Living polymerization of THF

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. 

Another synthetic approach based on pyridium salt photochemistry involves the use of alkoxy radicals which are formed in both direct and sensitized decomposition of pyridinium ions in free radical polymerization [78]. Obviously, polytetrahydrofuran (PTHF), terminated by JV-alkoxy pyridinium ions, can act as macrophotoinitiator for the polymerization of monomers such as methyl methacrylate (MMA) that readily polymerize by a free-radical mechanism. PTHF macrophotoinitiators were prepared by termination of living polymerization of THF by the corresponding IV-oxides, The well-defined macrophotoinitiators with exact functionalities, confirmed by H-NMR, UV-visible and g.p.e. analysis, were obtained. Upon irradiation of macroinitiators at suitable wavelengths, polymeric alkoxy radicals are produced. The overall process is shown for the pyridinium macrophotoinitiator in the following Scheme 21. 

This method requires not only strictly living conditions but also the quantitative conversion of the living growing centers into reactive end-groups. At present, living polymerization of THF and 1-t-butylaziridine have been used for this purpose, e.g. ... 

Termination of living polymerization of THF with sodium 4-oxy-TEMPO can be considered as a typical example of this... 

Kinetics. Details of the kinetics of polymerization of THF have been reviewed (6,148). There are five main conclusions. (/) Macroions are the principal propagating species in all systems. (2) With stable complex anions, such as PF , SbF , and AsF , the polymerization is living under normal polymerization conditions. When initia tion is fast, kinetics of polymerizations in bulk can be closely approximated by equation 2, where/ is the specific rate constant of propagation /is time [I q is the initiator concentration at t = 0 and [M q, [M and [M are the monomer concentrations at t = 0, at equiHbrium, and at time /, respectively. 

Burgess et al.I14) found that the living cationic polymerization of THF was initiated with the mixture of isopropyl iodide and AgC104. Therefore, Doi et al.104) have conducted the block copolymerization of polypropylene with THF by using an iodine-terminated polypropylene (4) which was synthesized via the reaction of living polypropylene (3) with I2. Iodine-terminated polypropylene (Kln = 16,500, Klw/Sln = 1.15) was... 

Triflic anhydride is an efficient initiator used in thfe living polymerization of oxolane 60), and bifunctional poly-THF oligomers can be further employed to initiate the polymerization of t-butylaziridine yielding polymers with aziridinium sites at both ends of their chain61 which can be reacted with methacrylic acid. 

To obtain a high molecular weight block or random copolymer of the oxonium ion type monomer and carbonium ion type monomer, experimental conditions must be such that termination or transfer reactions are minimized. The living nature of the cationic polymerization of THF (7) is well established, but it has been difficult to obtain a high polymer of styrene or DOL by cationic mechanism. In this paper we demonstrate the living nature of the polymerization of DOL and the high polymer of St-DOL copolymer. Using this technique, we were able to obtain a block copolymer of vinyl monomer and cyclic monomer. 

In some systems, for typical polymerization conditions, indeed kp kc and purely linear polymer is formed by itself. This is, for example, a case of cationic polymerization of THF [90]. After reaching monomer-polymer equilibrium, concentration of cyclic oligomers is undetectably low. Their concentration, however, increases if the living system is kept for a prolonged period of time (cf., Section III. A.3). 

Cationic polymerization of THF fulfills all the requirements of living polymerization. With several initiators, the initiation is relatively fast and quantitative, and propagation proceeds without transfer or termination. The dependence DP = ([M]0 - [M)e)/[I)0 holds up to high polymerization degrees. The only limitation is that, due to the reversibility of propagation, the molecular weight distribution is broadened and reaches the value of MjMn = 2 in equilibrium. Polymers with narrower MWD were obtained by terminating the polymerization at lower conversion [56]. 

Zirconium and hafnium complexes of an amine bis(phenolate) ligand with a THF side-arm donor have been shown to perform living polymerization of 1-hexene when activated with B(C6F5)3. The Zr catalysts (1) possess very high activities of 21 OOOgnunol cath. In neat olefin, the reaction is very exothermic and accompanied by various termination processes resulting in a higher polydispersity index than observed when the monomer is diluted in chlorobenzene. 

The chain tacticity of PMMA synthesized by GTP catalyzed by nucleophiles at different temperatures was analyzed by Webster and coworkers The syndiotactic content increases from 50% at 60 °C up to 80% at —90°C in THF, using tris(dimethylamino)sulfonium bifluoride [(Me2N)3S+ HF2 ] as catalyst . In contrast to the anionic polymerization of MMA, the stereoselectivity of GTP is less sensitive to solvent. It must be noted that PMMA is less syndiotactic when the GTP is catalyzed by nucleophiles rather than by Lewis acids . GTP was extended to the living polymerization of many acrylates and methacrylates, such as nBuMA, glycidyl-MA, 2-ethylhexyl-MA, Me3SiOCH2CH2-MA, sorbyl-MA, allyl-MA, lauryl-MA), acrylates (EA, BuA), acrylonitrile, methacrylonitrile and Al,A-dimethylacrylamide . 

Structures of the Rowing specie, assumed in lier works, have been directly observed by NMR in nmre recent studies. Taking the polymerization of THF as an example, the growing tertiary oxonium ion as an active ecies was postulated as one of the possible structures first by Meerwein and than by other groups describing the cationic polymerization of THF in terms of a living process... 

Triphenylmethyl, tropylium, xanthylium, acylium and diazonium salts have also been used successfully in the study of the polymerization of THF [50]. In fact the true equilibrium conversions were first demonstrated using p-chlorophenyl diazonium hexafluorophosphate [41, 115], and at the same time the polymerizations were shown to be living . The lower equilibrium yields [50] observed when other counter-ions are employed, e.g. SbClg, appear to arise because of a termination mechanism associated with the anions. Kinetic studies [50] of the polymerization of bulk THF with Phg C SbCl have established the apparent rate law... 

The ABA-type block copolymers B-86 to B-88 were synthesized via termination of telechelic living poly-(THF) with sodium 2-bromoisopropionate followed by the copper-catalyzed radical polymerizations.387 A similar method has also been utilized for the synthesis of 4-arm star block polymers (arm B-82), where the transformation is done with /3-bromoacyl chloride and the hydroxyl terminal of poly(THF).388 The BAB-type block copolymers where polystyrene is the midsegment were prepared by copper-catalyzed radical polymerization of styrene from bifunctional initiators, followed by the transformation of the halogen terminal into a cationic species with silver perchlorate the resulting cation was for living cationic polymerization of THF.389 A similar transformation with Ph2I+PF6- was carried out for halogen-capped polystyrene and poly(/>methoxystyrene), and the resultant cationic species subsequently initiated cationic polymerization of cyclohexene oxide to produce... 

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