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MacromolecularChemistry and Physics

DOI: 10.1002/macp.201300674

Networks Based on Poly(α-methylene-δ-valerolactone- co -δ-valerolactone): Crosslinking Through Free-Radical Vinyl Copolymerization

Olesja Stöhr, Helmut Ritter *

Unsaturated polyesters are synthesized via ring-opening copolymerization of α-methylene-δ-valerolactone and δ-valerolactone. These polyesters 4a–c are mixed with ethyl methacrylate (EMA), 2-hydroxyethyl methacrylate (HEMA), and α-methylene-δ-valerolactone (α-MVL), respectively. Then, crosslinking is carried out by free radical polymerization initiated by an azo-initiator. A second glass transition is found with incorporation of HEMA and α-MVL. These fi ndings indicate the formation of phase-separated polyester blocks crosslinked with the poly(meth)-acrylic-segments, respectively poly(α-methylene-δ-valerolactone) segments.

O. Stöhr, Prof. H. Ritter Institute of Organic Chemistry and Macromolecular Chemistry, Heinrich-Heine-University of Duesseldorf, Universitaetsstrasse 1, 40225 , Duesseldorf , Germany E-mail: h.ritter@uni-duesseldorf.de

the ROP of α-MVL and the crosslinking with 2-hydroxye-thyl methacrylate (HEMA), ethyl methacrylate (EMA), and α-MVL.

2. Experimental Section

2.1. Materials

δ-Valerolactone was received from Evonik Industries, dried over calcium hydride, distilled under reduced pressure, and stored over 4 Å molecular sieves under an argon atmosphere. 2,2′-Azobis(2,4-dimethyl valeronitrile) (V-65) was purchased from Wako Pure Chemicals Industries. Tin(II) 2-ethylhexanoate (Sn(Oct) 2 ), EMA, and HEMA were purchased from Aldrich and used without further purifi cation.

2.2. Measurements

1 H NMR and 13 C NMR spectra were obtained using a Bruker AVIII-600 spectrometer operating at 600 MHz for proton and 150 MHz for carbon NMR spectroscopy, using CDCl 3 as a solvent. The chem-ical shift ( δ scale) relative to tetramethylsilane (TMS) was cali-brated using the deuterium signal of the solvent as an internal standard. Infrared (IR) spectra were recorded on a Nicolet 6700 Fourier transform infrared (FTIR) spectrometer equipped with a diamond single bounce attenuated total refl ectance (ATR) acces-sory at room temperature. Differential scanning calorimetry (DSC) measurements were performed using a Mettler Toledo DSC 822

1. Introduction

The ring opening polymerization (ROP) of various lactones has been extensively studied in recent decades. There are many publications describing the ROP in presence of sev-eral catalyst systems such as metals, [ 1–7 ] lipases, [ 8,9 ] ani-onically, [ 10 ] or cationically, [ 11 ] respectively. A broad use in biomedical products such as tissue engineering scaffolds, drug delivery systems, and sutures, [ 12,13 ] is claimed. How-ever, the lack of functionalities of the backbone is the main reason for limited practical application of such materials.

Up to now, alpha-methylenelactones were polymerized in respect to the methylene group via free-radical mech-anism. Alpha-methylenelactones can be regarded as a cyclic analog of classical methyl methacrylates. However, there are only a few publications dealing with unsatu-rated polyesters prepared from alpha-methylenelactones via ROP. [ 14,15 ]

Surprisingly, we did not fi nd any information in litera-ture about ROP of α-methylene-δ-valerolactone (α-MVL). In this work, we thus wish to report our fi ndings about

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3.99 (t, 3 J = 5.9 Hz, 2H, –O–C H 2 –), 3.63 (m, 2H, –C H 2 –OH), 2.25 (m, 4H, –O–C H 2 –CH 2 –, CH 2 C–C H 2 –), 1.77−1.70 (m, 2H, CH 2 C–CH 2 –C H 2 –), 1.66−1.54 (m, 4H, –O–CH 2 –C H 2 –C H 2 –CH 2 –) Content of α-MVL in copolymer is about 4a ) 18 mol%, 4b ) 14 mol% and 4c ) 6 mol%; 13 C NMR (150 MHz, CDCl 3 , 20 °C, δ ): 173.6, 167.2, 139.9, 125.6, 64.6, 64.2, 62.5, 34.0, 28.8, 28.4, 27.6, 21.7; FTIR (dia-mond): ν = 2955, 2873 (m, CH 2 , CH), 1724 (s, C O, ester), 1634 (w, C C), 1134 cm −1 (s, C–O); SEC (THF): nM ( 4a ) = 3100 g mol −1 , D ( 4a ) = 1.69, nM ( 4b ) = 6300 g mol −1 , D ( 4b ) = 1.65, nM ( 4c ) = 4300 g mol −1 , D ( 4c ) = 1.64; T m( 4a ) = 17.1 °C, T m( 4b ) = 34.4 °C, T m( 4c ) = 40.9 °C.

