are known for condensation at a rate that depends on solvent, temperature and structure of the ester group (f-Bu esters being less reactive than Me esters). This undesired reaction makes the structural analysis of the chain-ends more complex . In order to get rid of any contribution of the chain in the structural analysis, unimeric, dimeric and oligomeric models of the chain-ends were considered. 

With the purpose of increasing the range of available block copolymers, comonomers other than methacrylates and acrylates can also be involved in sequential polymerization, provided that they are susceptible to anionic polymerization. Dienes, styrene derivatives, vinylpyridines , oxiranes and cyclosiloxanes are examples of such comonomers. The order of the sequential addition is, however, of critical importance for the synthesis to be successful. Indeed, the pX a of the conjugated acid of the living chain-end of the first block must be at least equal to or even larger than that of the second monomer. Translated to a nucleophilicity scale, this pK effect results in the following order of reactivity dienes styrenes > vinylpyridines > methacrylates and acrylates > oxiranes > siloxanes. 

An alternative method of initiation is through the use of the radical anion produced from the reaction of sodium (or lithium) with naphthalene. Such radical anions react with styrene by electron transfer to form styrene radical anions these dimerize to produce a dianion, which initiates polymerization as outlined in Scheme 14. One particular feature of this method is that polymerization proceeds outwards from the centre. Subsequent reaction of the living chains ends with another suitable monomer system produces a triblock copolymer. This is the principle by which styrene-butadiene-styrene triblock copolymers (formed when butadiene is polymerized in the same way. and styrene is added as second monomer) are produced commercially. This material behaves as a thermoplastic elastomer, since the rigid styrene blocks form cross-links at room temperature on heating these rigid styrene portions soften, allowins the material to be remoulded. ... 

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