6993
Macromolecules 1994,27, 6993-6995
Communications to the Editor Bimetallic Samarium(II1)Initiators for the Living Polymerization of Methacrylates and Lactones. A New Route into Telechelic, Triblock, and “Link-Functionalized”Polymers’
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Lisa S.Boffa and Bruce M.Novak.*+ Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, and The Center for Advanced Materials, Materials Sciences Division, Lawrence Berkeley Laboratories, Berkeley, California 94720 Received July 28, 1994 Revised Manuscript Received September 14, 1994
Bifunctional polymerization initiators-those with two 0 10 20 30 40 SO 60 70 80 90 103 covalently linked active sites-provide a number of unique monomer :initiator ratio strategies in polymer synthesis. The use of bisinitiators Figure 1. Molecular weight 88 a function of the monomer: dates back to Szwarc, who first demonstrated the utility initiator ratio for the polymerization of methyl methacrylate of the styrene dianion in “living” diene and a-olefin initiated using 111. Polymerizations were run in THF at -78 O C . polymerizations.2 Metal-based bisinitiators are less comCatalyst concentrations ranged from 1.75 X to 2.0X 10-2 M. mon; however, systems based on titanium? nickel: and aluminum5have been used to prepare a number of unusual mechanism of the polymerization proceeds via the formapolymers. tion of a bisinitiator. According to our mechanism, the In addition to possessing an inherent advantage over samarocene catalyst undergoes one-electron transfer to a monometallic species in the preparation of triblocks and of MMA forming a radical anion. Subsequent molecule telechelics,bisinitiators offer the possibility of synthesizing dimerization of these radical anions gives a bimetallic macromolecules with special functionality in the center of bisenolate, which then acts to initiate the polymerization their backbone (“link-functionalized”polymers). We are (Scheme 1). Since the two metal centers are now linked, interested in these polymers because chemical alteration the resultant polymer has a molecular weight twice that of the incorporated groups, such as cross-linking or predicted for monometallic polymerization, and the cleavage, may present new synthetic routes for the initiator appears to be 550% efficient. Reductive dimerproduction of star polymers, networks, biodegradables, izations of unsaturated compounds by Cp*zSm and its and functionalized materials. THF solvates are well known,ll and their abilityto initiate In an elegant study, Yasuda has shown that organopolymerization of ecaprolactone by electron transfer has samarium(II1) compounds of the type Cp*zSm-R (where been recently reported.12 Cp* = CsMe5 and R = hydride, alkyl, alkylaluminum) Evidence in support of this mechanism for MMA function as initiators for the living polymerization of polymerizations was obtained by monitoring the molecular methyl methacrylate (MMA)? Well-controlled polymerweight of poly(methy1 methacrylate) (PMMA) produced izations and block copolymerizationsof acrylates, lactones, and ethylene by these compounds have also been r e p ~ r t e d . ~ with I11 versus the monomer to initiator ratio (Figure 1). As expe~tedM,(obs)~~ was near that predicted for a linked, We are currently working to develop new bisinitiating bisinitiated polymerization. Bimetallic catalyst efficienpolymerization systems based on the lanthanides and cies as high as 96% are seen with the rigorous exclusion report in this Communication that the compounds Cp*2of air and water. Living polymerizations at monomer to Sm(C4Phz)SmCp*za(I)and [C~*~S~(P-V~-CHZCHCH-)IZ~ initiator (MI)ratios higher than 9 0 1 can also be achieved; (11) polymerize E-caprolactone and methacrylates, respechowever, the extremely sensitive nature of the organosatively, from both chain ends, in a well-controlled, living marium species results in an increasing degree of molecular fashion. We also discuss generation of a bimetallic weight inflation from calculated values. The polydisperbisenolate samarium initiator in situ and report the sities of the PMMA produced are near 1.1 and are not synthesis of triblock copolymers prepared in this manner. dependent on the monomer to initiator ratio. The polymerization of methyl methacrylate by Cp*2To improve initiator efficiencies and to expand the SmTHF21° (111) and other lanthanide(I1) complexes has utility of these lanthanide complexes, we have prepared been noted by Yasuda.6a*bAlthough the degree of control and isolated preformed bissamarium complexes and of these catalysts is about the same as with trivalent complexes, very low initiator efficiencies (