2.5. Crosslinking of Poly(α-methylene-δ-valerolactone- co -δ-valerolactone) with 2-Hydroxyethyl Methacrylate

General Procedure : 0.5 g of poly(α-methylene-δ-valerolactone- co -δ-valerolactone) 4 and 4.5 g of HEMA 6 were added to a round-bottom fl ask equipped with a septum and purged with argon for 20 min. After the polyester dissolved completely, 1 wt% radical initiator 2,2′-azobis(2,4-dimethyl)valeronitrile was added to the mixture. The reaction mixture was injected to a mold and heated for 24 h at 60 °C. The resulting crosslinked polymer was dried for 24 h in vacuum to remove residual monomer (50 °C and 8 × 10 −2 mbar). DSC: T g(1) = 29.7 °C, T g(2) = 107.1 °C.

2.6. Crosslinking of Poly(α-methylene-δ-valerolactone- co -δ-valerolactone) with Ethyl Methacrylate

General Procedure : 0.5 g of poly(α-methylene-δ-valerolactone- co -δ-valerolactone) 4 and 4.3 g of EMA 5 were added to a round-bottom fl ask equipped with a septum and purged with argon for 20 min. After the polyester was dissolved completely, 1 wt% radical initiator 2,2′-azobis(2,4-dimethyl)valeronitrile was added to the mixture. The reaction mixture was injected to a mold and heated for 24 h at 60 °C. The resulting crosslinked polymer was dried for 24 h in vacuum to remove residual monomer (50 °C and 8 × 10 −2 mbar). DSC: T g = 71.9 °C

2.7. Crosslinking of Poly(α-methylene-δ-valerolactone- co -δ-valerolactone) with α-Methylene-δ-valerolactone (α-MVL)

General Procedure : 0.5 g of poly(α-methylene-δ-valerolactone- co -δ-valerolactone) 4 and 4.3 g of α-MVL 3 were added to a round-bottom fl ask equipped with a septum and purged with argon for 20 min. After the polyester was completely dissolved, 1 wt% radical initiator 2,2′-azobis(2,4-dimethyl)valeronitrile was added to the mixture. The reaction mixture was injected to a mold and heated for 24 h at 60 °C. The resulting crosslinked polymer was dried for 24 h in vacuum to remove residual monomer (50 °C and 8 × 10 −2 mbar). DSC: T g(1) = 10.9 °C, T g(2) = 79.6 °C.

3. Results and Discussion

3.1. Synthesis of Unsaturated Polyesters

The ring-opening copolymerization of α-MVL 3 was carried out using tin(II)-2-ethylhexanoate (Sn(Oct) 2 ) as a catalyst

controller apparatus equipped with a sample robot TSO801RO. The apparatus was controlled over a temperature range between −40 and 250 °C at a heating rate of 20 K min −1 . Gas chromato-graphy/mass spectrometry (GC/MS) measurement was accom-plished on a Thermo Finnigan Trace DSQ system. The ionization occurred by means of electron impact (EI). Gel permeation chro-matography (GPC) was performed with a fl ow rate of 1 mL/min at room temperature and THF (HPLC Grade, unstabilized – Bio-solve) as eluent. The GPC System compromises a Scharmbeck SFD degasser (Gastor BG12), a FLOW pump (Intelligent PUMP AL-12) and a Scharmbeck SFD (Model S5200) sampler. For detection, a Waters 486 tunable absorbance detector and a Schambeck SFD RI 2000 detector were used. A set of columns packed with porous styrene-divinylbenzene-copolymer beads was used for separation of the analytes (MZ Analysentechnik GmbH, 1× guard column 100 Å, 3× columns with 10 000, 1000, and 100 Å). The system was calibrated with polystyrene standards with a molecular range from 575 g mol −1 to 3 114 000 g mol −1 . Toluene was added to the analytes as an internal standard.

