Polymerization, activation anionic

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. 

Addition polymerization through anionic active species. This is discussed in the next section. 

Nylon 4 is produced hy ring opening 2-pyrrolidone. Anionic polymerization is used to polymerize the lactam. Cocatalysts are used to increase the yield of the polymer. Carhon dioxide is reported to he an excellent polymerization activator. 

Monomers 1 (22), 2 (23) and 6 do not polymerize with anionic initiators or give low yields of ill-defined oligomers. Vinyl 2-furyl ketone 5 (24) is sensitive to anionic activation, but the products have low molecular weights. The most interesting results have been obtained with 3 and 4. 

A polymerization of a bulky methacrylate ester (e.g. trityl methacrylate) using an optically active anionic initiator can give an isotactic polymer, poly 1-methyl-1-[(trityloxy)carbonyl]ethylene of high optical activity owing to the formation of helical polymer molecules with units of predominantly one chirality sense. 

Styrene-1,3-butadiene-styrene (SBS) or styrene-isoprene-styrene (SIS) triblock copolymers are manufactured by a three-stage sequential polymerization. One possible way of the synthesis is to start with the polymerization of styrene. Since all polystyrene chains have an active anionic chain end, adding butadiene to this reaction mixture resumes polymerization, leading to the formation of a polybutadiene block. The third block is formed after the addition of styrene again. The polymer thus produced contains glassy (or crystalline) polystyrene domains dispersed in a matrix of rubbery polybutadiene.120,481,486... 

Anionic Catalysis Several bulky methacrylates afford highly isotactic, optically active polymers having a single-handed helical structure by asymmetric polymerization. The effective polymerization mechanism is mainly anionic but free-radical catalysis can also lead to helix-sense-selective polymerization. The anionic initiator systems can also be applied for the polymerization of bulky acrylates and acrylamides. The one-handed helical polymethacrylates show an excellent chiral recognition ability when used as a chiral stationary phase for high-performance liquid chromatography (HPLC) [97,98]. 

Figure 4.1. Flowsheet of production of a reactive mixture by activated anionic s-caprolactam polymerization (explanations of numbers are in the text).

Asymmetric PS stars of the type (PSA)n(PSB)n were also prepared by the divinyl-benzene (DVB) method [9]. Living PS chains, prepared by sec-BuLi initiation, were reacted with a small amount of DVB producing star homopolymers. The DVB core of the stars contains active anions which, if no accidental deactivation occurs, are equal to the number of the arms that have been linked to this core. These active sites are available for the polymerization of an additional quantity of monomer. Consequently further addition of styrene produced asymmetric star polymers... 

Though a vast number of studies on the characteristics of neodymium-mediated polymerizations were performed to the present day, only a few studies focus on the influence of the anion of the Nd precursor. As already mentioned in Sect. 2.1.1.2 Wilson systematically varied the structure of carboxylates and studied the influence on hydrocarbon solubility and on polymerization activity [183]. The dependence of polymerization activity on various halogenated Nd-carboxylates Nd(OCOR)3 (R = CF3, CCI3, CHCI2, CH2C1, CH3) was the target of a study by Kobayashi et al. [ 177]. 

Presently, the importance of Nd allyl compounds as intermediates in Nd carboxylate- and other Nd-based catalyst systems is widely accepted. As various Nd allyl compounds have been synthesized, characterized and successfully tested as polymerization catalysts this view is supported by solid experimental evidence (Sect. 2.1.1.5 and the references therein). Selected Nd allyl compounds exhibit significant polymerization activities without the addition of cocatalysts. In these cases the active species is neutral. But also cationic active Nd species are taken into consideration (Sect. 2.1.1.5) [288,291]. Cationic species also prevail in the presence of non-coordinating anions. 

Keys to the high polymerization activities of single-site catalysts are the cocatalysts. MAO is most commonly used and is synthesized by controlled hydrolysis of trimethyl aluminum. Other bulky anionic complexes which show a weak coordination, such as borates, also play an increasingly important role. One function of the cocatalysts is to form a cationic metallocene and an anionic cocatalyst species. Another function of MAO is the alkylation of halogenated metallocene complexes. In the first step, the monomethyl compound is formed within seconds, even at -60°C (69). Excess MAO leads to the dialkylated species, as shown by NMR measurements. For the active site to form, it is necessary that at least one alkyl group be bonded to the metallocene (70). 

Not all monomers are anionically polymerizable. Nevertheless, one can take advantage of the activity of the living ends to introduce reactive end groups at the extremity of homopolymers and then use such end groups to initiate the polymerization of anionically non polymerizable monomers. This method has been applied to the synthesis of copolymers with polyvinyl and polylactone blocks19 and of copolymers with polyvinyl and polypeptide blocks20-2S). One can at last use both anionic and cationic polymerization to prepare block copolymers of tetrahydrofuran with styrene or methylstyrene2. ... 

Flere MAO first generates the dimethyl complex 6.26 from 6.25. This reaction, of course, can also be brought about by Me3Al. It is the subsequent reaction (i.e., the conversion of 6.26 to 6.27 that is of crucial importance. The high Lewis acidity of the aluminum centers in MAO enables it to abstract a CH3 group from 6.26 and sequesters it in the anion, [CH3-MAO]. Although 6.27 is shown as ionically dissociated species, probably the anion, [CH3-MAO], weakly coordinates to the zirconium atom. It is this coordinatively unsaturated species, 6.27, that promotes the alkene coordination and insertion that are necessary for polymerization activity. 

Lactam polymerization with anionically activated monomer has its counterpart in the cationic processes of lactam polymerization. This type of mechanism has also been observed recently in some polymerizations of oxygen-containing heterocycles (see Chap. 4, Sect. 2.3)... 

The formation of aggregates with low to zero polymerization activity is quite general in ionic polymerizations. This statement can be further documented by the observation of centre aggregation during anionic polymerization of oxirane [100-103]... 

Taking into consideration the above concept, it is intuitively obvious that the simplest way to assess quantitatively acid-base properties of the anhydrous borates is to estimate the dependence of polymerization of anions in the borate structures on the sizes and valences of cations, and also on the ratio (N) of the total number of B03 triangles and BO4 tetrahedra (NB) to the total amount of cations (NM) contained in the borate structural formula N = Nb/Nm. From the crystallochemical point of view it is clear that the increase of the N-ratio increases the anion polymerization and the ratio Nb3+/No2. It leads to a decrease in the oxygen activity coefficient and simultaneously, to more acid properties of these compounds. Also, the value of n = nA/no (nA - number of triangles, - number of tetrahedra) increases (for N < 1), i.e. the ratio of the number of BO3 triangles to BO4 tetrahedra in the structures of compounds increases. 

One main difference between anionic polymerization and GTP has to be found in the amount of enolates active in polymerization. In anionic polymerization, all the chains are end-capped by an enolate, which is the case for only a small part of the chains in GTP consistent with the very good control of GTP even at room temperature. In this respect, Brittain and Dicker showed that prop/ term is by far higher in GTP (250) than in classical anionic polymerization ( prop/ term = 8) . In line with slow termination compared to propagation in GTP, Bandermann and coworkers found that the amount of the nucleophilic catalyst is essential to the polymerization control. Indeed, as far as the tris(piperidino)sulfonium bifluoride-mediated GTP of MMA in THF is concerned, the polydispersity index increases with the amount of catalyst . 

---