Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Synthesis of Highly Defined Graft Copolymers Using a Cyclic Acetal Moiety as a Two-Stage Latent Initiating Site for Successive Living Cationic Polymerization and Ring-Opening Anionic Polymerization Norifumi Yokoyama, Arihiro Kanazawa, Shokyoku Kanaoka, and Sadahito Aoshima* Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan S Supporting Information *
ABSTRACT: The synthesis of well-defined graft copolymers with designed intervals between branches was achieved using cyclic acetal moieties as two-stage latent initiating sites. A cyclic acetal was shown to initiate the living cationic polymerization of vinyl ethers (VEs), yielding a polymer with a hydroxy group at the α-end derived from the cyclic acetal. The newly generated hydroxy group was able to efficiently induce the subsequent ring-opening anionic polymerization of L-lactide (LLA), and a diblock copolymer with a narrow molecular weight distribution (MWD) was obtained. For the synthesis of a graft copolymer, a five-membered cyclic acetal moiety was introduced at the ω-chain ends of poly(VE)s, which was employed as the initiating site for the living cationic polymerization of VEs. Repeated polymerization and acetalization generated a macroinitiator with several hydroxy groups on the side chain of a poly(VE) backbone. Graft copolymers possessing branches with narrow MWDs and regular spaces between the branches were synthesized by the ring-opening polymerization of LLA using this macroinitiator.
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“grafting-through” method often suffers from low conversion of macromonomers due to the steric hindrance. Bottlebrush polymers can also be synthesized using this method.15 Recently, the synthesis of graft copolymers with highly defined structures has been achieved through both the elaborate design of synthetic strategies and the development of living polymerization reactions. The structure of a graft copolymer is defined by the following three parameters: (i) MW of the backbone chain, (ii) MW of the branched chains, and (iii) number of and distance between branching points. Highly precise control of these three parameters is critical to attaining desired functions. For example, slight differences in grafting positions have been reported to influence the morphology of graft copolymers in the bulk state.16 Motivated by these results, Paraskeva and Hadjichristidis17 achieved the synthesis of the “exact graft copolymer”, a graft copolymer consisting of a precisely defined backbone and branch structures, by combining living anionic polymerization and repetitive polymer coupling reactions. In addition, Hirao and co-workers18−24 developed synthetic methods for graft and star-shaped copolymers with precisely defined structures using repeated cycles of living anionic polymerization and postpolymerization reactions. For the precision synthesis of well-defined graft copolymers with precisely defined backbone and branch structures, the reactive sites of the prepolymers used in the synthesis must (1)
INTRODUCTION Graft copolymers have been studied as model branched polymers in terms of their properties in solution, such as the critical micelle concentration or viscosity, and in the bulk state, including their morphology.1 Recently, graft copolymers have attracted much attention for use in commercial applications as adhesives and as compatibilizing agents owing to the characteristics derived from their branched compact structure and many terminal functional groups.2 For the synthesis of graft copolymers, a variety of precise synthetic methods using prepolymers have been developed in recent years utilizing living/controlled polymerization techniques,3,4 while conventional studies have mainly employed transfer reactions of propagating species for branch formation.5 The methods are classified into three main strategies: “grafting-from”, “graftingonto”, and “grafting-through”. In the “grafting-from” method, a polymer with initiating sites on the side chains is used as a macroinitiator for the generation of graft chains. Graft copolymers are obtained in high yield without residual unreacted prepolymer because of the minimal steric hindrance of other graft chains.6,7 In addition, the “grafting-from” method can generate highly dense graft polymers like polymer brush.8,9 Graft copolymers consisting of a backbone and branch chains with well-controlled molecular weights (MWs) are synthesized by the “grafting-onto” method because graft copolymers are prepared by direct reaction between living linear polymers and a well-defined backbone polymer.10,11 In the “grafting-through” method, a polymer with a pendant polymerizable unit, or a macromonomer, is polymerized to prepare graft copolymers possessing well-defined graft chains.12−14 However, the © XXXX American Chemical Society
Received: December 9, 2017 Revised: January 17, 2018
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DOI: 10.1021/acs.macromol.7b02622 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 1. Synthetic Routes of Three-Arm Star-Shaped Polymers and Highly Defined Graft Copolymers
in Scheme 1) and subsequent anionic ring-opening polymerization of LLA (Scheme 1, top). Moreover, a polymer containing two or three hydroxy groups in the side chains, which was obtained by repeatedly introducing a cyclic acetal moiety followed by living cationic polymerization, was used as a macroinitiator in the synthesis of graft copolymers with exact numbers of branch chains and controlled chain lengths (Scheme 1, bottom).
