2,3-Dihydrofuran: A Special Vinyl Ether for Cationic ... - ACS Publications

Chapter 18

2,3-Dihydrofuran: A Special Vinyl Ether for Cationic Photopolymerization

Downloaded by UNIV LAVAL on April 21, 2016 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch018

Oskar Nuyken and Harald Braun Lehrstuhl für Makromolekulare Stoffe, Technische Universität München, Lichtenbergstrasse 4, D-85748 Garching, Germany

Mono- and bifunctionl monomers with 2,3-dihydrofuran moieties were synthesized by the Heck reaction of 2,5 -dihydrofuran and various arylbromides. The synthesized monomers were polymerized by typical cationic photoinitiators. Depending upon the type of monomers used, soluble and insoluble polymers were formed and the thermal properties were studied.

Homo- and copolymers of vinyl ethers are used in many commercial products. Reason for the great interest in new polymers with vinyl ether moieties, are their cheap synthesis and their excellent adhesion properties. Vinyl ethers possess double bonds with high electron densities due to their strong electron-donating alkoxy substituent. Consequently, they can not be polymerized by anionic or radical routes, but polymerize via a cationic mechanism. However, a common problem in the application of all linear vinyl ethers, is that they form small amounts of aldehyde during the polymerization (1). This can be avoided using 2,3-dihydrofuran, because of its ring structure. Figure 1 shows the possible formation of acetaldehyde by the cationic polymerization of ethyl vinyl ether.

© 2003 American Chemical Society

Belfield and Crivello; Photoinitiated Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

213

214 OH


Figure L Two possibilities for the formation of acetaldehyde by the cationic polymerization of ethyl vinyl ether. In the family of vinyl ethers, 2,3-dihydrofuran is known to be most reactive (2). Poly(2,3-dihydrofuran) was first synthesized by Barr et al. with boron trifluoride as initiator (3). The polymerization can also be carried out with other cationic initiators (4,5). An interesting alternative is the photoinduced cationic homopolymerization of 2,3-dihydrofuran initiated by (Ti -cyclopentadienyl)Fe(II)-(n -isopropylbenzene) hexafluorophosphate showing the high reactivity of 2,3-dihydrofuran towards cationic photoinitiators. The driving force of the polymerization of 2,3-dihydrofuran is its ring strain and cis-conformation at the double bond . The synthesis of unsubtituted 2,3-dihydrofuran is easy (7-11). The main synthetic routes are presented in the following scheme. 5

6

6

OU

Kobalt / Kieselgur

'

». 210°C-230°C

fl \ II J ΤΓ

+

H 0 2

+

H

2

Ag-Kat [O] ΔΤ Ca(OH)

r=\

Fe(CO) or KOC(CH )

ΔΤ

Ο

or Ru(PPh ) Cl

2

CL

χ = χ

α

Figure 2.

5

3 3

3 2

2

Ο

Different synthetic methods for 2,3-dihydrofuran.


215


Poly(2,3-dihydrofuran) showed excellent film forming properties and adhesion. 2,3-dihydrofuran type monomers were synthesized and examined with respect to their reactivity towards photochemically generated acids, since this is prerequisite for e.g. using such monomers in UV curable coatings. The synthesized compounds fulfill demands for practical applications since they undergo rapid polymerization which is finished within 2 minutes (100% conversion) upon photolysis. However, for application of those monomers to coatings it is essential to crosslink the organic layer, meaning a certain amount of bifunctional monomers must be added. Unfortunately, only a rather complicated synthesis for monomers with two 2,3-dihydrofuran moieties per molecule was available (12). Therefore, we had to look for an alternative route, which we found by reacting aryl dibromides with 2,5-dihydrofuran in the presence of a palladium catalyst (Heck reaction) (13). A series of mono and bifunctional 2,3-dihydrofuran compounds were synthesized using the Heck reaction. Analogous reaction conditions were applied. A typical reaction mixture contained 1 eq. arylbromide, 2 eq. 2,5dihydrofuran, 0.5 eq. tetrabutylammonium bromide, 1.2 eq. base: sodium acetate, and 0.01 eq. palladium catalyst, as depicted in Figure 3. >C^\ R

- \ 0 ) vrv

r=rv B

r

2.5Mol-%Pd(OAc) /2PPh 2

L >

/7^Z\

3

" DMF

r

- < 0 > - < ' y—y V^o

2.5 eq.NaOAc 110 °C, 18 h

Figure 3.

Heck reaction of 2,5-dihydrofuran with different arylbromides.

