Synthesis and Photopolymerization of 1-Propenyl Ether Functional

Chem. Mater. 1996, 8, 209-218

209

Synthesis and Photopolymerization of 1-Propenyl Ether Functional Siloxanes J. V. Crivello* and G. Lo¨hden Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180 Received July 13, 1995. Revised Manuscript Received October 11, 1995X

A variety of mono-, di-, and multifunctional 1-propenyl ether functional siloxanes were readily prepared in high yields by the transition-metal-catalyzed condensation of R-(1propenyl) ω-vinyl ethers with various linear and cyclic hydrogen functional siloxanes. Under these conditions, hydrosilation takes place regioselectively at the vinyl ether site of the R-(1propenyl) ω-vinyl ether. Using onium salt photoinitiators, these new monomers and oligomers undergo rapid polymerization under the influence of UV light. To study these very fast photopolymerizations, extensive use of Fourier transform real-time infrared spectroscopy was made. Employing this technique, the effects of monomer and photoinitiator structure on the rates of polymerization were studied.

Introduction A theme of continuing interest in this laboratory is the design and synthesis of multifunctional monomers that can be rapidly and efficiently photopolymerized. In previous publications1-4 from this laboratory, we have described the preparation and cationic photopolymerization of epoxy functional siloxanes. It was observed that siloxanes bearing epoxycyclohexyl groups display extraordinarily high reactivity in photoinitiated cationic polymerization, which makes them especially attractive for applications as electronic and decorative coatings, adhesives, and inks. Further, photo and electron-beam polymerized epoxycyclohexyl modified siloxanes typically display high glass transition temperatures as well as excellent mechanical properties and chemical resistance which make them especially suitable for composite applications.5 The high reactivity and excellent mechanical and chemical properties of epoxy functional siloxanes prompted us to consider what other kinds of modified siloxanes could be prepared to achieve the same or better characteristics. One class of monomers that exhibits even higher reactivity in cationic photopolymerizations are vinyl ethers.6 On the basis of these observations, Herzig and co-workers7 have prepared poly(dimethylsiloxanes) bearing pendant and terminal vinyl ether groups. An example of the route employed for their synthesis is depicted in eq 1. An R,ω-Si-H functional poly(dimethylsiloxane) is condensed with trimethylolpropane trivinyl ether in the presence of a platinum catalyst to give nominally the tetrafunctional vinyl terminated oligomer. However, since the hydrosilation reaction occurs equally and * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, November 15, 1995. (1) Crivello, J. V.; Lee, J. L. J. Polym. Sci., Polym. Chem. Ed. 1990, 28, 479. (2) Crivello, J. V.; Lee, J. L. Chem. Mater. 1989, 1, 445. (3) Crivello, J. V.; Lee, J. L. ACS Symp. Ser. 1989, 417, 398. (4) Crivello, J. V.; Schroeter, S. H. U.S. Patent 4,026,705, 1977, to General Electric. (5) Crivello, J. V. U.S. Patent 5,260,349, 1993, to General Electric. (6) Crivello, J. V.; Lee, J. L.; Conlon, D. A. J. Radiat. Curing 1983, 1, 6. (7) Herzig, Ch.; Deubzer, B. Proc. Radtech Asia Conf. 1993, 99.

0897-4756/96/2808-0209$12.00/0

(1)

without selectivity at the three vinyl ether groups, a variety of higher functional oligomers are also simultaneously produced. Under such circumstances, it is difficult to control the synthesis of multifunctional vinyl ether functional siloxanes to avoid gelation during synthesis and to design poly(dimethylsiloxane) resins that possess a high degree of vinyl ether functionality. The objective of this investigation was to develop alternative hydrosilation chemistry that would allow the synthesis of well-characterized siloxanes bearing vinyl ether or other similarly highly reactive functional groups. Experimental Section General Techniques. 1,1,1,3,3-Tetramethydisiloxane, phenyldimethylsilane, methyltris(dimethylsiloxy)silane, phenyltris(dimethylsiloxy)silane, and 1,3,5,7-tetramethylcyclotetrasiloxane (D4H) were used as purchased from Hu¨ls America, Inc. Poly(dimethylsiloxane-co-methylhydrosiloxane) random copolymer (average structure MHD60D5HMH; i.e., Si-H terminated, 60:5 mol ratio dimethylsiloxy:methylhydrogen siloxy repeat units; Mn ) 4874 g/mol), an epoxy functional poly(dimethylsiloxane) (GE-9500), the Karstedt catalyst, and UV9380C (bis(4-dodecylphenyl)iodonium hexafluoroantimonate) were obtained as gifts from General Electric Silicones. l,4Butanediol, allyl bromide, tetra-n-butylammonium bromide, and tris(triphenylphosphine)ruthenium(II) dichloride [(Ph3P)3RuCl2] and tris(triphenylphosphine)rhodium(I) chloride [(Ph3P)3RhCl] were purchased from the Aldrich Chemical Co. and used without further purification. 4-Vinyloxybutan-1-ol and 2-vinyloxyethanol were obtained as samples from International Specialty Products Division of the GAF Corp., while 6-(vinyloxy)hexan-1-ol, 4-[(vinyloxy)methyl]cyclohexylmethanol and 2-(2-(vinyloxyethoxy)ethanol were obtained from the BASF Corp. The cationic photoinitiator (4-(n-decyloxy)phenyl)phen-

