Classical Living Anionic Polymerization

For classical living anionic polymerizations and other systems in which all of the chains are active all of the time, i eq= and thus [CE] = [ P ], and... 

New approaches based on the introduction of reactive species into reaction mixtures that tend to cap the growing chains reversibly allow, in many cases, production of well-defined polymers and copolymers with narrow polydispersi-ties. Up to few years ago, such a possibility was unobtainable by a classical free radical process. The proposed principle of control of macroradical reactivity is very interesting conceptually, and represents a very powerful tool to prepare block copolymers with well-controlled structures. However, it is clear that the true living character as demonstrated by some anionic polymerizations is still not obtained and much more work needs to be done to understand and control this new process better. 

In the study of anionic copolymerization it is possible to use two types of approach. The first method is the use of the classical copolymer composition equations developed for free radical polymerization. The second is unique to anionic polymerization and depends on the fact that for living systems it is possible to prepare an active polymer of one monomer and to study its reaction with the second monomer. The initial rate of disappearance of one type of active end, or the appearance of the other type (usually determined spectroscopically) or the rate of monomer consumption gives directly the reactivity of polymer-1 with monomer-2. It is in principle possible to compare the two methods to see if additional complications occur when both monomers are present together. 

In classic anionic polymerization reactions, the deactivation of living anions at the chain ends during the polymerization process is often considered as an unfavored side reaction and this has to be minimized, since it leads to an undesired termination and a broad molecular weight distribution of the aimed polymer. However, a controlled deactivation of these living species can be useful in preparing hierarchic polymer... 

The anionic polymerization methacrylic acid esters using such classical initiators as bulky RLi requires low temperatures and has other disadvantages. A breakthrough was reported by Webster, et al, at Dupont in 1983. They used 0-silyl ketene ketals as an initiator in combination with nucleophilic catalysts (fluorides) at room temperature. The process also has two other positive features 1) The polymers are living prior to aqueous workup, and 2) The molecular weight distribution is very narrow (D=M /Mn 1.2) ... 

Here, we report on the synthesis and characterization of PMMA-poly(/i-butylacrylate)-PMMA (MBuM) symmetric triblock copolymers, as a new family of thermoplastic elastomers. These compounds have be prepared by a novel route based on controlled radical polymerization [4]. Compared to classical anionic living polymerization [5], this new route, sketched in Scheme la, appears particularly appealing since the triblock copolymers are prepared in a two-step process instead of the usual three steps required in anionic polymerization. In the latter case, as butylacrylate cannot be polymerized via a living process, the MBuM symmetric triblocks have to be synthesized by sequential copolymerization of er butylacrylate (tBuA) and MMA followed by the transalcoholysis of the tBu esters with n-butanol... 

Two different types of (PS-b-PBDh) diblock can be presently synthesized. The first one by classical anionic initiation (s-buty1-lithium) and "living" propagation of the (PS-b-PBD) copolymer (8), followed by the hydrogenation procedure described here as discussed above, the resulting product will be close to a (PS-b-LLDPE) copolymer. The second one came from the discovery (9) of a "living" polymerization of butadiene into a pure (99 %) 1,4 polymer by a bis n allylnickel-tri-fluoroacetate) coordination catalyst, followed by styrene polymerization unfortunately, the length of the polystyrene block is limited (to a M.W. of ca. 20,000) by transfer reactions. 

Polymerization of styrene initiated by n-butyl lithium in benzene was investigated by a spectrophotometric technique by Worsfold and Bywater156). Concentration of polysty-ryl anions was monitored by their absorbance at 334 nm, while the concentration of the unreacted styrene was determined by its absorbance at 291 nm. The results are shown graphically in Fig. 23. Concentration of polystyryl anions increases with time and eventually reaches its asymptotic value, being constant afterwards. This observation indicates the stability of these species. On the other hand, the concentration of styrene decays in sigmoidal fashion. This classic study unequivocally demonstrated the living character of the resulting polymers, and therefore it was justified to identify the rate of increase of the absorbance at 334 nm with the rate of initiation. 

The NIR in situ process also allowed for the determination of intermediate sequence distribution in styrene/isoprene copolymers, poly(diene) stereochemistry quantification, and identification of complete monomer conversion. The classic one-step, anionic, tapered block copolymerization of isoprene and styrene in hydrocarbon solvents is shown in Figure 4. The ultimate sequence distribution is defined using four rate constants involving the two monomers. NIR was successfully utilized to monitor monomer conversion during conventional, anionic solution polymerization. The conversion of the vinyl protons in the monomer to methylene protons in the polymer was easily monitored under conventional (10-20% solids) solution polymerization conditions. Despite the presence of the NIR probe, the living nature of the polymerizations was maintained in... 

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