Poly(Vinyl Alcohol) Block Copolymer: Preparation from a Bifunctional Initiator
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Polymer Journal, Vol. 36, No. 3, pp. 182—189 (2004) Amphiphilic Poly("-caprolactone)–Poly(vinyl alcohol) Block Copolymer: Preparation from a Bifunctional Initiator y Jin ZHOU, Akinori TAKASU, Yoshihito INAI, and Tadamichi HIRABAYASHI Department of Environmental Technology and Urban Planning, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan (Received September 8, 2003; Accepted December 15, 2003) ABSTRACT: Synthesis of a new block copolymer, Poly("-caprolactone)-block-Poly(vinyloxytriethylsilane) (PCL- b-PVOTES) was examined by using a bifunctional initiator 4-(2-Hydroxyethoxy)benzaldehyde (4-HEBA) or (5- Hydroxymethyl)furfural (5-HMF) which was responsive to both living ring-opening polymerization (ROP) and aldol-type group-transfer polymerization (Aldol-GTP). The structures of the resulting block copolymers were con- firmed by 1H NMR, IR, and size-exclusion chromatography (SEC). Desilylation of this PCL-b-PVOTES copolymer by acidic methanol resulted in an aimed amphiphilic block copolymer, namely Poly("-caprolactone)-block-Poly(vinyl alcohol) (PCL-b-PVA). KEY WORDS Poly("-caprolactone)-block-Poly(vinyloxytriethylsilane) / Poly("-caprolactone)- block-Poly(vinyl alcohol) / Bifunctional Initiator / Aldol-type Group-Transfer Polymerization (Al- dol-GTP) / Ring-Opening Polymerization (ROP) / Amphiphilic Block Copolymer / Recently, there are rapidly growing needs for bio- al.5 Concerning the biodegradability, David et al. degradable materials for various kinds of applications has reported that even the PCL-rich blends were not owing to people’s concerns with environmental prob- degraded by microorganisms from a compost of lems. Many scientists have extensively studied the house-hold refuse, while pure PCL films was com- synthesis and commercialization of aliphatic polyes- pletely assimilated over periods of 600–800 h.9 So ters mainly because of their potential for providing de- the synthesis of PCL-PVA block copolymer attracted gradability.1 Poly("-caprolactone) (PCL) is one of the our attention to expect that they would show different biodegradable polyesters responsible to use as both biodegradation behavior from the blends depending ecological polymer and biomedical polymer.1 Further- on the chain length of two blocks. more, PCL with a high molecular weight can be Usually, block copolymers are prepared by sequen- formed into fibers or blown films having similar me- tial addition of monomers to single active species or chanical properties with low-density polyethylene.2 by coupling reaction of the preformed functional However, PCL exhibits a melting temperature close polymers.10 In our previous work,11 we have succeed- to 60 C, representing a severe limitation for many ed in synthesis of Poly("-caprolactone)-block- daily applications. This weakness might be overcome Poly{vinyloxy(tert-butyl)dimethylsilaneg, (PCL-b- when a PCL is used together with polymers having PVOTBDMS), through a coupling method. Each high melting point and other excellent physical prop- block unit could be prepared by ring-opening poly- erties. When such hybrid (co)polymers are achieved, merization (ROP) of CL and aldol-type group-transfer the good biodegradability of PCL can be successfully polymerization (Aldol-GTP) of VOTBDMS, respec- applied in practice. tively. The aimed PCL-b-PVA copolymer could be Poly(vinyl alcohol) (PVA) is one of water soluble obtained by desilylation from PVOTBDMS unit in polymers and the only vinyl-type biodegradable syn- the original block copolymer. In the above case, how- thetic polymer, even though its biodegradation rate ever, two blocks were bound through a rather unstable under natural environmental conditions is rather semi-acetal structure. slow.3 PVA is extensively applied in biomedical field In order to obtain PCL-b-PVA copolymer with owing to its non-toxicity and excellent biocompatibil- more stable chemical bond, a new route will be inves- ity. As the simplest way, PVA is often selected to tigated in the present work. As illustrated in Scheme 1, blend with other biodegradable polymers for re- the new synthetic route involves two polymerization search.4–7 Especially, the flexibility and the hydrophi- processes started from a single initiator responsible licity of the brittle polyesters are expected to be im- to ROP of "-CL as well as Aldol-GTP of vinyloxytrie- proved after blending with PVA.8 The morphology thylsilane (VOTES). The PCL-b-PVA copolymer with of PCL/PVA blends has been studied by David et a stable chemical bond involving an aromatic ring can yTo whom correspondences should be addressed (E-mail: [email protected]). 