© 1996 American Chemical Society

210

Chem. Mater., Vol. 8, No. 1, 1996

Crivello and Lo¨ hden

Table 1. r-(1-Propenoxy)-ω-(vinyloxy)alkanes elemental analysis compound

notation

yield (%)

boiling point

1-(1-propenoxy)-2-(vinyloxy)ethane

IP

96

58 °C/7 mmHg

1-(1-propenoxy)-4-(vinyloxy)butane

IIP

89

52 °C/0.2 mmHg

1-(1-propenoxy)-6-(vinyloxy)hexane

IIIP

91

63 °C/0.15 mmHg

1-(1-propenoxy)-5-(vinyloxy)-3-oxapentane

IVP

78

68 °C/0.15 mmHg

trans-1-(1-propenoxymethyl)-4(vinyloxy)methylcyclohexane

VP

87

93 °C/0.2 mmHg

found calc found calc found calc found calc found calc

C (%)

H(%)

65.55 65.60 69.23 69.19 71.61 71.70 62.80 62.77 74.34 74.24

9.49 9.44 10.24 10.32 10.99 10.94 9.34 9.46 10.52 10.54

yliodonium hexafluoroantimonate was prepared as described previously.8 Routine infrared spectra were obtained on a Buck Scientific Model 500 spectrometer. Gas chromatographic analyses were performed on a Hewlett-Packard HP-5890 gas chromatograph equipped with a high-performance capillary HP-1 (5% phenylmethyl silicone) column and a flame ionization detector. 1H NMR spectra were obtained using a Varian XL 500 MHz spectrometer at room temperature in CDCl3. Elemental analyses were performed by Quantitative Technologies, Inc., Whitehouse, NJ. Synthesis of r-(1-Propenoxy)-ω-(vinyloxy)alkanes. The following experimental procedures are typical of those used for the preparation of R-(1-propenoxy)-ω-(vinyl)oxyalkanes. Preparation of 1-(Allyloxy)-2(2-(vinyloxy)ethoxy)ethane (1(Allyloxy)-5-(vinyloxy)-3-oxapentane) (IVA). Into a 100 mL round-bottom flask fitted with an overhead mechanical stirrer, addition funnel, reflux condenser, and nitrogen inlet was dispersed 20 g (0.5 mol) of sodium hydride (60% dispersion in paraffin wax) in 200 mL of dry toluene. To this solution was slowly added 0.52 mol of 2-(allyloxy)ethanol until the evolution of hydrogen had ceased. After addition had been completed, there was added to the resulting clear solution 63.9 g (0.60 mol) of 2-chloroethyl vinyl ether. The reaction mixture was heated at 200 °C for 10 h. Thereafter, the reaction mixture was cooled and poured into 300 mL of distilled water. Using a separatory funnel, the organic layer was recovered, and the solvent and unreacted volatile starting materials were removed on a rotary evaporator. The residue, a colorless oil, was distilled under reduced pressure (bp 64 °C/8 mmHg) to give the desired 1-(allyloxy)-2-(2-(vinyloxy)ethoxy)ethane in 56% yield. General Procedure for the Preparation of R-(Allyloxy)-ω(vinyloxy)alkanes IA-VA. Combined in a 500 mL three necked flask were 0.5 mol of the R-hydroxy-ω-(vinyloxy)alkane, 78.64 g (0.65 mol) of allyl bromide, 26 g (0.65 mol) of NaOH, 10 g (0.31 mol) of tetra-n-butylammonium bromide, and 200 mL of toluene. The reaction flask was equipped with a thermometer, overhead stirrer, and a reflux condenser. Initially the solution was stirred under a nitrogen atmosphere for 15 min and then heated to 90 °C using a heating mantle. After 16 h, the reaction mixture was cooled to room temperature and poured into 300 mL of water. The organic layer was separated and washed twice with distilled water to remove the quaternary ammonium salt phase transfer catalyst. After removal of the solvent on a rotary evaporator, the residue was distilled under reduced pressure and then further purified by flash column chromatography on silica gel using a 1:9 mixture of ethyl acetate and hexane. This procedure effectively removed traces of starting alcohol as indicated by the absence of an OH band in the infrared. General Procedure for the Isomerization of R-(Allyloxy)-ω(vinyloxy)alkanes to R-(1-Propenoxy)-ω-(vinyloxy)alkanes Example: 1-(1-Propenoxy)-4-(vinyloxy)butane (IIP). Into a 100 mL flask fitted with a reflux condenser and magnetic stirrer were added 34 g (0.23 mmol) of 1-(allyloxy)-4-(vinyloxy)butane and 0.03 g (0.32 µmol) of (Ph3P)3RuCl2. This mixture was heated and stirred for 8 h at 130 °C. Analysis by 1H NMR of the product shows complete conversion of the allyl group to

