Chapter 22
Photoinitiated Polymerization Downloaded from pubs.acs.org by YORK UNIV on 12/03/18. For personal use only.
Synthesis and Photoinitiated Cationic Polymerization of Organic-Inorganic Hybrid Resins Ki Yong Song , Ramakrishna Ghoshal , and James V . Crivello 1
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Semiconductor R&D Center, SEC, Samsung Electronics Company, Ltd., San #24 Nongseo-Ri, Kiheung-Eup, Yongin-City, Kyungki-Do, Korea Polyset Company, P.O. Box 111, Mechanicville, NY 12118 Department of Chemistry, New York Center for Polymer Synthesis, Rensselaer Polytechnic Institute, 110 8 Street, Troy, NY 12180 2
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A streamlined synthetic scheme was developed for the preparation of a novel series of cationically polymerizable epoxy-functional organic-inorganic hybrid resins. The reaction sequence consisted first, of the regioselective rhodium catalyzed monoaddition of an α,ω-Si-H difunctional siloxane to a vinyl epoxide. The second step involves the addition of the remaining Si-Η bond of the latter adduct to vinyl trimethoxysilane. Finally, an ion exchange catalyzed sol-gel hydrolysis of the resulting trialkoxysilane functional epoxide yields the desired silicone-epoxy resins bearing pendant epoxy groups. These resins undergo facile U V and thermally induced cationic photopolymerization in the presence of onium salt photoinitiators to give crosslinked organic-inorganic hybrid glassy matrices. The mechanical and thermal properties of polymerized resins were determined and were correlated with the structures of the resins.
© 2003 American Chemical Society
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Introduction In recent years, organic-inorganic hybrid resins have received much attention in the literature due to their unique electrical, optoelectronic and mechanical properties. Much work is currently focused on the synthesis of crosslinked glass-like matrices composed of Si-O-Si linkages employing sol-gel techniques that involve the acid or base catalysed hydrolysis of tri- or tetraalkoxysilanes (i-J). Despite the many potential applications of organicinorganic hybrid resins, traditional sol-gel techniques for their synthesis have a number of drawbacks which have limited their use. First, sol-gel chemistry is slow, often days and weeks are required for the hydrolysis of the alkoxysilane to proceed to completion. During hydrolysis and subsequent condensation of the resulting silanols, a highly porous, crosslinked matrix resin is formed. If the hydrolysis is carried out too rapidly,fractureof the fragile matrix often occurs. Due to the loss of the alkyl groups as alcohols during hydrolysis, there is always considerable shrinkage of the resin matrix. Densification of the initially formed porous matrix by calcination is often required to produce specimens with good cohesion and mechanical strength. Lastly, the modification of sol-gel matrices through the incorporation of functionallized alkoxysilane precursors is sometimes difficult due to difference in the hydrolysis rates of the components or to their phase separation during the cohydrolysis step. In this laboratory, we have been addressing some of the problems outlined above by exploring several new approaches to the synthesis of siloxane-based hybrid organic-inorganic resins. Previously (4) we have employed the two-step methodology depicted in Scheme 1 for the preparation of these resins. First, the sol-gel condensation of a precursor such as I containing both a trialkoxysilane and an epoxy functional group (eq. 1) was carried out using an ion exchange resin as a catalyst. The resulting product Π is an oligomer with a backbone composed of both cyclic and linear siloxane groups and bearing pendant epoxy groups. The structure of the resin shown in equation 1 is idealized. Typically, oligomers with up to 12-15 repeat units can be prepared in which greater than 85% hydrolysis of the alkoxy groups has taken place. Despite the fact that a trialkoxysilane is employed in this reaction, the simultaneous hydrolysis and condensation reactions can be carried out in a controlled manner such that no crosslinking or gel formation takes place. Termination of the reaction can be achieved by simply removing the catalyst by filtration. In a subsequent step, these oligomers can be polymerized via a ringopening polymerization of the pendant epoxy groups. For example as shown in equation 2, the oligomers undergo very rapid crosslinking with formation of organic-inorganic hybrid networks ΙΠ when irradiated with U V light in the presence of an onium salt photoinitiator.