182 Amphiphilic Poly("-caprolactone)–Poly(vinyl alcohol) Block Copolymer HOR CH O R: CH2 CH2 O R: H2C (4-HEBA) , O (5-HMF) Living ROP Aldol-GTP O CH =CH-OSiEt 2 3 O Route 2 Route 1 H O R CH O HO CH O R CH=O m n O O O Et3Si SiEt3 PCL-CH=O PVOTES-OH Masking with Aldol-GTP acetyl chloride CH2=CH-OSiEt3 CH CO O CH O Living ROP 3 O R O m O O Aldol-GTP CH2=CH-OSiEt3 O CH H/CH3CO R CH=O O n m O O O SiEt3 SiEt3 PCL-b-PVOTES Desilylation H/CH CO O R CH CH=O 3 O n m OH O OH PCL-b-PVA Scheme 1. Approach route for PCL-b-PVA copolymer started from a bifunct-ional initiator (4-HEBA or 5-HMF). be obtained after desilylation with little damage by of the two block segments, and new applications may side reactions. arise from these designed properties. Although it was well known that the best result in molecular weight regulation was achieved12 through EXPERIMENTAL Aldol-GTP of VOTBDMS as a monomer, the strong acid condition needed for desilylation of VOTBDMS Materials block often caused hydrolysis of ester linkages in PCL "-Caprolactone ("-CL) was dried over CaH2 and block. We used VOTES as monomer in this work for distilled under reduced pressure just before use. Vi- its compromise between the stability of silyl group nyloxytriethylsilane (VOTES) was synthesized ac- during polymerization and easiness for desilylation cording to the procedure reported by Jung and Blum,13 even though it showed no good regulation in molecu- dried over CaH2 for 24 h, and distilled under reduced lar weight as VOTBDMS. As bifunctional initiator, pressure just before use. Two initiators, 4-(2-hydro- aromatic aldehyde having hydroxyl group must be xyethoxy)benzaldehyde (4-HEBA) and (5-hydroxy- fit for our purpose. Two types of initiator, benzalde- methyl)furfural (5-HMF), as well as stannous octa- hyde and furfural derivatives are examined in this noate (Sn(Oct)2) as a catalyst were commercially work. Hydrophilicity, thermal behavior, and biodegra- available in highly pure state and used without further dation rate of the PCL-b-PVA copolymers must be purification. Zinc bromide (ZnBr2) was purified by controlled and balanced by changing the chain length sublimation just before use. Dichloromethane Polym. J., Vol. 36, No. 3, 2004 183 J. ZHOU et al. (CH2Cl2), diethyl ether (Et2O), tetrahydrofuran PCL synthesized above, polymerization of VOTES (THF), and toluene were purified by distillation before was carried out under Aldol-GTP conditions. A glass use. tube with a magnetic stirring chip was flamed and purged-and-filled with nitrogen for three times. Mac- Measurements roinitiator dissolved in CH2Cl2 and the solution of 1 H NMR spectra were measured by Bruker ZnBr2 in Et2O were charged in the glass tube cooled DPX200 (200 MHz) spectrometer. Number-average at À78 C under nitrogen atmosphere. Then VOTES molecular weight (Mn) and polydispersity index of was added into the mixture. The polymerization was the molecular weight (Mw=Mn) were determined by carried out at 0 C with stirring. After the predeter- size-exclusion chromatography (SEC) with a series mined time, a mixture of methanol and triethylamine of Tosoh G2000-, G3000-, G4000-, and G5000-HXL (1:1 vol) was added to make the catalyst inactive. columns, used THF as an eluent and calibrated with The reaction mixture was precipitated by methanol. polystyrene standards. IR spectra were recorded in The crude polymer was purified by extraction with KBr disk on a JASCO FT/IR-400 spectrometer. n-hexane so as to remove PVOTES homopolymer. The final product was dried under vacuum at ambient ROP of "-CL Initiated by 4-HEBA or 5-HMF temperature until a constant weight. Initiator 4-HEBA (or 5-HMF) and Sn(Oct)2 dis- solved individually in toluene were charged in a dried Preparation of PCL-b-PVA Copolymer by Desilyla- flask under nitrogen atmosphere. Then the calculated tion of PVOTES Block amount of "-CL monomer according to the predeter- One hundred milligrams of PCL-b-PVOTES co- mined molar ratio to initiator was added to the flask. polymer were dissolved in a mixture composed of The polymerization was carried out on an oil-bath at 10 mL of THF and 1 mL of MeOH. Then 1 N HCl 100 C. After a prescribed time, the polymer was pre- aqueous solution was added in 1–2 mol% of VOTES cipitated from n-hexane and dried in vacuo to a con- units. After 48–72 h, the precipitate was recovered stant weight. by pouring reaction solution into n-hexane, and dried in vacuo to a constant weight. Acetylation of Hydroxyl End Group of PCL In a flame dried three-neck flask, PCL dissolved in RESULTS AND DISCUSSION THF and excess amounts of triethylamine were added under nitrogen atmosphere. Then excessive acetyl ROP of "-CL Initiated from 4-HEBA or 5-HMF; Mak- chloride in THF was added dropwise to the above ing Formyl-Terminated Macroinitiators (PCL–CH=O) PCL solution under cooling on an ice bath. The reac- In order to synthesize PCL-b-PVOTES copolymer, tion mixture was kept stirring at room temperature for preparation of macroinitiator (PCL–CH=O) having 24 h under nitrogen stream, and then condensed to re- formyl group at its terminal was first investigated in move THF partly.