the 1-propenyl ether group. The product was distilled under reduced pressure (bp 52 °C/0.2 mmHg) to give 32.8 g (96% yield) of the desired 1-(1-propenoxy)-4-(vinyloxy)butane (IIP). Similarly, the other R-(allyloxy)-ω-(vinyloxy)alkanes were isomerized to the corresponding R-(1-propenoxy)-ω-(vinyloxy)alkanes. Given in Table 1 are the isomerization conditions and yields for these compounds. Synthesis of 1-Propenyl Ether Functional Silicone Monomers by the Hydrosilations of R-(1-Propenoxy)-ω(vinyloxy)alkanes. The hydrosilations for all the R-(1propenyl)-ω-(vinyloxy)alkanes with Si-H functional silanes and siloxanes were carried out in a similar manner. The following two examples are typical for the preparation of the 1-propenyl ether functional siloxanes. Example 1. Synthesis of IP2. To 2.69 g (20 mmol) of 1,1,3,3tetramethyldisiloxane and 4.96 g (42 mmol) of 1-(1-propenoxy)4-(vinyloxy)ethane in a 50 mL round-bottom two-necked flask there were added 10 mL of dry toluene. To this solution was added 1 µL of the Karstedt catalyst (3% Pt complex solution in xylene).9 The reaction mixture was stirred under nitrogen initially at 50 °C. The resulting reaction exotherm carries the temperature to approximately 100 °C. After 16 h, the reaction was complete, the reaction mixture was cooled to room temperature, and the solvent was removed on a rotary evaporator. In this case, the product was further purified by distillation under reduced pressure (0.05 mm) at 130 °C in a Bu¨chi microdistillation apparatus. There were obtained 7.19 g (97% yield) of the difunctional siloxane monomer IP2. Example 2. Synthesis of IVP2. In a manner analogous to that described above, 0.67 g (5 mmol) of 1,1,3,3-tetramethyldisiloxane and 2.01 g (12 mmol) of 2-(1-propenoxy)-5-(vinyloxy)-3-oxapentane (IVP) were combined in a 25 mL roundbottom flask containing 10 mL of dry THF and 0.9 mg (1 µmol) of (Ph3P)3RhCl (Wilkinson’s catalyst). The reaction flask was equipped with a magnetic stirrer, reflux condenser, and a CaCl2 drying tube and then heated and stirred at 60 °C for 16 h. After removal of the solvent using a rotary evaporator, the residue was subjected to vacuum distillation in a Bu¨chi microdistillation apparatus. There were obtained 2.15 g (92% yield) of IVP2 with a boiling point of 210 °C at 0.05 mmHg. High-boiling 1-propenyl ether functional siloxane monomers and oligomers which could not be purified by distillation were stripped of solvents and starting materials under high vacuum for several hours. All monomers were subjected to analysis by 1H NMR and by elemental analysis. Data for the monomers prepared in this article are given in Table 2. Photopolymerization of 1-Propenyl Ether Functional Siloxanes. Thin-Film Photopolymerizations. 1-Propenyl ether functional monomers and oligomers containing 0.5 mol % (4(n-decyloxy)phenyl)phenyliodonium hexafluoroantimonate were spread as a thin (approximately 25 µm) films onto glass plates and then irradiated using a Fusion Systems, Inc. Laboratory UV cure processor fitted with a microwave-activated 300 W UV lamp aligned perpendicular to the travel of the conveyor belt and mounted at a distance of 10 cm from the belt. The monomers and oligomers exhibited very high rates of polymerization and required less than 1 s of irradiation to produce a tackfree film.

(8) Crivello, J. V.; Lee, J. L. J. Polym. Sci., Polym. Chem. Ed., 1989, 27, 3951.

(9) Karstedt, B. D. U.S. Patents 3,715,334, 1973; 3,775,452, 1973; and 3,814,730 to General Electric.

