Reactivity of Vinyl Ethers and Vinyl Ribosides in UV-Initiated Free

Jul 28, 2010 - Université de Reims Champagne Ardenne, Institut de Chimie Moléculaire de ... Moulin de la Housse, BP 1039, 51687 Reims Cedex 2, Franc...
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Biomacromolecules 2010, 11, 2415–2421

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Reactivity of Vinyl Ethers and Vinyl Ribosides in UV-Initiated Free Radical Copolymerization with Acceptor Monomers Loic Pichavant, Ce´line Guillermain, and Xavier Coqueret* Universite´ de Reims Champagne Ardenne, Institut de Chimie Mole´culaire de Reims, CNRS UMR 6229, Moulin de la Housse, BP 1039, 51687 Reims Cedex 2, France Received May 30, 2010; Revised Manuscript Received July 9, 2010

The reactivity of various vinyl ethers and vinyloxy derivatives of ribose in the presence of diethyl fumarate or diethyl maleate was investigated for evaluating the potential of donor-acceptor-type copolymerization applied to unsaturated monomers derived from renewable feedstock. The photochemically induced polymerization of model monomer blends in the bulk state was monitored by infrared spectroscopy. The method allowed us to examine the influence of monomer pair structure on the kinetic profiles. The simultaneous consumption of both monomers was observed, supporting an alternating copolymerization mechanism. A lower reactivity of the blends containing maleates compared with fumarates was confirmed. The obtained kinetic data revealed a general correlation between the initial polymerization rate and the Hansen parameter δH associated with the H-bonding aptitude of the donor monomer.

Introduction The use of biomass as a source of renewable monomers is an economically and environmentally promising alternative to new synthetic polymers. Several studies on step-growth polymerization are currently being investigated.1 In the approach conducted by our group, we wished to explore the potentialities of free radical polymerization applied to unsaturated monomers derived from hydroxylic compounds of renewable origin.2 Free radical polymerization is indeed a key method for obtaining synthetic polymers but is surprisingly less investigated for designing original monomers from biomass. As a major source of molecules with various structural features and a large potential for chemical modification, glucides have been modified for use as monomers or prepolymers suited for free radical polymerization. Acrylate as well as styrene derivatives of carbohydrates have been prepared by various acylation or glycosidation reactions for obtaining hydrophilic polymeric stationary phases for analytic purposes, as well as for designing original hydrosoluble carriers and biocompatible adhesives for biomedical applications.3-5 However, for a better respect of the green chemistry principles,6 conversion of the selected biosourced precursors into target monomers should be preferentially achieved with particular attention to atom economy. Accordingly, unsaturations can be formed by elimination reactions achievable at numerous positions of common carbohydrates.7 A series of papers exemplifies this type of approach where the polymerizable double bonds were generated in the exo- or endocyclic position of the pyranoid or furanoid structure of the monosaccharide.8-10 The reactivity of the resulting monomers has been investigated in free radical polymerization with various conventional vinylic and acrylic comonomers. On the whole, conventional free radical polymerization proceeds with rather low overall yields for all studied monomers. The structural features of the latter exert a negative influence on the steric hindrance as well as on the electron density and polarization of the unsaturated glucide * Corresponding author. Tel/Fax: 33 3 26913338. E-mail: xavier. [email protected].

Scheme 1. Possible Mechanisms for the Copolymerization of Acceptor Monomers with Vinyl Ethers Yielding Alternating AM-VE Polymers

derivatives. This in turn affects the polymerizability of unsaturated monomers during the key propagation step, not to mention the competitive transfer reactions. A more promising option consists of introducing a polymerizable group by minimal functionalization, introducing an unsaturated group with two or three carbon atoms. Indeed, allyl functions can be easily introduced into glucide precursors by Fischer glycosidation, the Williamson reaction, or both. In a formally simpler modification scheme,11 vinyl ethers can be synthesized from hydroxylic compounds by addition to acetylene or by transvinylation. This class of electron-rich unsaturated monomers does not satisfactorily homopolymerize by free radical mechanism.12 Their copolymerization proceeds efficiently with electrondeficient ethylenic compounds as butenedioic derivatives, yielding alternating copolymers,13 either via a charge transfer complex (CTC) formed from the two comonomers14 or by the predominance of the cross-propagation. The latter possibility corresponds, in the Lewis-Mayo model, to reactivity ratios r1 and r2 for each monomer both equal to zero or negligible compared with unity (Scheme 1).15-17 We have focused the present work on the comparative evaluation of a series of model monomers derived from hydroxylic compounds of increasing structural complexity and carrying a single vinyl ether or vinyloxy function for assessing the polymerizability of corresponding glucides. The electron-rich vinyloxy monomers were copolymerized with diethyl fumarates (DEFs) and maleates so as to favor a donor-acceptor synergy in a cross-propagation mechanism. Fumarate diesters of mono or polyhydroxylic compounds can