Nearly complete conversion based on arylbromide was achieved after 18h. The reaction offers a simple route to prepare 2,3-dihydrofuran derivatives in one step. The method can also be applied to thesythesis of monomers with two 2,3dihydrofuran moieties, that can be used as crosslinkers. Different synthesized monomers are shown in Figure 4. The pure monomers could be obtained only by several successive column chromatography steps.

Photopolymerization of 2,3-dihydrofuran dérivâtes 10 mmol of the monomer and 0.4 mmol of the photoinitiator contained in a quartz flask were dissolved in 5ml methylene chloride at roomtemperature and irradiated from an Ushio UXM 200H Hg-Xe high pressure lamp with a light intensity of 200 mW cm" . At the end of irradiation, the solution was poured into ten-fold excess of methanol andfilteredoff. The polymers were then dried 2


Belfield and Crivello; Photoinitiated Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2003. l,4-(Bis-2,3-dihydrofuraii-3-yl)-beiizene

Figure 4. Synthesized 2,3-dihydrofuran derivatives.

4,4'-(Bis-2^-dihydrofuraii-3-yl)-biphenyl

î ,6-Bis[(2,3-dîhydroftiran-3yl>4-ph€nyIJ-2,5-dioxahexane (n=l); 140-Bis[(23-dihydrofuran-3yl)-4-phenyl]-2,9-dioxidecane(n-5).

7,8


217


Figure 5.

Cationic polymerization of 2,3-dihydrofuran derivatives

at 60°C for 24 h at 10 mbar. In case of bifunctional monomers, a mixture of 9 parts 2,3-dihydrofuran and one part of the bifunctional monomer is used. At the end of irradiation insoluble networks are formed. Regardless of their functionality, photoinitiated cationic polymerization of all monomers, was completed within 2 min by using a triphenylsulphonium salt (14). The typical cationic formation of poly-2,3-dihydrofuran is shown in Figure 5. Insoluble network polymers were formed when the formulations contained bifunctional monomers (7-10). Molecular weights, molecular weight distributions and glass transition and decomposition temperatures of the monofunctional polymers obtained, are summarized in Table I. Although the molecular weights of the polymers were rather low, free standing films from these polymers could easily be formed by solvent casting. Moreover, their adhesion on glass and metal is a good indication for their possible application in coatings. Glass transition temperatures of the polymers were in the range between 124°C and 157°C depending on the substituent of the 2,3-dihydrofuran derivative. As expected, better thermal stabilities were obtained with the crosslinked polymers (T = 341-391°C) compared to that of poly-2,3dihydrofuran 1 (T = 311 ° C ) . d

14

d

Table I. The glass transition and decomposition temperatures, and molecular weights of the uncrosslinked polymers.

Source: Reproduced with permission from right 2001 Oskar Nuyken.)

Polym. Prepr. 2001, 42(2), 779-780. Copy


218

Conclusions 2,3-Dihydrofuran type monomers can be prepared by a versatile synthetic procedure in a one-step reaction (Heck reaction) and the monomers were shown to be polymerizable by photoinduced cationic polymerization.

References 1. Murad E. J. Am. Chem. Soc. 1961, 83, 1327.


2. Nuyken, O.; Raether, R.B.; Spindler C.E. Des. Monomers Polym. 1998, 1(1), 97. 3. Barr D.A.; Rose J.B. J. Chem. Soc.; 1954, 3766. 4. Kim J.B.; Cho I. J. Polym. Sci., Part A: Polym. Chem., 1989, 27, 3733.

5. Sanda F.; Matsumoto M. Macromolecules, 1995, 28, 6911. 6. Nuyken O.; Aechtner S. Polym. Bull., 1992, 28, 117. 7. Dimroth P.; Pasedach H. Angew. Chem. 1960, 72, 865. 8. Eliel E.L.; Nowak B.E.; Daignault R.H.; Badding V.G. J. 30, 2441.

Org. Chem.1965,

9. Havel J.J.; Chan K.H. J. Org. Chem. 1976, 41, 513. 10. Vogel E.; Gunther H. Angew. Chem. 1967, 79, 429. 11. Eberbach W.; Buchhardt B. Chem. Ber. 1978, 11, 3665. 12. Nuyken O.; Spindler C.; Raether R.B. J. M. S.- Pure Appl. A34(12), 2389. 13. Jeffery T.; David M. Tetrahedron Let., 1998, 39, 5751. 14. Nuyken O; Braun H. Des. Monomers Polym., 2001, 4(1), 19.


Chem., 1997,

2,3-Dihydrofuran: A Special Vinyl Ether for Cationic ... - ACS Publications

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