1-Propenyl Ether Functional Siloxanes

Chem. Mater., Vol. 8, No. 1, 1996 211

Table 2. Synthesis of 1-Propenyl Ether Functional Siloxanes by the Hydrosilation of r-(1-Propenoxy)-ω-vinyloxyalkanes (CH2dCHOYOCHdCHCH3)a nota- yield reactant tion (%)

silane

%C

%H

78 found calc 82 found calc 78 found calc 91 found calc 85 found calc 97 found calc

68.00 68.13 54.67 55.34 51.31 51.49 54.80 55.42 50.67 51.03 -

9.14 9.15 9.72 9.75 9.31 9.26 8.75 8.74 8.59 8.56 -


69.70 69.81 58.47 59.14 55.11 55.39 58.36 58.60 55.24 55.52 -

9.71 9.65 10.53 10.55 9.83 9.84 9.24 9.33 9.27 9.32 -

98 found calc IIIP2 97 found calc IIIP3M 89 found calc IIIP3P 95 found calc IIIP4 98 found calc IIIPM 99 found calc

71.07 71.19 61.78 62.10 57.79 58.49 60.87 61.18 58.46 58.97 -

10.12 10.06 10.90 10.82 10.39 10.31 9.82 9.81 9.68 9.90 -


65.63 66.19 54.45 55.19 51.94 52.00 55.28 55.01 50.74 51.49 -

9.19 9.15 9.62 9.68 9.31 9.24 8.80 8.90 8.65 8.68 -


72.78 72.60 63.99 64.93 61.30 61.42 62.91 63.70 61.58 62.18 -

9.89 9.87 10.26 10.53 10.14 10.08 9.68 9.64 9.73 9.69 -

Ph(CH3)2SiH

IPa

IP1

[H(CH3)2Si]2O

IPa

IP2

CH3Si[OSi(CH3)2H]3 PhSi[OSi(CH3)2H]3 [Si(CH3)H-O]4 (D4H)

IPa

IP3M

IPa

IP3P

IPa

IP4

IPa

IPM

Ph(CH3)2SiH

IIPa

IIP1

[H(CH3)2Si]2O

IIPa

IIP2


IIPa

IIP3M

IIPa

IIP3P

IIPa

IIP4

IIPa

IIPM

Ph(CH3)2SiH

IIIPa

IIIP1

[H(CH3)2Si]2O

IIIPa

CH3Si[OSi (CH3)2H]3 PhSi[OSi(CH3)2H]3 [Si(CH3)H-O]4 (D4H)

IIIPa

CH3 H Si O CH3

CH3 H Si O CH3

CH3 H Si O CH3

CH3 CH3 (SiO)5 (Si•O)60

Si H

H

CH3

CH3

CH3


Si H

H

CH3

CH3

CH3

IIIPa IIIPa


Si H

H

CH3

CH3

CH3

IIIPa

Ph(CH3)2SiH

IVPb

IVP1

[H(CH3)2Si]2O

IVPb

IVP2


IVPb

IVP3M

IVPb

IVP3P

IVPb

IVP4

IVPb

IVPM

Ph(CH3)2SiH

VPb

VP1

[H(CH3)2Si]2O

VPb

VP2


VPb

VP3M

VPb

VP3P

VPb

VP4

VPb

VPM

CH3 H Si O CH3


Si H

H

CH3

CH3

CH3

elemental analysis

propriate IR band due to the polymerizing group while simultaneously irradiating the thin film sample with UV light. Measurements were performed on a Midac, Corp. Fourier transform infrared spectrometer equipped with a UVEXS Co SCU 110 UV lamp fitted with a flexible liquid optic cable. All studies were conducted using broad band, unfiltered UV light at an intensity of 18-19 mW/cm2. Samples were prepared by placing the liquid monomer containing 0.25 mol % per 1-propenyl ether equivalent of either (4-(n-decyloxy)phenyl)phenyliodonium hexafluoroantimonate or 2 wt % bis(4-dodecylphenyl)iodonium hexafluoroantimonate (UV-9380C) between two poly(vinylidine chloride) films and then mounting the sandwich in 5 cm × 5 cm slide frames. The progress of the polymerizations of the 1-propenyl ether functional silanes and siloxanes were determined quantitatively by monitoring the decrease of the 1-propenyl ether double bond peaks at 16601670 cm-1. Similarly, the polymerization of an epoxy functional silane (GE-9500) was monitored by following the 3000 cm-1 bond due to the epoxy C-H bond. The data were collected on a Bit-Wise Co. 486 PC computer, reduced and plotted as conversion versus time curves with the aid of a Galactic Industries Corp. Grams 386, Version 3.0 software package. Light-intensity measurements were made with the aid of an International Light Co. Control-Cure Radiometer.

