Brief Review of Advances in Developments of Living Polymers by Michael Szwarc Hydrocarbon Research Institute, University of Southern California テトラヒドロフランのような極性非プロトン溶媒の中でナトリウムナフタリンのラジカルアニオンからス チレンモノマーへの1電子移動,それに続くスチレンのラジカルアニオンの2量化を開始反応とするリビン グアニオン重合が発見され,分子量のそろった単分散ポリスチレンやブロックコポリマーの合成はリビング アニオン重合によって可能となった.それから35年が過ぎたが,ビニルモノマーのカチオン重合法による リビングポリマー,あるいは金属ポルヒリンを用いた環状エーテル類のリビングポリマーの合成など,リビ ングポリマーの合成は大きく発展してきた.ここではリビングポリマーの開発の進歩について述べる. Polymerization is referred to as living when the free of homo polymers, were the only products. growing polymers retain their capacity of growth Thus, a novel method of synthesis of well character for a time sufficiently long to allow the operator to ized block-polymers was discovered. complete a contemplated task, i.e. when the reaction Synthetic advantages of living polymerization is propagated with virtual exclusion of termination and chain transfer. It is beneficial if all the polymer- Living polymerization provides synthetic polymer ic molecules start their growth simultaneously, chemists with powerfull tools for design polymer because then the resulting polymers are of uniform chains of requested specification, namely : size. (1) A control of the molecular mass of the for- The feasibility of such a polymerization was med polymers and a method of producing polymers demonstrated 35 years ago by simple and most of uniform size. convincing experiments' described here briefly. The (2) A new technique for preparing block- reactor, shown in Fig. 1, contained 60 ml of-3 .10' polymers of desired sequence of blocks of requested M THF solution of sodium naphthalenide (the initi- size and composition with exclusion of homo- ator) to which 10 gr. of styrene was added by polymers. crushing a breakseal. The polymerization ensued instantly and was over in seconds. The viscosity of the resulting solution was estimated by the time of fall, —5 s, of a plunger placed in a side tube filled with the polymer solution by tilting the reactor. After a while, additional 10 gr of styrene in 60 ml of THF was added. The subsequent polymerization was over again in seconds and the viscosity of the solution increased, as indicated by the increased of the time of fall of the plunger to 20 s. Since the concentration of the polymer did not change, this proved that the previously formed polymers retained their activity and grew longer on the addi- tion of the monomer. Repetition of these experi- ments using isoprene as a second monomer Equipment for demonstrating living character confirmed these conclusions since block-polymers, of polymerization. 798高 分 子40巻12月 号(1991年) (3) A method of attachment of desired func- butadiene-styrene-butadiene nor the di-block poly- tional groups to one or both ends of a polymer chain mers behave in this uncommon way. Similar prop- since the addition of a proper reagent easily con- erties are claimed for the cationically prepared verts the active end of a living polymer into a indene-iso-butene-indene ter-block polymers'. desired functional group''. Films of di-blocks of styrene and butadiene of a The control of molecular mass of the produced sufficiently high molecular mass have irridescent polymers is of obvious advantage. Living polymers colors', although the homo-polymers are colorless. can be "fed" by monomer until they attain the The color depends on the molecular mass of the desired size. Their number average momecular blocks and changes reversibly on touching the film, mass increases proportionally with conversion, pro- This phenomenon results from the incompatibility vided that all of them begin their growth simultane- of the blocks which, being bonded and unable to ously. This requirement is met when the rate of separate, form paralel lamellae 1000 A thick. initiation is faster of or equal to the rate of propaga- These act as an optical grating producing interfer- tion. Alternatively, polymers of uniform molecular ence colors. Such an effect is not shown by the size are formed by feeding with monomer the previ- blends of the homo-polymers since being not linked ously prepared "monomeric" or oligomeric living by chemical bond they can separate into large irreg- polymers. For example, cumyl potassium, a living ular domains. monomeric "poly-a-methylstyrene", grows to a liv- Another example revealing the effect of linking ing n-meric poly- a-methylstyrene on addition of incompatible blocks by chemical bond is shown by a-methylstyrene. The complex of 1-phenylethyl the di-block films casted from good solvents for one chloride with tin tetrachloride, CH3CH (Ph) Cl • block but poor for the other. The same styrene- SnC14 is a living monomeric "polystyrene"' yielding butadiene di-block forms a flexible film when casted on feeding with styrene a living n-meric--- from a solvent good