10.1021/bm1005883  2010 American Chemical Society Published on Web 07/28/2010

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Scheme 2. Photolysis of 2-Hydroxy 2-Methyl Phenylpropanone (PI)

be obtained from biosourced feedstock,18 allowing the formation of various copolymers with a high content in renewable carbon. Photochemical initiation with 2-hydroxy 2-methyl phenylpropanone (Scheme 2) was used for studying the free radical polymerization because of more convenient experimental aspects with regard to conventional thermal initiation and because of the potential use of this type of copolymerization in radiation curing applications.19 Our interest was focused on the consumption rates of both donor- and acceptor-type monomer functions and particularly on the influence of the structure of the donor monomer and of the stereochemistry of the butenedioate of the copolymerization reactivity. Previous investigations on the free radical polymerization of acrylates and methacrylates monomers have shown the importance of dipolar effects and H-bonding on the reaction kinetics.20,21 The series of vinyloxy monomers represented in Scheme 3 was purposely selected to incorporate a variety of structural features for a detailed structure-reactivity investigation.

Experimental Part Materials. Commercial monomers, diethyl fumarate (DEF), diethyl maleate (DEM), butyl vinyl ether (BVE) and butyl acrylate (BuA) were purchased from Sigma Aldrich and used as received. 4-Hydroxy-1vinyloxybutane (HBVE for hydroxybutyl vinyl ether) was obtained as a gift sample from ISP. The mono- and polyhydroxylic compounds used as starting materials as well as the reagents and catalysts for synthesizing the unsaturated monomers were reagent grade chemicals purchased from Sigma Aldrich. The intermediate products and the synthesized monomers were purified by column chromatography on silica gel and afforded a satisfactory set of spectroscopic and analytical data. (See the Supporting Information.)

Pichavant et al. Analytical Methods. IR spectra were recorded on a Bruker Alpha-T FTIR spectrometer. 1H and 13C NMR spectra were obtained on a Bruker 250 MHz spectrometer in CDCl3 used as solvent with tetramethylsilane as an internal standard. Electrospray ionization mass spectra (ESI-MS) were obtained on a hybrid tandem quadrupole/time-of-flight (Q-TOF) Waters instrument. Elemental analyses were performed with a PerkinElmer CHN 2400 apparatus, and analysis fell within (0.4% of calculated values. Irradiation of Solvent-Free Monomer Blends. The reactive blends were prepared from the donor and acceptor monomers mixed in equimolar proportion. The photoinitiator (PI), 2-hydroxy-2-methyl-1phenylpropanone (Additol HMPP, Cytec), was added to the homogeneous blend so as to obtain a final 5 wt % PI concentration. A controlled volume (15 µL) of the photosensitive blend was cast between two NaCl plates (plate thickness 7 mm, reactive layer thickness between 15 and 20 µm). Photopolymerization experiments were carried out at a temperature of ca. 30 °C in Bio-Link irradiation system (Vilber Lourmat) equipped with 365 nm fluorescent tubes (8 mW cm-2). After application of each incremental UV light dose, the samples were analyzed by FTIR spectroscopy in the transmission mode for comparison with the unirradiated sample.