2.3. Synthesis of α-Methylene-δ-valerolactone

General Procedure : [ 16 ] To a stirred suspension of sodium hydride (4.8 g of 60% oil dispersion, 200 mmol) in 250 mL of diethyl ether under argon, absolute ethanol was slowly added (1.17 mL, 20 mmol), immediately followed by a mixture of δ-valerolactone (18 g, 180 mmol) and ethyl formate (16.3 g, 220 mmol). The mix-ture was added over 1 h to give a steady refl ux and evolution of H 2 . After the addition was completed, the suspension was stirred for additional 1 h. After cooling to room temperature, the resulting solid was fi ltered and washed with diethyl ether several times and dried in vacuum. Paraformaldehyde (27.3 g, 900 mmol) was added to the solid in 400 mL of dry THF under argon. The suspension was heated immediately for 1 h. The sus-pension was cooled down to 10 °C using an ice-water bath and quenched with aqueous sat. K 2 CO 3 solution (100 mL). The organic layer was separated and the aqueous layer was extracted three times with diethyl ether. The organic fractions were combined, washed with brine, dried, and evaporated to dryness to afford the crude lactone. Purifi cation occurred by fl ash chromatography (ethyl acetate/n-hexane 1:5) to yield (57%) of colorless liquid.

1 H NMR (600 MHz, CDCl 3 , 20 °C, δ ): 6.40 (q, J = 1.8 Hz, 1H), 5.54 (q, J = 1.9 Hz, 1H), 4.41–1.33 (m, 2H), 2.68–2.62 (m, 2H), 1.97–1.90 (m, 2H); 13 C NMR (150 MHz, CDCl 3 , 20 °C, δ ): 165.3, 134.0, 127.9, 69.5, 27.9, 23.0; FTIR (diamond): ν = 2956, 2856 (m, CH 2 , CH), 1713 (s, C O, ester), 1624 cm −1 (C C); GC/MS: (EI) m/z: [M + ] = 112.

2.4. Synthesis of Poly(α-methylene-δ-valerolactone- co -δ-valerolactone) 4a–c

General Procedure : To the mixture of α-MVL and δ-valerolactone (ratio 1:3, 1:5, and 1:9) in a round-bottom fl ask fl ushed with dry argon, 1 mol% of Sn(Oct) 2 was added as a catalyst. The fl ask was immersed in oil bath at 130 °C for 72 h. After cooling down to room temperature, the reaction mixture was dissolved in 5 mL chloro-form, precipitated twice in a cold n-hexane, and dried in vacuum. Yield ( 4a 53%, 4b 76%, 4c 88%). 1 H NMR (600 MHz, CDCl 3 , 20 °C, δ ): 6.16 and 5.55 (each s, C H 2 ), 4.08 (d, 2 J = 5.7 Hz, 2H, O–C H 2 –),

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to unchanged composition of incorporated monomer units. Principally, the exo-vinylidene can polymerize radi-cally. The results of GPC measurements show molecular weights nM of the copolymers 4a – c in the range of 3000 up to 7000 g mol −1 .

DSC analysis was performed to show assumed infl u-ence of methylene function of 3 on the crystallinity of the copolyester chains 4a–c (Table 1 ). As expected, the incorporated double bond reduces the van der Waals and dipole–dipole interaction between the polyester chains due to steric effects. The higher the amount of 3 in copolymers 4a–c the lower the melting point of the copolymers. Accordingly, polyesters 4a–c show an endo-thermic melting peak between 17 °C (18 mol% 4a ) up to 41 °C (6 mol% 4c ). However, the low melting point of the copolymer 4a at 17 °C is the reason for soft, elastic proper-ties at room temperature.

4. Free-Radical Crosslinking of Copolyesters

The double bond incorporated into the copolyester 4b can be used for further modifi cations. Thus, we used the exo-vinylidene groups of the copolyester 4b for free radical

(Figure 1 ) yielding the copolyesters 4a– c . The molar con-tent of 3 incorporated into the unsaturated copolyester 4a – c was calculated by integration of the 1 H NMR signals of the double bond of 3 (singulets at δ = 5.55 ppm and 6.16 ppm, Figure 2 ) in comparison to the integrated signals of the hydrogen atoms of the methylene group of 1 (triplet at 3.99 ppm, Figure 2 ) in the copolymer. As expected, the exo-vinylidene group remained intact after ROP. This cor-responds to the fact that a quantitative yield of ROP leads

Figure 1. a) Synthesis of α-methylene-δ-valerolactone α-MVL [ 16 ] and b) ring-opening copolymerization of α-methylene-δ-valerolactone 3 with δ-valerolactone 1 .