for polybutadiene but bad for CH,CH (Ph) Cl SnCl., polystyrene. The metallated poly-styrene, whereas a glassy film is formed in the iso-butyric ester, (CH3) 2C (COOCE13) , Cat+, is a reverse case. In the former solvent polystyrene living monomeric "polymethyl methacrylate"4 form- blocks form granulae in a continuous matrix of the ing on the addition of the monomer a living-- flexible poly-butadiene, whereas polystyrene is the CH2C- (CH,) (COOCH3) , Cat+ polymethyl metha- continuous phase in the latter solvent. crylate. Nuyken5 used recently a stable solution of Many di-block polymers are effective emulsifying CH3CH (OR) I complexed with quaternary ammo- agents, especially when composed by hydrophilic nium salt to produce, by feeding it with monomer, a and hydrophobic blocks. Graft and comb-shape similarly complexed living iodide of polyvinyl ether, polymers are prepared by a modified living polymer etc. In all these cases the resulting polymers are of technique', and many of them are most useful as uniform size since all of them start their growth adhesive and elastomers. Similar procedures' lead simultaneously. to star-shape polymers valuable in control of Living polymerization is unique in allowing prep- rheological properties of lubricants and as an agent aration of block polymers of desired architecture, strengthening various polymeric materials. free of homo-polymers. The styrene-butadiene- Much interest is shown nowaday in well defined styrene is a commercial ter-block polymer produced macro-cyclic polymers prepared by linking the by living anionic technique having the interesting active ends of hi-functional living polymers with and useful thermo-plastic properties. It is moldable appropriate "linking" agents, e.g. di-carbanions of at higher temperatures but behaves as a tough living linear polystyrene converted into rings by a, cross-linked rubber at ambient conditions'. This w-dibromo-p-xylene or dimethyldichlorosilane'°. remarkable and reversible change of its properties is Their rheological behavior differs drastically from caused by the formation of rigid micro-spheres of that of linear polymers of the same degree of poly- glassy polystyrene on cooling the melt, and these act merization, for example their respective glass as giant cross-links. Significantly, neither the temperarature decreases with the degree of poly- 高分子40巻12月 号(1991年)799 merization" whereas it increases for linear poly- their rate of propagation is slown down on the mers. Significantly, it makes a difference whether addition of crown ethers or cryptates" that convert cyclic polymers of Mn-10,000 dalton are linked by the tight pairs into the loose ones. The retarding ortho rather than by para-dibromoxylene". effect of these powerful cation solvating agents Numerous functional polymers were prepared indicates the participation of push-pull effect, the from living anionic or cationic polymers. The prob- anion and cation cooperate in the monomer addition lems encountered in the preparation of functional process. polymers by anionic technique were discussed in a A more convincing evidence for the operation of recent review by Quirk". Novel kind of functional push-pull mechanism is provided by the reaction of polymers, referred to as macromers, were reported ethylene oxide with anions of substantially delocal- 11 years ago". Such polymers are terminated by ized charge. The undissociated ion-pairs of 9- units acting as monomers, e.g., methylfluorenyl" or fluoradenyl" cleave the oxir- ane faster than their free ions, the addition of ----CH=CHPh or --CH=C(CH3)C0 OCH3, cryptates retards the reaction, whereas the addition and could be copolymerized with small monomers of common cation salts, that convert the free anion yielding polymers with predetermined branches. into ion-pairs, accelerates its course. Polymers endowed with two functional end-groups, Living anionic polymerization of polymethyl known as telechelics, are most useful as chain exten- methacrylates is more complex, the active anions tion agents. For example, the diols formed by the form allylic-enolates solvated by penultimate poly- cationic ring-opening polymerization of cyclic mer units". Consequently, two centers may be ethers are used in preparation of urethane foams. involved in the propagation : the C- or the 0-. The Interesting amphilic gels were prepared by polymer- participation of 0- leads to some side reactions izing acrylic monomers with bi-macromers of which destroy the living character of this polymer- cationicaly produced living polyisobutene terminat- ization, especially in low polarity solvents. These ed on both ends by acrylic units. Such gels swell in undesired reactions are eliminated, or diminished, water and in organic solvents", and the character of by the