Results and Discussion Synthesis of Vinyloxy Monomers. The comparative study of the O-vinyl riboses and one of their allyl analogs required the synthesis of well-defined monomers isolated as pure compounds. Only BVE and 4-hydroxy butylvinyl ether (HBVE) are commercially available. Hexyl-vinyl-ether (VHE) and racemic 2,2-dimethyl-4-(vinyloxymethyl)-1,3-dioxolane, also named 1-O-vinyl-2,3-O-isopropylidene glycerol (VIG), were synthesized in 65 and 72% yield, respectively, by transvinylation from ethyl-vinyl-ether (EVE) to 1-hexanol and 1,2-O-isopropylidene glycerol (solketal), as shown in Scheme 4. The reaction was conducted in a mixture of EVE/dichloromethane 4:3 v/v (5 mL mol-1 of hydroxylic groups to be etherified), as depicted in Scheme 3 for 12 h at 20 °C.22,23 The catalyst, (1,10-phenanthroline)Pd(OOCCF3)2, was generated in situ from Pd(OOCCF3)2 so as to obtain a 2 mol % concentration of the Pd complex with respect to the amount of hydroxylic groups to be etherified. The addition of triethylamine

Scheme 3. Formulas of the Donor and Acceptor Monomers, Selected for the Structure-Reactivity Investigation

Reactivity of Vinyl Ethers and Vinyl Ribosides Scheme 4. Synthesis of Model Vinyl Ethers Derived from N-Hexanol and from Solketal

(four- to six-fold excess with respect to Pd) enabled us to prevent the acetal formation catalyzed by free trifluoroacetic acid in the catalytic cycle.24 5-O-vinyl-2,3-O-isopropylidene-D-ribofuranose (VIR) was synthesized in four steps from D-ribose. At first, allyl-2,3-Oisopropylidene-D-ribofuranoside (AIR) was synthesized by Oglycosidation of D-ribose with allyl alcohol,25 followed by the cetalization of the C2 and C3 positions in acetone under slightly acidic conditions.26,27 Then, the allylic function at the anomeric position C1 was hydrolyzed under mild conditions in the presence of aqueous Pd (II) chloride and Cu (I) chloride, as catalysts.28,29 Finally, 5-O-vinyl-2,3-O-isopropylidene-D-ribofuranose was obtained by transvinylation of the primary alcohol in the C5 position of the carbohydrate with BVE and 4,7diphenyl-1,10-phenanthroline-Pd(CF3CO2)2 complex (2 mol %) at a relative concentration 200:1 and 0.02:1 with respect to the amount of hydroxylic groups to be vinylated, respectively.

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Triethylamine (four- to six-fold excess with respect to Pd salt) was added before the introduction of sugar substrate to prevent acetylation.30 The reaction was conducted at 80 °C for 12 h with a final 24% yield (Scheme 5). Vinyl-2,3,5-tri-O-methyl-D-ribofuranoside (VTMR) was synthesized in four steps starting from D-ribose. At first, the pentose was glycosylated in methanol under acid conditions. Then, the hydroxylic groups were methylated by the Williamson reaction31 in the presence of methyl iodide under alkaline conditions. The methoxy group in the anomeric position was removed in dioxane solution with aqueous hydrochloric acid (0.5%) to yield 2,3,5tri-O-methyl-D-ribofuranose after 24 h.32,33 Vinylation by exchange at the C1 position with BE, under the conditions described for VIR, afforded VTMR in 24% yield (Scheme 6). The allyloxy analog allyl-2,3,5-tri-O-methyl-D-ribofuranoside (ATMR) was obtained by Fischer glycosidation of tri-O-methylD-ribofuranose in allyl alcohol (yield 97%). At room temperature, all unsaturated ethers of Scheme 2 are liquids under normal conditions and form homogeneous blends when mixed with DEF or with DEM in equimolar proportions. The selected type I PI, 2-hydroxy-2-methyl-1-phenylpropanone, was introduced in the mixed monomers at a concentration of 5 wt %. FTIR Monitoring of the Copolymerization of Vinyl Ether Monomers with Diethyl Fumarate and Diethyl Maleate. The photosensitive formulations were treated in the form of thin liquid films sandwiched between two NaCl plates by exposure to a 365 nm light source. The effective part of the incident light produces absorption in the long wavelength edge

