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Biomacromolecules 2008, 9, 2072–2080
Complex Macromolecular Chimeras Anastasis Karatzas, Hermis Iatrou, and Nikos Hadjichristidis* Department of Chemistry, University of Athens, Panepistimiopolis, Zografou, 15771, Athens, 15771, Greece
Kyouichi Inoue, Kenji Sugiyama, and Akira Hirao* Polymeric and Organic Materials Department, Graduate School of Science and Engineering, Tokyo Institute of Technology, H-127, 2-12-1, Ohokayama, Meguro-ku, Tokyo, 152-8552, Japan Received March 28, 2008; Revised Manuscript Received May 2, 2008
By combining two living polymerizations, anionic and ring opening (ROP), the following novel multiblock multicomponent linear and miktoarm star (µ-star) polymer/polypeptide hybrids (macromolecular chimeras) were synthesized: Linear, PBLL-b-PBLG-b-PS-b-PBLG-b-PBLL; 3µ-stars, (PS)2(PBLG or PBLL), (PS)(PI)(PBLG or PBLL); 4µ-stars, (PS)2[P(R-MeS)](PBLG or PBLL), (PS)2(PBLG or PBLL)2 [PS, polystyrene; PI, polyisoprene; P(R-MeS), poly(R-methylstyrene); PBLG, poly(γ-benzyl-L-glutamate); and PBLL, poly(-tert-butyloxycarbonylL-lysine)]. The procedure involves (a) the synthesis of end- or in-chain amino-functionalized polymers, by anionic polymerization high vacuum techniques and appropriate linking chemistry and (b) the use of the amino groups for the ROP of R-amino acid carboxyanhydrides (NCAs). Molecular characterization revealed the high molecular weight and compositional homogeneity of the macromolecular chimeras prepared. The success of the synthesis was based mainly on the high vacuum techniques used for the ROP of NCAs, ensuring the avoidance of unwanted polymerization mechanisms and termination reactions.
Introduction Conventional block copolymers are able to self-assemble either in bulk (microphase separation) or in selective solvents (micellization) leading to different nanostructures.1 By replacing one or more blocks by polypeptide chains, the number of the self-assembled structures is increased, mainly because of the unique ability of polypeptides to organize into R-helix or β-sheet motifs.2 These polymer/polypeptide hybrid block copolymers are called molecular chimeras,3 a term borrowed from Greek mythology. Chimera (in Greek χιµRιFR) was an awesome firebreathing monster with the head of a lion, the body of a goat, and the tail of a serpent. Biosciences started using the term chimera long ago to describe an organism composed of tissues that are genetically different. The variety of the nanostructures could become even richer and more complex in multiblock multicomponent (e.g., terpolymers) or in nonlinear (e.g., miktoarm stars, µ-stars)4 chimeras. Therefore, the synthesis of well defined complex chimeras adds another dimension to the suprastructural hierarchy of the block copolymers with potential biomedical applications.5 The most common way to prepare polypeptides is via the ring-opening polymerization (ROP) of R-amino acid N-carboxyanhydrides (NCAs) using primary amines (e.g., n-hexylamine) as initiators. The same holds for hybrid-polypeptides, only that the initiating NH2 group is attached to a conventional macromolecule rather than to a small molecule. The livingness of the ROP of NCAs is necessary for the preparation of welldefined polypeptide-based materials in particular with complex architectures. * To whom correspondence should be addressed. Tel.: +302107274330 (N.H.); +81-3-5734-2131 (A.H.). Fax: +302107221800 (N.H.). E-mail:
[email protected] (N.H.);
[email protected] (A.H.).
