Polymer living tetrahydrofuran

New Aspects of the Chemistry of Living Tetrahydrofuran Polymers Initiated by Trifluoromethane Sulfonic Anhydride... 

The polymerization kinetics of alkali salts of living vinyl polymers In ethereal solvents, such as tetrahydrofuran CD, tetrahydropyran (2), dlmethoxyethane Q), oxepane (4) and dloxane... 

Macroinitiators such as polymeric Li, Na, and K alkoxides can also be used for the initiation of the six-membered cyclic carbonate polymerization. Thus, besides living vinyl polymers, hydroxyl group-terminated polymers of poly (tetrahydrofuran) (PTHF), poly(oxyethylene), and poly(dimethylsiloxane) (PDMS) were transformed to alkoxides by treatment with scc-BuLi or K-naphthalene and used as initiators. The use of these macroinitiators enables the identification of side reactions, as shown by Keul and Hocker for polystyrene lithium (PS Li ). The addition of the macro initiator to the monomer, to maintain a high excess of monomer, minimizes side reactions. Transformation of the polystyryl... 

Rempp has used a living PS precursor of low molar mass that is made either in a polar solvent, tetrahydrofuran (THF) [82], or in a nonpolar solvent [57]. Here again a living precursor initiates the polymerization of a small amount of DVB, whereupon the cores are formed, as in an arm-first process (Scheme 8). Each core contains as many active sites as there are branches surrounding it. In spite of their low molar mass, the protection the branches exerts on the cores is efficient and prevents the formation of aggregates. The living star polymer solution is molecularly dispersed. Subsequently, the active sites located in the cores are used to initiate the polymerization of another suitable monomer, whereupon a new set of branches (of different chemical nature or not) is generated from the cores. 

In ionic polymerizations termination by combination does not occur, since all of the polymer ions have the same charge. In addition, there are solvents such as dioxane and tetrahydrofuran in which chain transfer reactions are unimportant for anionic polymers. Therefore it is possible for these reactions to continue without transfer or termination until all monomer has reacted. Evidence for this comes from the fact that the polymerization can be reactivated if a second batch of monomer is added after the initial reaction has gone to completion. In this case the molecular weight of the polymer increases, since no new growth centers are initiated. Because of this absence of termination, such polymers are called living polymers. 

The first living polymer studied in detail was polystyrene polymerized with sodium naphthalenide in tetrahydrofuran at low temperatures ... 

With conventional techniques and electrolytes, it was not possible to obtain living anions because they are rapidly protonated by tetraalkylammonium salts and residual water. The first report of the production of living polymers by an electrolytic method has to be attributed to Yamazald et al. [247], who used tetrahydrofuran as solvent, and LiAlH4 or NaAl(C2H5)4 as electrolyte for the polymerization of a-methylstyrene. A similar technique was used to polymerize styrene as well as derivatives [248-252]. 

The polymerization vessel is illustrated in Pig. 8. Ampoules A and B contained dilute and concentrated benzene solutions of the seed polymer, respectively, and ampoules C and D contained tetrahydrofuran (or dimethoxyethane) and a benzene solution of the monomer. The polymerization vessel was connected to a high-vacuum line, evacuated, flamed and sealed off. Then the break-seal on ampoule B was crashed, and the whole vessel was washed with the concentrated solution of living polymer. All the solutions were brought to E and the vessel was washed by condensing the benzene using a pad at Dry Ice-methanol temperature. The solvent was distilled into F from E and E was sealed off. The seals on A, C, and D were broken, and the dilute solution of living polymer was collected into G by turning the whole vessel upside down,... 

The living polymer technique has been applied recently in studies of equilibria tetrahydrofuran polytetrahydrofuran. This polymerization proceeds by a cationic mechanism (15) the propagation being described by the equation... 

Various agents may initiate such a polymerization, e.g., Ph3C+, SbClg, whose action was investigated by Bawn and his coworkers (16). They also were first to recognise the formation of living polymers in tetrahydrofuran... 

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. 

Electrolytically initiated polymerization may either depend on a direct electron transfer between electrode and monomer, or on the formation of an intermediate which interacts with a monomer molecule in a fast chemical step, thus creating a chain initiator. As an example of the former type of process, the formation of a living polymer from the cathodic polymerization of a -methylstyrene by electrolysis in sodium tetraethylaluminate - tetrahydrofuran may be cited 639 whereas a typical case of the latter type is the anodic polymerization of vinyl monomers by electrolyzing them together with sodium acetate in aqueous solution 63 7,640) Here it is assumed that acetate ion is discharged to form an acetoxy or methyl radical which attacks the monomer molecule in a fast chemical step. 

The study of the spectra of living polymer systems is valuable from a more practical point of view and indicates that the term has some limitations. At room temperature all the polymer-lithium compounds in hydrocarbon solvents show spectra which are stable for considerable time intervals. At elevated temperatures spectral changes occur at least for polystyryllithium, which indicate that isomerization reactions are occurring 4). Most of them display instability in solvents containing appreciable amounts of more polar constituents such as tetrahydrofuran. This effect was first noticed for poly-sty rylsodium 11) and has been attributed to the elimination of sodium hydride, followed by a subsequent reaction to form the more stable substituted allyl anion 21). 

The necessary, but not sufficient criterion of the living character of polymerization is the possibility of preparation of high molecular weight polymers (M > 10s). This has been achieved in several systems in cationic ring-opening polymerization, e.g., in the polymerization of some cyclic ethers 3,3-bis(chloromethyl)oxetane, tetrahydrofuran, 1,3-dioxo-lane, and 1,3,5-trioxane. 

In the polymerization of 1,3-dioxolane and tetrahydrofuran it has been shown additionally that concentration of active centers is constant throughout the polymerization (both by direct determination and from analysis of polymerization kinetics). In some other polymerizations, believed to proceed as living processes, only the moderate molecular weights regions (M < 105) were studied thus, for example, no very high molecular weight polymers were obtained in the polymerization of oxazolines. 

All the approaches described have been used to prepare functional polymers by cationic ring-opening polymerization. From this point of view, groups of monomers that have been investigated most are cyclic ethers (tetrahydrofuran), cyclic acetals (1,3-dioxolane), cyclic imines (N-f-butylaziridine), and oxazolines, i.e., these monomers for which the living conditions can be approached. 

---