Scheme 5. Synthesis of Monohydroxylic Ribose Vinyl Ether

Scheme 6. Synthesis of Hydroxyl-Free Riboside Monomers VTMR and ATMR

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πO-vinyl ) 1 -

πDEF ) 1 -

Figure 1. FTIR monitoring of the photopolymerization of DEF for increasing irradiation time (PI content 5 wt %, 365 nm light, dose ranging from 0 to 24 J cm-2, 8 mW cm-2).

of the n-π* band inducing weak absorption of incident photons (ε ≈ 100 L mol-1 cm-1). The incoming light penetrates uniformly into the sample and generates free radicals under soft irradiation conditions without photoisomerization of fumarate or maleate esters included in the mixtures. The samples were maintained at moderate temperature (T ≈ 30 °C) all along the treatment without significant evaporation of low boiling points monomers and without excessive risk of thermally activated transfer reactions possibly in competition with the main free radical processes. Preliminary experiments were performed to assess the reactivity of each type of comonomer in the absence of its donor or acceptor counterpart under the standard conditions defined for the study. The vinyl ether monomer HBVE was shown to be almost unreactive at low dose, reaching ∼15% conversion after exposure to a 20 J cm-2 irradiation dose. Among the two families of butenedioates, only dialkyl fumarates are known to undergo homopolymerization, the Z maleate isomers requiring photochemical or amine-mediated isomerization prior to taking part in propagation.34,35 The wavelength of the UV source (365 nm) does not allow the photoisomerization process to take place for the DEM monomer, almost unreactive under the selected conditions. However, the FTIR spectra recorded along the irradiation of DEF show the gradual decrease in the absorbance at 775 cm-1 that corresponds to the out-of-plane deformation of DEF conjugated unsaturation (Figure 1). The reaction is completed after exposure durations as high as 1500 s under the 8 mW cm-2 source. In the presence of a vinyloxy comonomer, the time scale for completing the reaction with DEF or DEM as the acceptor was typically ranging from 50 to 500 s, depending on the donor monomer structure. FTIR monitoring at two or more wavenumbers allowed us to determine the progress of the reaction for the comonomers. The series of spectra in Figure 2 illustrates the situation with the HVE-DEF mixture where a single band is observed for the vinyl ether in the monitoring domain. In another situation, the vinyl ether of VTMR appears in the form of two bands (Figure 3), possibly because of the presence of the R and β epimeric forms of the modified ribofuranoside, clearly evidenced by 1H and 13C NMR. With the blends including DEF, the bands of the two monomers are isolated with baseline points easily defined in the plots of the superimposed spectra. The fractional conversion ratios of the vinyl ether (πO-vinyl) and of DEF (πDEF) calculated from the measured variations of absorbance, according to eqs 1 and 2, respectively36

t ∞ A810cm -1 - A810cm-1 0 ∞ A810cm -1 - A810cm-1 t ∞ A775cm -1 - A775cm-1 0 ∞ A775cm -1 - A775cm-1

(1)

(2)

Because of the presence of two absorption bands centered at 815 and 835 cm-1 in the spectrum of pure DEM, overlapping is observed in all blends containing this acceptor monomer. The spectrum of Figure 4 illustrates this feature for the HBVE-DEM blend. Manipulation of spectral data is needed to separate the contributions of the vinyl ether and of the maleate in the absorbance at 810-815 cm-1 using the proportionality relation of eq 3 t t A810cm -1(DEM) ) 1.2 × A835cm-1(DEM)

(3)

Determination of Monomer Reactivity in Vinyloxy Monomer-DEF or Vinyloxy Monomer-DEM Mixtures. A representative profile obtained with HVE-DEF blend as the donor monomers is presented in the plot of Figure 5. The slope of the linear segment of each kinetic profile was used to determine the absolute rate of consumption (Rp)0 of both monomers. The simultaneous disappearance of the two comonomers observed during the polymerization whereas the monomers do not homopolymerize significantly confirms the occurrence of an alternating copolymerization, as previously established and

Figure 2. FTIR monitoring of the photopolymerization of HVE and DEF in equimolar amounts for increasing irradiation time (PI content 5 wt %, 365 nm light, dose ranging from 0 to 4 J cm-2, 8 mW cm-2).

Figure 3. FTIR monitoring of the photopolymerization of VIR and DEF in equimolar amounts for increasing irradiation time (PI content 5 wt %, 365 nm light, dose ranging from 0 to 0.8 J cm-2, 8 mW cm-2).

