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Procedures for Homogeneous Anionic Polymerization

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Procedures for Homogeneous Anionic Polymerization JOURNAL OF RES EARCH of the Notional Bureau of Standards - A. Physics and Chemistry Vol. 70A, No.5, September-October 1966 Procedures for Homogeneous Anionic Polymerization Lewis J. Fetters* ~ Institute for Materials Research , National Bureau of Standards, Washington, D.C. 20234 I (May 11, 1966) This report is a revie w of the procedures and equipment used in the preparation of polyniers of predi ctable molecular weights and narrow molecular weight distributions. The monomers considered I in detail are styrene, a-methyl styre ne, isopre ne, and butadiene. ~ Key Words: Homogeneous anionic polymerization, monodis perse, polystyrene, poly-a-methylsty­ I rene, polyisopre ne, and polybutadiene. l ! 1. Introduction molecular weight distributions may be represented 1 by the Poisson function, Organometallic actuated addition polymerizations of unsaturated compounds (vinyl, diene, etc.) in hydro­ carbon and ether media have received considerable attention. Many of these polymerizations are unusual K=1,2,3,4, . in that spontaneous termination is avoidable through the judicious choice of experimental conditions; a where W(K, XlIo ) denotes the weight fraction of Kmers I feature first fully appreciated by Szwarc [1, 2).1 Of residing in a mixture of chains of number average considerable experimental value is the fact that the de­ chain length Xno' For these systems, the weight-to­ activation of these "living" polymercarbanions may be number average chain length ratio is given as: accomplished at the leisure of the experimenter by the selective addition of a reagent suited to meet a par­ ticular need. This retention of chain-end activity is a feature that facilitates investi gations of the thermo­ Such distributions have not been achieved, or even ap­ dynamics of polymerization processes, simplifi es proached, until quite recently with monomers such as studies of polymerization kineti cs, and permits styrene [4- 15], O'-methylstyrene [16, 17], isoprene r synthesis of novel macromolecules. [18-21], and butadiene [21 -24]. This was due prin­ These systems can therefore consist of active chains cipally to various factors which combined to veil the which will continue to incorporate monomer until true possibilities of these systems. These factors monomer depletion is accomplished. Hence, if each included the presence of impurities or solvents which center of propagation is afforded equal opportunity of growth, the resulting product will possess a homo- could terminate or transfer with the growing chains, a slow initiation reaction, a depropagation step (equi­ I geneous molecular weight [3] . As a result, many ;. studies of homogeneous anionic polymerization have librium polymerization) or the presence of interchange I reactions between polymer chains. The latter two I had as a foremost goal the synthesis of polymers of phenomena are encountered in the polymerization predictable molecular weight and predeterminable of cyclic siloxanes molecular weight di stributions_ Obviously, this goal [25, 26]. Hence, the fulfillment of the following conditions can only be attained in the absence of any constituent is necessary if precise control over molecular weight capable of reacting with these carbanions, e.g., acids, and molecular weight distribution is to be achieved: alcohols, 'or the usual atmospheric components. (a) rigorous exclusion of substances capable of causing The general case of a termination-free system re­ termination or transfer; (b) uniformity of reaction ceived treatment by Flory [3], who showed that the conditions with regard to temperature and concentra­ growth of a fixed number of active chains leads to tion of reactants; (c) a high ratio of the apparent molecular weight distributions approaching mono­ initiation to propagation rates; and (d) the absence of dispersity, i.e., the Poisson distribution. These depropagation steps and bond interchange reactions. In view of increasing interest in the study of bulk and solution properties of polymers with narrow dis­ * Nat ional Academy of Sciences·National Researc h Council Postdoctoral Resident tributions of molecular weights, as well as those of Research Associate. I Figures in bra ckets indicate the lit erature references a l th e end of this paper. precisely constructed copolymers and "star" macro- 421 molecules, the purpose of this communication is to resultant sample will possess the "most probable" review the techniques by which the preparation of molecular weight distribution. ~ these polymers may be achieved. Results from various The monofunctional organolithium-initiated polym­ laboratories have indicated that the high-vacuum erization of styrene and other monomers in hydro­ technique provides the most reliable method for the carbon media offers several distinct advantages over successful attainment of these goals. As will be dis­ reactions carried out in tetrahydrofuran. The slow cussed later, among those who have realized the poten­ propagation