Results and Discussion Monomer Synthesis and Characterization. Work in this laboratory has resulted in the development of a novel approach to the synthesis of very highly reactive cationically polymerizable silicon-containing monomers and oligomers. As reported previously,7 vinyl ethers undergo hydrosilation reactions to give predominantly the β-substituted ethers as shown in eq 2. Thus, it is Pt

ROCHdCH2 + R3SiH 9 8 ROCH2CH2SiR3 (2) catalyst not possible to prepare well-characterized vinyl ether functional siloxanes by the hydrosilation condensation of multifunctional Si-H compounds with di- and multifunctional vinyl ethers. Similarly, attempts to preferentially hydrosilate compounds bearing both vinyl ether and allyl ether groups were unsuccessful. While the allyl ethers undergo more rapid hydrosilation than vinyl ethers, the difference in their rates is not sufficient to preclude some competitive addition at both sites. Clearly, we required an intermediate compound bearing two different functional groups; one that undergoes preferential hydrosilation and another that undergoes facile cationic polymerization. It has been noted in this laboratory10 that despite their close structural and electronic similarity to vinyl ethers, 1-propenyl ethers fail to undergo hydrosilation reactions to any appreciable extent (eq 3). At the same (3)

lyst used. c Wilkinson’s catalyst used.

time, we have observed that 1-propenyl ether monomers undergo photoinitiated cationic polymerization at rates comparable to those of vinyl ethers.11 We have taken advantage of these two key observations to design a series of novel highly reactive mono-, di-, and multifunctional 1-propenyl ether functional siloxanes. To achieve our goal, intermediates bearing a vinyl ether

Fourier Transform Real-Time Infrared Spectroscopy (FTRTIR). More definitive photopolymerization data were obtained using Fourier transform real-time infrared analysis (FT-RTIR). This method involves the monitoring of an ap-

(10) Crivello, J. V.; Yang, B.; Kim, W.-G. J. Macromol. Sci., Pure Appl. Chem., in press. (11) Crivello, J. V.; Jo, K. D. J. Polym. Sci., Polym. Chem. Ed., 1993, 31, 1483.

CH3 H Si O CH3


Si H

H

CH3

CH3

CH3

a

Y ) (IP) -(CH2)2-; (IIP) -(CH2)4-; (IIIP)-(CH2)6-; (IVP) b Karstedt cata-(CH2)2-O-(CH2)2-; (VP) CH2 CH2

212



Figure 1. The 500 MHz 1H NMR monomer IIP3P in CDCl3.

Figure 2. FT-IR spectra for the photopolymerization of monomer IP4P using 0.75 mol % IOC10 as photoinitiator.

group at one end of the molecule and a 1-propenyl ether group at the other. Such compounds can be prepared by straightforward procedures as described below. Starting with an alkanediol, R-(vinyloxy)-ω-hydroxyalkanes can be readily obtained in good yields by treatment with acetylene in the presence of a base such as KOH.12 KOH

HOYOH 9 8 CH2dCHOYOH HCtCH

(4)

In a subsequent reaction, the facile allylation of the R-vinyloxy-ω-hydroxyalkanes was carried out under (12) Schildknecht, C. E. Vinyl and Related Polymers, John Wiley & Sons: New York, 1952; p 594.

phase transfer conditions to give the corresponding R-allyloxy-ω-vinyloxy alkanes as depicted in eq 5. (C4H9)4N+Br-

CH2dCHOYOH + CH2dCHCH2X 9 8 NaOH X ) Cl or Br CH2dCHOYOCH2CHdCH2 (5) Last, the R-(allyloxy)-ω-vinyloxyalkane was isomerized (eq 6) to the desired R-(1-propenoxy)-ω-(vinyloxy)alkane precursor in high yields using (Ph3P)3RuCl2 as the catalyst. The structures of the R-(1-propenoxy)-ω-(vinyloxy)alkanes prepared according to the above described synthesis are shown in Table 1.