Reactivity of Vinyl Ethers and Vinyl Ribosides

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Table 1. Influence of the Structure of Donor Monomer on the Reactivity in Photochemically Induced Copolymerization of Equimolar Mixtures of Vinyloxy Monomers with DEF and DEM as the Acceptor Monomera (Rp)0b -1 -1

(mmol kg s )

Figure 4. FTIR monitoring of the photopolymerization of HBVE and DEM in equimolar amounts for increasing irradiation time (PI content 5 wt %, 365 nm light, dose ranging from 0 to 1 J cm-2, 8 mW cm-2).

Figure 5. Kinetic profiles of the photopolymerization of HVE (O) and DEF (*) in equimolar amounts (PI content 5 wt %, dose ranging from 0 to 4.5 J cm-2, 8 mW cm-2).

Figure 6. Kinetic profiles of the photopolymerization of the vinyloxy monomers in equimolar amounts with DEF (PI content 5 wt %, dose ranging from 0 to 4.5 J cm-2, 8 mW cm-2).

well-documented for analogous pairs of monomers.12 The differences in the progress of conversion for the two types of unsaturations was indeed lower than 3% and within the accuracy of the conversion values for any of the studied comonomer pairs. The profiles were plotted using the average value measured for the donor and acceptor monomers. The curves of Figure 6 correspond to the six vinyloxy models copolymerized with DEF. All polymerizations were driven to almost complete conversion under the selected conditions. The quantity of prime importance for comparing the different systems is the initial polymerization rate (Rp) deduced from the slope of the linear segment of the steady regime observed for each profile. By comparing the data gathered in Table 1, the influence of the structure of the donor and acceptor monomers can be discussed at first on a qualitative basis. The reactivity is obviously higher for the series based on DEF for a given vinyl monomer. Maleates are known to be less reactive than fumarates a` verifier in donor-acceptor copolymerization.37,38 Ranking the vinyl ethers according to their reactivity leads to the same order in both series of acceptor monomers: HVE < BVE ≈ VIG < VTMR < VIR < HBVE. The data can be interpreted by associating the presence of an aliphatic chain

(Rp)0b (mmol kg

-1

δ Hc -1

s )

vinyloxy monomer

comonomer DEF

comonomer DEM

BVE HBVE HVE VIG VIR VTMR

17 76 9 14 51 31

14 42 13 19 31 21

0.5

δPc 0.5

δ Dc

(MPa )

(MPa )

(MPa0.5)

6.1 13.6 5.2 6.6 12.1 9.1

4.6 7.1 4.0 5.5 9.7 15.8

15.5 13.1 15.4 20.5 20.4 20.6

a [PI] 5 wt %, neat films, 365 nm light, dose ranging from 0 to 5 J cm-2, 8 mW cm-2. b Polymerization rate of each monomer reacted in equimolar amounts. c Hansen solubility parameters calculated with the groupcontribution method of ref 49.

(HVE, BVE) with lower reactivity, the presence of hydroxy groups (VIR, HBVE) with the higher reactivity, and the presence of alkoxy groups in aprotic molecules (VIG, VTMR) with intermediate reactivity. Some recent reports have addressed this type of issue and brought some clarifications on the effect of aprotic polar groups39-41 as well as of hydroxylic42-44 and carboxylic45 functions on the reactivity of photopolymerizable monomers. This literature teaches that intermolecular associations can act at various levels of the complex polymerization mechanism: (i) the photoinitiation step can be sensitive to the polarity of the medium, with changes in the absorption spectrum and on the quantum yield for the formation of key species;46,47 (ii) propagation can also be affected by a modification of the intrinsic reactivity of the free radical and of the monomer reactants, depending on the presence of polar functional groups borne by monomers or by other components in the polymerization medium;39,42,45 (iii) associated molecules can form a shield around the propagating free radical, limiting to some extent the accessibility for addition to a monomer;42 (iv) preassociation of monomers through polar interactions or H-bonding can induce locally a higher concentration of reactants and possibly their auto-organization that would favor fast reaction diffusion.42 Other possible effects are based on free radical chain kinetics, with an enhanced autoacceleration effect if monofunctional monomers with a secondary polar function monomers exhibit a partial difunctional character or if the associations between the formed polymer and the growing chains dramatically reduce the termination rate constant even at low conversion level. In the present donor-acceptor system involving pairs of complementary monomers reacted in the bulk, the specific interaction through polar effects can exert an additional effect favoring the formation and the stability of the charge transfer complexes. An attempt to evidence the presence of a charge transfer complex by UV-visible spectroscopy of donor-acceptor blends is currently in progress. An investigation of the influence of the relative content in donor and acceptor monomers on the polymerization rate will be carried out to explore this point. Structure-Reactivity Relationship. We have examined the possible correlation between the observed polymerization rates with the Hansen solubility parameters (HSP).48 The HSP concept is indeed attractive for the present discussion because of its ability to quantify the participation of a molecular entity interacting with the surrounding medium on the basis of separate contributions for dispersive (δD) and dipolar forces (δP) as well as for H-bonding (δH).