rates [30] permit the addition of the mono­ tial of these tephniques have been Bywater and Wors­ mer to the solvent-initiator solution in a batchwise 1 fold, Wenger, Szwarc, Rempp, and Morton. fashion. Hence, any impurities will react on mixing The monomers considered herein are styrene, a­ with the initiator. If a fraction of the total initiator is methylstyrene, isoprene, and butadiene. The polym­ destroyed, the molecular weight distribution will not erization initiators described are the isomers of butyl­ be broadened, although an increase in the molecular lithium. These monofunctional initiators provide weight will occur. The effect on the distribution of several advantages over the electron transfer initiators, a slow termination reaction, comparable in speed to e.g., sodium biphenyl or sodium naphthalene. Addi­ the comsumption of monomer, has received considera­ tion of an electron transfer initiator to an ether solution tion by Coleman et aI., [31]. Fortunately, for certain -;; of monomer results in the formation of an ion-radical monomers the occurrence of such reactions is avoid­ [1, 2] which eventually undergoes a combination reac­ able through the proper selection of solvent and by tion with the resultant generation of a polymeric the application of the procedures to be outlined shortly. species possessing a carbanion at each end. In pre­ paring the difunctional species, the accidental deacti­ vation of a fraction of the carbanions will result in 2. Experimental Techniques a bimodal molecular weight distribution [5, 7, 16], i.e., the material will possess a portion that will have one­ 2.1 . High Vacuum Apparatus half the molecular weight of the remaining polymer. For the case of the polymerization of styrene in an A high vacuum apparatus, figure 1, permits the ether such as tetrahydrofuran (THF), complications attainment of the rigorous experimental conditions arise as a consequence of the rapidity of the propaga­ necessary to prevent the termination of a reaction by tion reaction [27, 28]. In order to maintain homo­ the presence of contaminants. Therefore, all mono­ geneous reaction conditions, it is necessary to add mer and solvent purifications and polymerizations are monomer very slowly, either by distillation or dropwise carried out on the vacuum line, or in sealed evacuated addition, to a rapidly agitated solution. The presence vessels. The vacuum apparatus is composed of an t of trace impurities in the monomer will cause a oil pump (1) working in concert with a mercury dif­ broadening of the molecular weight distribution be­ fusion pump (2). In a leak-free system pressures in cause of continuous termination of chain ends through­ the range of 10- 5 to 10- 6 mm Hg are obtainable. A out the duration of the polymerization. This assumes liquid nitrogen trap (3) is used to condense any con­ a constant rate of replenishment of any impurity. densable gases so that the pressure registere·d on the Szwarc has shown [29] that when an anion deactivator McLeod gage (4) is exclusively that of noncondensable is present in the monomer at concentrations sufficient species. The nitrogen trap also prevents the passing to annihilate the entire initial polymercarbanion popu­ of mercury vapor from the diffusion pump into the lation by the time the monomer supply is exhausted, the main manifold (5). FIGURE 1. High vacuum apparatus. 422 --------- The pressure release (6) is a tube over 76 c m in length with one end connected to the vacuum appara­ .. tus and the other end submerged in a mercury pool. When substances with low boiling points, e.g., buta­ diene, ethylene oxide, etc., are used in vacuum systems I there exists the possibility that pressure may build up to the point of fracturing the apparatus or bJow in O" temporarily attached pieces of equipment off the lin e~ The purpose of the pressure release is to relieve s uch " pressure in the apparatus and still maintain an air­ ti ght system. Furthermore, the fall of the mercury column provides a vis ual indication of the onset of , such a pressure increase. a . The Main Manifald '" A manifold of this type consists of a 4 to 6 cm diam­ eter section of Pyrex glass tubing, clamped in place on a metal supporting frame. The vacuum stopcocks con­ necting the main manifold to the secondary one possess A stopper bores of 6 to 10 mm. The stopcocks placed in B the secondary m anifold are those with 4 mm stopper FIGURE 2. Monomer-solvent ampoules. bores. The connecting tubes of the secondary mani­ f?ld are .m~de of 12 mm diam heavy wall Pyrex tubing, sIn ce thI S IS about the largest size of glass tubin" that is practical in constructing a large and ri gid pi:ce of apparatus where proper ann ealing is impracti cal. Apiezon N hydrocarbon grease is recommended for Figure 2 depicts typical monomer-solvent ampoules. use in the main manifold whereas sili con O"rease should The break seals are usually those made from 12 mm be used in the secondary manifold s to~coc k s si nce diam tubing. A conve ni ent s ize of ground glass joint this lubricant is hi ghly resistant to the passage of ether is the 14/35 type.
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