Chem. Mater., Vol. 8, No. 1, 1996 213

Figure 3. FT-RTIR curves for the photopolymerization of monomers based on the hydrosilation of IP with various silanes using IOC10 as photoinitiator. (Ph3P)3RuCl2

CH2dCHOYOCH2CHdCH2 98 CH2dCHOYOCHdCHCH3 (6) The compounds appearing in Table 1 are liquids that were purified either by distillation or by flash column chromatography. It should be noted that the R-(1propenoxy)-ω-(vinyloxy)alkanes consist of an approximate 60:40 mixture of trans and cis isomers resulting from the geometrical isomerism of the 1-propenyl ether double bond. Due to the close similarity of their boiling points, no attempt was made to separate the isomers. These compounds bearing two different cationically polymerizable groups within the same molecule are an interesting class of ambifunctional monomers whose cationic polymerization will be reported in a latter publication.13 The R-(1-propenoxy)-ω-(vinyloxy)alkanes in Table 1 undergo exclusive regioselective hydrosilation reaction at the vinyl ether group. A general reaction depicting this is shown in eq 7. When multifunctional siloxanes Pt or Rh

CH2dCHOYOCHdCHCH3 + R3SiH 9 8 catalyst R3SiCH2CH2OYOCHdCHCH3 (7) are employed in this reaction, multifunctional 1-propenyl ether functional siloxanes are readily produced. The structures of a series of 1-propenyl ether functional monomers prepared during the course of this investigation are given in Table 2. It is noteworthy that in all cases the hydrosilation reactions were carried out in the (13) Crivello, J. V.; Lo¨hden, G. J. Polym. Sci., Polym. Chem. Ed., submitted.

presence of a platinum or a rhodium catalyst employing either exact stoichiometry or with a slight excess of the R-(1-propenoxy)-ω-(vinyloxy)alkane. In no case was there any indication that competitive hydrosilation at the 1-propenyl group occurred. This was particularly evident by the absence of cross-linking in the hydrosilation of the Si-H functional siloxane oligomers to produce soluble, free flowing copolymers 1PM-VPM bearing pendant 1-propenyl ether groups (eq 8). This

(8)

hydrosilation method is very versatile and mono-(IP1VP1), di-(IIP2-VP2), tri-(IP3M-VP3M; IP3P-VP3P), cyclic tetrafunctional (IP4-VP4) monomers and multifunctional (1PM-VPM) oligomers can be readily prepared under mild conditions. For these reactions, two different transition-metal catalysts were employed; Karstedt’s catalyst,9 which is a platinum complex of 1,1,2,2-tetramethyldivinyldisiloxane and Wilkinson’s catalyst, (Ph3P)3RhCl2. It has been found that although the hydrosilation of the vinyl ether group is slow in toluene, it may be markedly accelerated when the reaction is carried out in THF. When Wilkinson’s catalyst was used, it was found necessary to conduct the reaction in the presence of oxygen to activate the catalyst.

214



Figure 4. FT-RTIR curves for the photopolymerization of monomers based on the hydrosilation of IIP with various silanes using IOC10 as photoinitiator. Table 3. Photopolymerization of 1-Propenyl Ether Functional Monomers and Oligomers photoinitiator Rp/[M0]a photoinitiator Rp/[M0]a monomer concn (mol %) (s-1) monomer concn (mol %) (s-1) IP IP1 IP2 IP3M IP3P IP4 IPM IIP1 IIP2 IIP3M IIP3P IIP4 IIIP1 IIIP2

0.5 0.25 0.5 0.75 0.75 1.0 0.5 0.25 0.5 0.75 0.75 1.0 0.25 0.5

0.43 0.11 0.26 0.29 0.20 0.24 0.05 0.45 0.35 0.23 0.26 0.11 0.25 0.34

IIIP3M IIIP3P IIIP4 IVP1 IVP2 IVP3M IVP3P IVP4 VP1 VP2 P3M VP3P VP4

0.75 0.75 1 0.25 0.5 0.75 0.75 1.0 0.25 0.5 0.75 0.75 1.0

0.22 0.28 0.22 0.12 0.15 0.05 0.21 0.25 0.13 0.20 0.11 0.18 0.04

a Determined from the intial slope of the conversion versus irradiation time curves.

With the exception of the 1-propenyl ether functional siloxane oligomers 1PM-VPM which were slightly viscous oils, the di-, tri-, and tetrafunctional monomers prepared during this investigation were low-viscosity, colorless, odorless oils. The structure of the monomers and oligomers were determined by means of their 1H NMR spectra which were straightforward and diagnostic for these compounds. For example, Figure 1 gives the 1H NMR spectrum of monomer IVP3P. Further confirmation of the structure of these materials was provided by elemental analysis, and those data are given in Table 2. Evaluation of the Photoinitiated Cationic Polymerization of 1-Propenyl Ether Functional Siloxanes. It has been found that the 1-propenyl ether functional siloxanes are readily and rapidly photopolymerized by exposure to UV irradiation in the presence

of a variety of diaryliodonium salts as cationic photoinitiators. This is depicted in eq 9, where S represents a siloxane containing group and X- is a nonnucleophilic anion.