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Figure 7. Initial polymerization rates as a function of the Hansen parameter δH for the photopolymerization of the donor monomers in equimolar amounts with DEF (0) and DEM (O) (PI content 5 wt %, dose ranging from 0 to 5 J cm-2, 8 mW cm-2).

We have primarily considered the two parameters δP and δH as possible descriptors of the donor monomer, the dispersive contributions δD being considered as much less relevant for interpreting the present reactivity behavior. The corresponding δx values (x ) H, P, D) were calculated by the groupcontribution method of Stephanis and Panayiotou49 according to eqs 4-6

∑ NiCi + ∑ MjDj + 7.9793) MPa0.5 δP ) ( ∑ NiCi + ∑ MjDj + 7.3548) MPa0.5 δD ) ( ∑ NiCi + ∑ MjDj + 17.3231) MPa0.5 δH ) (

(4) (5) (6)

where Ci is the contribution of the first-order group i that appears Ni times in the compound and Dj is the contribution of the secondorder group j that appears Mj times in the compound. The groupcontribution model proposed by the authors takes into consideration second-order groups based on the ABC framework,50 claimed for its relevance for accurate H-bonding contributions. The corresponding HSP values (δx) of the vinyloxy monomers evaluated in this study are collected in Table 1. The plots representing the dependence of the steady polymerization rates for the vinyl ether monomers copolymerized with DEF or DEM as a function for their δH Hansen parameters suggest the existence of a monotonous correlation for the two series of experiments (Figure 7). The attempt to correlate the polymerization rates with the δP Hansen parameters lead to erratic variations. The correlation is approximately linearized by representing the variation of ln(Rp)0 with δH, whereas a simple and monotonous relation with the polar term δP can be ruled out for the DEF and DEM series, as shown in Figures 8 and 9, respectively. These results encourages us to examine further with a more precise methodology the effects of hydrogen bonding on the reactivity of various types of monomers polymerized by a free radical process. In particular, it is worth examining whether the dependence of (Rp)0 or of other quantity measuring monomer reactivity on Hansen parameter δH can be established for monofunctional acrylates and methacrylates. The physical meaning of the descriptor δH indeed encompasses various interactions as electron exchange that may be relevant for the present donor-acceptor systems. For a given set of experimental conditions, the steady polymerization rates of can be provisionally expressed by eq 7 where the pre-exponential factor A and the constants b and k

(Rp)0 ) Aacceptor × exp(kacceptor × δH,donor + b)

Figure 8. Logarithmic plot of the initial polymerization rates as a function of the Hansen parameters δH (2) and δP (O) for various vinyloxy donor monomers in equimolar amounts with DEF (PI content 5 wt %, dose ranging from 0 to 5 J cm-2, 8 mW cm-2).