Scheme 1. Complex Polymer/Polypeptide Hybrids (Macromolecular Chimeras) Synthesized in this Work
Attempts to synthesize well-defined living polypeptides with amino initiators have been plagued, for more than 50 years, by unwanted polymerization mechanisms and termination reactions.6 This problem has been overcome by the use of organonickel7 or ammonium chloride initiators8 and very recently by hexamethyldisilazane.9 To the best of our knowledge, only two examples of complex hybrid synthesis have been reported. The first involves the preparation of polypeptide-based linear multiblock multicomponent hybrids10,11 by applying the organonickel method, developed by Deming. The second addresses the synthesis of 3-miktoarm star hybrids, PS(PBLG)2, by a combination of ATRP and ROP.12,13 Recently, we have employed high vacuum techniques (HVT) to create living conditions for the amino-initiated ROP of NCAs.
10.1021/bm800316w CCC: $40.75 2008 American Chemical Society Published on Web 06/12/2008
Complex Macromolecular Chimeras Scheme 2. Flask for the Preparation of Amino-Macroinitiator Solution
Biomacromolecules, Vol. 9, No. 7, 2008
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(PS)(PI)(PBLG or PBLL) (3µ-stars); (PS)2[P(R-MeS)](PBLG or PBLL), (PS)2(PBLG or PBLL)2 (4µ-stars), where PS is polystyrene, PI is polyisoprene, P(R-MeS) is poly(R-methylstyrene), PBLG is poly(γ-benzyl-L-glutamate), and PBLL is poly(-tert-butyloxycarbonyl-L-Lysine).
Materials and Methods
Scheme 3. Reactor for the Synthesis of Polymer/Polypeptide Hybrids
Apparently with this method, the unwanted polymerization mechanism (activated monomer) is either avoided or is insignificant, thus favoring the desired normal amine mechanism.14 With this approach, a wide variety of both linear and star homoand copolypeptides, having controlled molecular and architectural characteristics have been synthesized in ∼100% yields.15 Subsequent studies on their self-assembly properties, in bulk and in water, revealed novel supramolecular structures.16–18 Our methodology was applied, along with anionic polymerization and using HVT, for the synthesis of the following novel complex macromolecular chimeras shown in Scheme 1: PBLLb-PBLG-b-PS-b-PBLG-b-PBLL (linear); (PS)2(PBLG or PBLL),
Materials. sec-Butyllithium (sec-BuLi, 1.4 M in heptane, Aldrich Japan) was used as received. Both styrene and R-methylstyrene, after washing with 5% aq NaOH and water, were distilled over CaH2 under reduced pressure and finally distilled in the presence of dibutylmagnesium (ca. 3 mol %) on a vacuum line. Isoprene, after washing with 5% aq NaOH and water, was distilled over CaH2 under nitrogen and finally distilled in the presence of n-butyllithium (ca. 1 mol %) at 0 °C on a vacuum line. 1-(3-Bromopropyl)-2,2,5,5-tetramethyl-aza-2,5disilacyclopentane (1; Aldrich) was distilled over CaH2 on a vacuum line. THF was refluxed over sodium wire, distilled over LiAlH4 under nitrogen, and then distilled from its sodium naphthalenide solution under high vacuum conditions (10-6 Torr). Sodium naphthalenide, the difunctional initiator, was synthesized from sodium and naphthalene according to our previous report.19 1-[4-(4-Bromobutyl)phenyl]-1phenylethylene (DC4Br),20 1,1-bis(3-chloromethylphenyl)ethylene (DCl2),21,22 and 1,4-bis(1-phenylethenyl)benzene (D2)23 were synthesized as previously reported. Synthesis of both chain-end-functionalized polyisoprene (PI-D) and in-chain-functionalized polystyrenes (PS-DPS) with 1,1-diphenylethylene moieties were described elsewhere.24–28 The synthesis of γ-benzyl-L-glutamate N-carboxyanhydride (Glu-NCA) and -tert-butyloxycarbonyl-L-lysine N-carboxyanhydride (Lys-NCA) was performed according to well-known literature procedures.14,15 After three crystallizations, under vacuum, the product was dissolved in purified N,N′-dimethylformamide, DMF, and the resulting monomer solution was transferred under vacuum to a sealed apparatus with precalibrated ampoules and was kept at -20 °C, until use. The polymerization solvent, DMF (