(9)

Critical to the success of these photopolymerizations is the solubility of the onium salt photoinitiator in these nonpolar monomers. It has been found that diaryliodonium salts bearing long alkoxy or alkyl side chains are particularly well suited for the polymerization of these materials because of their enhanced solubility in the monomers. For this study, we have employed the two diaryliodonium salt photoinitiators shown below:

Initially, the various monomers shown in Table 2 were evaluated by conducting photopolymerizations in thin films. Accordingly, 25 µm (1 mil) films were drawn onto glass plates and irradiated with broad-band UV light using a Fusion Systems 300 W microwave-actuated medium-pressure mercury arc lamp. The apparatus was fitted with a conveyor that can be controlled to pass samples under the lamp at controlled speeds to provide different exposure doses. It was observed that with the exception of the monofunctional monomers, IP1-VP1,


Chem. Mater., Vol. 8, No. 1, 1996 215

Figure 5. FT-RTIR curves for the photopolymerization of monomers based on the hydrosilation of IIIP with various silanes using IOC10 as photoinitiator.

Figure 6. FT-RTIR curves for the photopolymerization of monomers based on the hydrosilation of IVP with various silanes using IOC10 as photoinitiator.

all the di- and multifunctional monomers and oligomers were converted to tackfree films at the highest speed of

the conveyor (97.5 cm/s, 3.2 ft/s) which corresponds to a UV dose of less than 78 mJ/cm2.

216



Figure 7. FT-RTIR curves for the photopolymerization of monomers based on the hydrosilation of VP with various silanes using IOC10 as photoinitiator. Table 4. Photopolymerization of 1-Propenyl Ether Functional Poly(dimethylsiloxane) Oligomers monomer

photoinitiator concn (wt %)

Rp/[Mo]a (s-1)

conversion at 300 s (%)

IPM IIPM IIIPM IVPM VPM UV-9100

1 1 1 1 1 2

0.09 0.07 0.11 0.05 0.11 0.03

100 100 100 100 100 87

a Determined from the intial slope of the conversion versus irradiation time curves.

The rapid photopolymerization of these novel monomers can be monitored using Fourier transform realtime infrared spectroscopy (FT-RTIR). The decrease in the IR bands in the region 1660-1680 cm-1 assigned to the cis- and trans-1-propenyl ether double bonds was followed continuously as a function of time while the sample was simultaneously irradiated with UV light. Thus, this method directly monitors the disappearance of the reactive 1-propenyl ether groups as the polymerization proceeds. The polymerization studies described here were conducted at a low light intensity of 18-19 mW/cm2 and at a photoinitiator concentration of 0.25 mol % IOC10 per 1-propenyl ether group to slow them sufficiently to allow kinetic analysis. In the case of the oligomeric 1-propenyl ether functional poly(siloxane)s, a photoinitiator (UV-9380C) concentration of 2 wt % was employed. Figure 2 shows the evolution of the FT-IR spectra obtained in the photoinduced cationic polymerization of monomer IVP3P at a repetition rate of 1 scan/s. In this figure, the absorptions for the trans- (1660 cm-1) and cis- (1670 cm-1) 1-propenyl ether groups present in the monomer can be clearly delineated. As polymerization

proceeds over the course of 50 s, both bands markedly decrease nearly to zero with the major decrease occurring within the first 10 s. By inspection of this figure, it can also be noted that the cis 1-propenyl ether is more reactive than the trans isomer. This observation has been noted previously.13 In Figure 3 are plotted the FT-RTIR conversion versus irradiation time curves for the six siliconcontaining monomers in which IP was used as the derivatizing R-(1-propenoxy)-ω-(vinyloxy)alkane. Also included for comparison in this figure is a curve for the photopolymerization of IP itself. Table 3 presents data extracted from these curves. IP undergoes very rapid photoinduced cationic polymerization and proceeds to high conversion (90%). However, the conversion of the silicon-containing monomers is even higher and is quantitative in every case. This is contrary to the general observation that the conversion of functional groups of a monomer in photopolymerization decreases as the functionality of a monomer increases.14 One explanation for this result might be that the shrinkage taking place during these very rapid photopolymerizations can occur more slowly than the polymerization itself. The difference between the rate of polymerization and the rate of shrinkage is typically greater the faster the polymerization proceeds. This has been experimentally demonstrated by Kloosterboer15,16 and results in additional free volume available to reactants during all stages of the polymerization. Additional free volume (14) Moore, J. E. UV Curing Science and Technology; Pappas, S. P., Ed.; Technology Marketing: Stamford, CN, 1978; p 134. (15) Kloosterboer, J. G.; van de Hei, G. M. M.; Gossink, R. G.; Dortant, G. C. M. Polym. Commun. 1984, 25, 322. (16) Kloosterboer, J. G.; Lippits, G. J. M. J. Imag. Sci. 1986, 30, 177.