(7)

Figure 9. Logarithmic plot of the initial polymerization rates as a function of the Hansen parameters δH (2) and δP (O) for various vinyloxy donor monomers in equimolar amounts with DEM (PI content 5 wt %, dose ranging from 0 to 5 J cm-2, 8 mW cm-2).

are dependent on the nature of the acceptor monomer, and where the Hansen parameter δH is the convenient descriptor for the vinyloxy donor. It would be worth examining whether the dependence on Hansen parameter δH of (Rp)0 or of other quantity measuring monomer reactivity can be extended to other donoracceptor pairs as well as to monofunctional acrylates and methacrylates carrying secondary functional groups. The physical meaning of the descriptor δH indeed encompasses various interactions as electron exchange that may be relevant for not only the present donor-acceptor systems but also other systems. Comparison with Other Types of Monomers. The ribosebased vinyloxy monomers VTMR and VIR were compared with an allyloxy analog, ATMR, as comonomer polymerized with DEF. n-Butyl acrylate (BA) was also homopolymerized under the same conditions for comparison with a conventional monomer. The obtained conversion profiles are presented in Figure 10. We have previously reported on the nonalternating copolymerization of allyl ethers with DEF.2 The acceptor monomer is consumed faster than the allyl group in the ATMR-DEF mixture with (Rp)0,DEF ) 3.0 mmol kg-1 s-1 and (Rp)0,ATMR ) 1.3 mmol kg-1 s-1. The vinyloxy derivatives of ribose exhibit a much higher reactivity with (Rp)0,VTMR+DEF ) 63 mmol kg-1 s-1 and (Rp)0,VIR+DEF ) 104 mmol kg-1 s-1 to be compared with the reactivity of a reference acrylate monomer undergoing homopolymerization ((Rp)0,BA = 200 mmol kg-1 s-1). In an extension of this work, we are currently examining the formation of

Reactivity of Vinyl Ethers and Vinyl Ribosides

Figure 10. Kinetic profiles of the photopolymerization of the equimolar VIR-DEF (0) blend compared with butyl acrylate (O), VTMR-DEF blend (4), and equimolar ATMR(2)-DEF(b) blend (PI content 5 wt %, dose ranging from 0 to 4.5 J cm-2, 8 mW cm-2).

networks by cross-linking copolymerization of multifunctional monomers derived from model glucides.

Conclusions This work allows the quantitative assessment of the reactivity of vinyloxy monomers derived from polyols and ribosides in donor-acceptor free radical copolymerization with DEF and DEM. The kinetic results confirm the high reactivity of vinyl ethers in free radical copolymerization with butadienoates. We have also observed the expected higher reactivity of blends including DEF compared with those based on DEM. We have established a correlation between the structure of vinyloxy monomers and the polymerization rate by using the Hansen parameter δH as the descriptor of the donor monomer. The obtained data provide the basis of a tentative model that shall be further studied for possible extension. Sugar-derived vinyloxy monomers exhibit a higher reactivity than alkyl vinyl ether, offering new potentialities in the current search for alternative biosourced monomers. Acknowledgment. We thank the Conseil Regional de Champagne Ardenne and the Centre National de la Recherche Scientifique for the studentship granted to L.P. Financial support by Conseil Regional Champagne Ardenne, MENESR, and EUFEDER Programme (CPER Project PlAneT) is gratefully acknowledged. Supporting Information Available. Description of the synthesis of noncommercial allyloxy and vinyloxy monomers as well as NMR data of isolated products. This material is available free of charge via the Internet at http://pubs.acs.org.

References and Notes (1) Linderblad, M. S.; Liu, Y.; Albertsson, A. C.; Ranucci, E.; Karlsson, S. In Polymers from Renewable Resources; Springer: Berlin, 2001. (2) Pichavant, L.; Guillermain, C.; Duchiron, S.; Coqueret, X. Biomacromolecules 2009, 10, 400–407. (3) Buchholz, K.; Yaacoub, E.; Warn, S.; Skeries, B.; Wick, S.; Boeker, M. Ger. Pat. Appl. 4408391, 1995. (4) Deppe, O.; Glumer, A.; Yu, S.; Buchholz, K. Carbohydr. Res. 2004, 339, 2077–2082. (5) Black, W. A. P.; Dewar, E. T.; Rutherford, D. J. Chem. Soc. 1963, 9, 4433–4439. (6) Anastas, P.; Warner, J. In Green Chemistry: Theory and Practice; Oxford University Press: Oxford, U.K., 1998. (7) Yaacoub, E. J.; Wick, S.; Buchholz, K. Macromol. Chem. Phys. 1995, 196, 3155–3170.

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