have sufficiently high reactivity, (2) remain intact throughout the isolation and purification procedures, and (3) be introduced quantitatively into the prepolymer. In this regard, acetal moieties are possible candidates as reactive sites in prepolymers because of their stability under most nonacidic conditions and the simplicity of their introduction into polymers. In cationic polymerization, acetal compounds have been used as effective initiators of the living polymerization of vinyl ethers (VEs).25−27 Recently, our group demonstrated that quantitative initiation reactions from an acyclic acetal proceeded in the living cationic polymerization of VEs28 or styrene derivatives29 using oxophilic metal chlorides such as TiCl4. This method was applicable to the synthesis of block or graft copolymers derived from monomers with completely different reactivities using a well-defined macroinitiator containing acetal moieties. Additionally, hydroxylation of the α-end without a postpolymerization reaction was achieved using a cyclic acetal with trimethylsilyl iodide (TMSI) in the cationic polymerization of VE.25 Thus, cyclic acetals with oxophilic Lewis acids may also initiate the living cationic polymerization of VEs or styrene derivatives, yielding well-defined polymers with a hydroxy group at the α-end. In this study, graft copolymers with precisely defined structures were synthesized through a combination of the living cationic polymerization of VEs and ring-opening anionic polymerization of L-lactide (LLA) using cyclic acetal moieties as “two-stage latent initiating sites”. Polylactide is expected to be applied in various fields because of its characteristics such as crystallinity and good biocompatibility.30−32 First, the cationic polymerization of isobutyl VE (IBVE) was examined using several cyclic acetals as initiators. Subsequently, the synthesis of block copolymers through the anionic ring-opening polymerization of LLA was examined using poly(IBVE) that contained a hydroxy group at the α-end, which was obtained by polymerization with a suitable cyclic acetal as a macroinitiator. In addition, a three-arm star-shaped polymer was prepared using the cyclic acetal moiety that had been introduced at the ω-end of poly(IBVE) (A in Scheme 1) as a two-stage latent initiating site for the living cationic polymerization of IBVE (B
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EXPERIMENTAL SECTION
Materials. IBVE (TCI; >99%), isopropyl VE (IPVE; Wako; >97.0%), 2-methyl-1,3-dioxolane (MDOL; TCI; >98.0%), 1,3dioxolane (DOL; Wako; >99.0%), and 2,2-dimethyl-1,3-dioxolane (DMDOL; TCI; >98.0%) were distilled twice over calcium hydride. 2Benzyl-1,3-dioxolane (BnDOL; TCI; >98.0%), 2,6-di-tert-butylpyridine (DTBP, Aldrich; 97%), and 1,8-diazabicyclo[5.4.0]-7-undecene (DBU; TCI; >98.0%) were distilled twice over calcium hydride under reduced pressure. 2-Methyl-1,3-dioxane (MDOX) was prepared by reacting paraldehyde (TCI; >98.0%) with 1,3-propanediol (TCI; >98.0%) according to a previously published method33 and was then distilled twice over calcium hydride. L-(−)-Lactide (LLA; TCI; >98.0%) was recrystallized twice from ethyl acetate and toluene and vacuum-dried for more than 3 h prior to use. Ethyl acetate (Wako; >99.5%) was dried overnight over 3A and 4A molecular sieves and distilled twice over calcium hydride. Toluene (Wako; >99.5%), dichloromethane (CH2Cl2; Wako; >99.0%), and hexane (Wako; 96.0%) were dried using solvent purification columns (Glass Contour; Solvent Dispensing System). The adduct of IBVE with acetic acid [IBEA; CH3CH(OiBu)OCOCH3] was prepared from the addition reaction of IBVE with acetic acid according to the literature method.34 LiBH4 (Aldrich; 2.0 M solution in tetrahydrofuran) was used as received. TiCl4 (Aldrich; 1.0 M solution in toluene) and EtAlCl2 (Wako; 1.0 M solution in hexane) were used without further purification. A stock solution of ZrCl4 in ethyl acetate was prepared from anhydrous ZrCl4 (Aldrich; 99.99%). Imidazole (Nacalai Tesque; 99%) and tert-butyldimethylchlorosilane (tBuMe2SiCl; Nacalai Tesque; ≥98%) were used as received. All chemicals, except for toluene, dichloromethane, hexane, imidazole, and tBuMe2SiCl, were stored in brown ampules under dry nitrogen. Polymerization Procedure. Cationic Polymerization of IBVE Using MDOL as an Initiator. The polymerization was performed under a dry nitrogen atmosphere in a glass tube with a three-way B
DOI: 10.1021/acs.macromol.7b02622 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Mn = 577−1.09 × 106, Mw/Mn ≤ 1.1). The absolute weight-average MW was determined using a GPC system composed of a pump (Viscotek VE 1122), two polystyrene gel columns [TSKgel GMHHRM × 2 (exclusion limit MW = 4 × 106; bead size = 5 μm; column size = 7.8 mm i.d. × 300 mm); flow rate = 0.7 mL/min], and a Viscotek TDA 305 triple detector [refractive index, laser light scattering (λ = 670 nm; 90° RALS and 7° LALS), and differential pressure viscometer] (eluent: THF). NMR spectra were recorded using a JEOL JNM-ECA 500 spectrometer (500.16 MHz for 1H). Matrixassisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was conducted on a Shimadzu/Kratos AXIMACFR spectrometer (linear mode; voltage, 20 kV; pressure,