Chem. Mater., Vol. 8, No. 1, 1996 217

Figure 8. FT-RTIR study and comparison of the photopolymerization of 1-propenyl ether functional poly(dimethylsiloxane)s with epoxycyclohexyl functional poly(dimethylsiloxane) UV-9100 using UV9380C as photoinitiator.

provides increased mobility to the functional groups and chain ends during polymerization and ultimately results in higher conversions. Another reason for the high conversion of functional groups is the high conformational mobility of the functional groups within the monomers which is due to the presence of the siloxane group. Ultimately, this results in a lowering of the Tg. During polymerization, the Tg usually rises as the crosslink density increases but even in the final state of the polymerization, the Tg may be depressed below the temperature at which the photopolymerization takes place. The cationic photopolymerizations of the 1-propenyl ether functional siloxanes described in this investigation are highly exothermic. Using an infrared optical pyrometer, temperatures of 90-100 °C of some samples were recorded during photopolymerization. This temperature increase results in additional chain mobility and an increase in the free volume due to the thermally induced acceleration of the rate of polymerization. In Figure 3, the FT-RTIR curve of the functionalized oligomer, IPM, shows a long induction period and a rather slow polymerization as compared to the other monomers. Both observations can be attributed to the poor solubility of the photoinitiator, IOC10, in the oligomer. It should, however, be noted that despite this, the conversion of the propenyl ether groups is still quantitative after 300 s irradiation. A very similar pattern of reactivities can be observed in Figure 4 and Table 3 with the series of monomers based on 1-(1-propenoxy)-4-(vinyloxy)butane (IIP). For these monomers, a very short induction period was noted; however, the rates of polymerization (Rp/[M0]) were comparable to the previous series. The tetrafunctional monomer, IIP4, has the highest viscosity and also gives slowest rate of polymerization among the monomers.

Analogous results obtained for the series of monomers based on IIIP, IVP and VP, respectively, are displayed respectively in Figures 5-7 and in Table 3. Along with the same trends noted above, the monomers synthesized using the latter two intermediates polymerize more slowly than the former monomers. One possible reason is these monomers were prepared using Wilkinson’s catalyst which contains triphenylphosphine ligands instead of the usual Karstedt catalyst which bears nonbasic (vinylsiloxane) ligands. Triphenylphosphine is sufficiently basic to inhibit polymerization by deactivating the photogenerated acid as well as the growing chain ends. To eliminate the problem of photoinitiator solubility in the 1-propenyl ether functional oligomers (IPMVPM), FT-RTIR studies were conducted using the commercially available photoinitiator UV-9380C. This initiator contains approximately 45% of bis(dodecylphenyl)iodonium hexafluoroantimonate in 2-ethyl-1,3hexanediol. The solubility of this photoinitiator is quite good in all five prepolymers. For comparison, the analogous commercially available epoxycyclohexyl functional poly(dimethylsiloxane) oligomer UV-910017,18 was included in the study. Figure 8 and Table 4 show a direct comparison of these functionalized silicone oligomers. As the data clearly indicate, the photopolymerization of the novel 1-propenyl ether functional poly(dimethylsiloxane) oligomers proceeds much faster than UV-9100. The conversions of the 1-propenyl ether groups measured at 300 s for these former oligomers are also quantitative while for the latter epoxycyclohexyl functional oligomer, the epoxide conversion is 87%. It should also be noted that to achieve comparable rates, (17) Riding, K. Proceedings RadTech '90-North America Conference, 1990; p 377. (18) Eckberg, R. P. Proceedings Radtech '88-North America Conference, 1988, p 576.

218


2 wt % of the photoinitiator solution (0.95 wt % diaryliodonium salt) had to be added to UV-9100. Conclusions A novel series of cationically photopolymerizable siloxane monomers and oligomers has been prepared by the regioselective hydrosilation of several different R-(1propenyl)-ω-(vinyloxy)alkanes with various Si-H containing precursors. Regioselective hydrosilation, of these bifunctional compounds takes place exclusively at the vinyl ether sites thereby giving 1-propenyl ether functional siloxanes in high yields. These novel mono-


mers and oligomers exhibit very high reactivity in cationic photopolymerization. An unusual feature of the photopolymerizations is that they proceed to essentially quantitative conversion of the 1-propenyl ether groups independent of the functionality of the starting monomer. The novel monomers and oligomers may have interesting applications as photocurable coatings. Acknowledgment. The authors would like to thank the General Electric Co. for their financial support of this work. CM9503240

Synthesis and Photopolymerization of 1-Propenyl Ether Functional

Recommend Documents