Tetrahydrofuran, living polymerization

The intrinsic viscosities of the polymers prepared in tetrahydrofuran increased throughout the experiment. This system thus exhibits some of the aspects of living polymerization—that is, catalyst activity over an extended period, and increasing viscosity average molecular weights with added amounts of monomer. The rather broad molecular-weight distributions of these polymers, however, differentiates this system from that of the classical case in which polymerization proceeds in the complete absence of a termination process. 

A well-defined diblock copolymer of propylene and tetrahydrofuran (THF) was synthesized on the basis of the transformation of living polypropylene end (3) to cationic end (9) which initiates the living polymerization of THF, as schematically represented by reaction (46)104). 

In the 1980s, a synthetic method to produce AB block copolymers of propylene and tetrahydrofurane (THF) was proposed [29]. Polypropylene-fi/ock-poly(THF) was prepared by a combination of living polymerization of propylene with a V(acac)3 catalyst and the living polymerization of THF. Its synthesis was based on the transformation of living polypropylene chain ends to cationic ones, which initiated the living polymerization of THF. 

Table 2 also indicates that the nucleophiles effective for vinyl ethers are relatively mild, when compared with those for isobutene (cf., Section V.B.2). In fact, stronger bases lead to inhibition or severe retardation of polymerization [36,64] ketones aldehydes, amides, acid anhydrides, dimethyl sulfoxide (retardation) alcohols, aliphatic amines, pyridine (inhibition). The choice of nucleophiles is determined by their Lewis basicity (as measured by pKb, etc. [64,103]), and this factor determines the effic-tive concentrations of the nucleophiles. For example, the required amounts of esters and ethers decrease in the order of increasing basicity (i.e., a stronger base is more effective and therefore less is needed) [101,103] tetrahydrofuran < 1,4-dioxane ethyl acetate < diethyl ether. On the other hand, for amines not only basicity but also steric factors play an important role [142] thus, unsubstituted pyridine is an inhibitor, while 2,5-dimethylpyridine is an effective nucleophile for controlled/living polymerization, although the latter is more Lewis basic. 

Blue solutions of potassium in dimethoxyethane or tetrahydrofuran initiate polymerization of styrene77. With the former solvent the reaction was quantitative, both at 0° and - 70 °C, yielding living polystyrene. Termination was observed in tetrahydrofuran presumably it was caused by some slowly reacting impurities. The authors attributed this initiation to diamagnetic (e )2 however, the diamagnetic blue species are the K anions which act as the electron-transfer agents. 

Living polymerization of azo monomers is one of the most effective ways to prepare well-defined azo BCs. Generally, a monodispersed macroinitiator should be prepared first. It is then used as an initiator for the subsequent polymerization of azo monomers. Finkelmann and Bohnert (1994) first reported the synthesis of LC-side chain AB azo BCs by direct anionic polymerization of an azo monomer. As shown in Scheme 12.1, the polymerization of polystyrene (PS)-based diblock copolymers was carried out from a PS-lithium capped with 1,1-diphenylethylene (DPE), whereas the poly(methyl methacrylate) (PMMA)-based diblock copolymers were prepared by addition of methyl methacrylate (MMA) monomers to the living azo polyanion, obtained by reaction of l,l-diphenyl-3-methylpentylithium (DPPL) with the azo monomer in tetrahydrofuran (THF) at lower temperature. By this method, a series of well-defined azo BCs were obtained with controlled molecular weights and narrow polydispersities (Lehmann et al., 2000). 

Water and compounds with active hydrogen must be excluded from the reaction medium. Oxygen, on the other hand, does not interfere with the reaction. Tetrahydrofuran, acetonitrile, and aromatic solvents are commonly used in polymerizations catalyzed by nucleophiles. Chlorinated solvents and dimethylformamide are utilized in many reactions catalyzed by electrophiles. Living polymerizations of methacrylate esters can be carried out at 0 to 50 C. The acrylate esters, however, require temperatures below 0 °C for living, group-transfer polymerizations, because they are more reactive and can undergo side reactions. 

Describe the termination reaction in tetrahydrofuran polymerization, including living polymerization. 

The cationic polymerization of tetrahydrofuran is used commercially to produce a,CD-dihydroxypoly(tetramethylene oxide) (PTMO glycol). Although this polymer is not used by itself as an elastomer, it is used as one of the elastomeric block components for preparation of segmented thermoplastic polyurethane [133] and thermoplastic polyester [134] elastomers. The cationic polymerization of tetrahydrofuran (THF) is a living polymerization under proper experimental conditions [135-139], i.e., it does not exhibit any termination step, very much like the analogous anionic polymerizations which are discussed in Section VIII. However, these polymerizations are complicated by the fact that the ceiling temperature, where the free energy of polymerization is equal to zero, is estimated to be approximately 83 2°C in bulk monomer solution [140] therefore, the polymerization is reversible and incomplete conversion is often observed, especially in the presence of added solvent. For... 

The termination reactions in tetrahydrofuran polymerizations can depend upon the choice of the counterion, particularly if the reaction is conducted at room temperature [60]. In many reactions, the chains continue to grow without any considerable termination or transfer [63, 64]. Some refer to this process as living polymerization, thus in polymerizations of tetrahydmfuran [65] with PFe or... 

Various approaches have been taken to improve the properties of fluorosilicones. In this connection, Kobayashi et al. [17] synthesized several methyl-3,3,3-trifluoropropylsiloxane (F)-dimethylsiloxane (D) random and block copolymers. The random copolymers were prepared by equilibrium copolymetization starting from a mixture of cyclic F and D siloxanes with potassium silanolate as the catalyst The F-D block copolymer was prepared by sequential anionic living polymerization of strained cyclotrisiloxanes using a much weaker catalyst, lithium silanolate. The prepared copolymers were soluble in both tetrahydrofuran (THF) and... 

Propagation proceeds with complete consumption of monomer and the propagating anionic centers remain intact as long as one employs solvents such as benzene, n -hexane and tetrahydrofuran which are inactive in transferring a proton to the propagating anion. These polymerizations, referred to as living polymerizations, are terminated when desired by the deliberate addition of a proton source such as water or alcohol. 

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