O

O

1. NaH

2. HCOOC2H5

Et2O

O

OO Na

O

O

H2CO

THF

HO O

O

m

O

On

H

1 2 3

4

O

O

1

O

O

3

mn +Sn(Oct)2

a)

b)

Figure 2. 1 H NMR spectrum of poly(α-methylene-δ-valerolactone- co -δ-valerolactone) 4b in CDCl 3 .

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copolymerization with (meth) acrylates EMA 5 and HEMA 6, and α-MVL 3 to yield novel hybrid networks (Figure 3 ). HEMA 6 and EMA 5 were chosen because of their structural similarity to α-MVL 3 . Principally, the elemental composi-tion of HEMA 6 only differs from α-MVL 3 through 2H- and 1O-atoms, respectively. Correspondingly, the atomic com-position of EMA 6 , differs from 3 only through 2H-atoms (Figure 4 ). The obtained hybrid network (i) is not crystalline ( T g = 71.9 °C) (Table 2 ) and the brittle material is absolutely transparent (Figure 5 ). However, the networks (ii) and (iii) show some crystalline properties, which is the reason for their turbidity (Figure 5 ). Additionally, the hybrid network (ii) shows two glass transition temperatures at T g = 10.9 °C (copolyester 4b ) and T g = 79.6 °C (poly(HEMA)). The values of hybrid network (iii) were found at T g = 29.7 °C (copoly-ester 4b ) and T g = 107.1 °C for (poly(α-MVL)). This fi nding indicates that the matrix type polyester phase separates from poly(HEMA) and poly(α-MVL), respectively. These

Table 1. Results of the copolymerization of α-MVL 3 and 1 at 130 °C for 72 h.

Entry Molar feed ratio [α-MVL]:[VL]

Incorporation molar ratio a) α-MVL:VL [mol%]

nnMM b) [g mol −1 ]

D b) T m c) [°C]

4a 1:3 18:82 3.1 × 10 3 1.69 17.1

4b 1:5 14:86 6.3 × 10 3 1.65 34.4

4c 1:9 6:94 4.3 × 10 3 1.64 40.9

a) Calculated from 1 H NMR spectroscopy data; b) Determined by SEC in THF relative to polystyrene standards; c) DSC measurements third heating cycle −50 to 180 °C; 10 K min −1 .

Figure 3. Scheme of architecture of the networks. Crosslinked copolyester 4 with (i) EMA 5 , (ii) α-MVL 3 and (iii) HEMA 6 using V-65 as initiator.

Figure 4. Formal transformation of α-MVL to HEMA and EMA.

Figure 5. Photographs of the disc-shaped polymer samples.

Table 2. Composition of the hybrid networks and the DSC char-acterization data.

Entry 4b wt% 3 wt% 5 wt% 6 wt% T g(1) a) [°C]

T g(2) a) [°C]

(i) 10 90 – – 71.9 –

(ii) 10 – 90 – 10.9 79.6

(iii) 10 – – 90 29.7 107.1

a) DSC measurements third heating cycle −50 to 150 °C; 30 K min −1

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phase-separated materials are not transparent because of different refractive index of the polymer components led to visible light scattering.

5. Conclusion

For the fi rst time, α-MVL was copolymerized with δ-valerolactone via ROP yielding crosslinkable unsaturated polyester. These polyesters were crosslinked with HEMA, EMA, and α-MVL, respectively. The obtained material is fully transparent in the case of EMA 5 as crosslinker. In contrast to that, the hybrid-networks (ii) and (iii) are not transparent. Since a second glass transition was found with incorporation of HEMA 6 and α-MVL 3, respectively, a phase-separated polyester-block is responsible for the tur-bidity of the hybrid-networks.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Received: October 30, 2013 ; Revised: December 18, 2013; Published online: January 29, 2014; DOI:10.1002/macp.201300674

Keywords: crosslinking reaction; hybrid networks; α-methylene-δ-valerolactone; ring opening copolymerization; unsaturated polyester

Macromol. Chem. Phys. 2014, 215, 426−430

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