255 Scheme 1
The above synthetic approach has the advantage that the normally slow solgel hydrolysis condensation can be carried out in a rapid but controlled fashion using the ion-exchange catalyst. In addition, the strategy of initially carrying out the sol-gel reaction removes most of the shrinkage in the first step. The crosslinking photopolymerization of the pendant epoxy functional oligomer Π takes place rapidly with rather small shrinkage. No further densification of the polymerized specimen is required. The present paper describes the development of a new, generally applicable synthetic approach that can be employed to prepare a family of silicone-epoxy resins using simple, readily available precursors. These resins undergo facile cationic photopolymerization to yield organic-inorganic hybrid resins with a wide range of properties.
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Experimental General Synthesis of a-Epoxy-o-Trialkoxysiloxanes and oligomers The synthetic procedures given below are typical of those used for the preparation of the a-epoxy-ca-trialkoxysiloxanes and the silicone epoxy oligomers shown in Table 1. Preparation of I-[2-(3{7-Oxabicyclo[4A.0]heptyl}ethyl]-3-[2^ ethyl] -1,1,3,3-tetramethyldisiloaxne (Villa) A mixture of 20.0g (0.15 mol) of distilled 1,1,3,3-tetramethyldisiloxane, 18.5g (0.15 mol) of 3-vinyl-7-oxabicyclo[4.1.0]heptane, 20mL of dry toluene were charged into a 250-mL round bottom flask fitted with a reflux condenser and thermometer. There were added 30 mg of Wilkinson's catalyst, and the reaction mixture was heated at 60°C for 4 h in an oil bath. After this time, the *H-NMR spectrum showed that the vinyl peaks at 5.5 ppm had disappeared. Next, there were added 22.2g (0.15 mol) of distilled vinyltrimethoxysilane. After 6 hours at 90°C, the infrared absorption at 2160cm" (Si-Η group) had completely disappeared. Removal of the solvent under reduced pressure gave the desired product, (54,6g, 90% yield) of colorless oil, which was characterized by H-NMR and confirmed to be monomer Villa. 1
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Sol-Gel Condensation of 2-(3,4-Epoxycyclohexylethyl)trimethoxysilane A mixture of 20g (0.081 mol) of la, 4.4g (0.24 mol) of deionized water, 0.8g of Amberlite IRA-400 ion exchange resin and 10g of isopropanol were placed in a 100 mL round-bottom flask equipped with a magnetic stirrer and a reflux condenser. The condensation reaction was carried out at the reflux temperature, 82°C, for 6 hours. Thereafter, the solution was cooled,filteredand passed though a silica gel column using ethyl acetate as the eluent. The solvent was removed under vacuum, and a viscous liquid oligomer (16.3g) lb was obtained. *H-NMR (CDC1 ) 6(ppm) 0.5, CH -Si; 1.2-2.2, aliphatic protons; 3.1, epoxy CH; 3.48, OCH . 3
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Sol-Gel Condensation of2-(3{7-Œabicyclo[4JM]heptyl}ethyl-[2-trimethoxysilylethylj-methylphenylsilane A mixture of 6.0g (0.015mol) of Vila, 0.82g (0.0456 mol) of deionized water, 0.24g of Amberlite IRA-400 ion exchange resin and 3.0g of n-propanol were placed in a 100 mL round-bottom flask equipped with a magnetic stirrer
257 and a reflux condenser. The hydrolysis-condensation reaction was carried out at the reflux temperature, 97°C, for 24 hours. After the solution was filtered, it was passed though a silica gel column using ethyl acetate as the eluent. The ethyl acetate was removed in vacuum and a viscous liquid oligomer (4.9g) Vllb was obtained. *H-NMR (CDC1 ) ô(ppm) 0.2-2.2, CH -Si and CH -Si and aliphatic protons; 3.1, epoxy CH; 3.48, OCH ; 7.4, aromatic protons. 3
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Sol-Gel Condensation of l-[2-(3{7-Œabicyclo[4AM]heptyl}ethyl]-3-[2^^ methoxysilylethyl/-i, i , 5,3-tetramethyldisiloaxne To a 75mL pressure vesselfittedwith a magnetic stirrer were charged 10g (24.6mmol) of VHIa, 1.33g (73.8mmol) of deionized water, 0.4g of Amberlite IRA-400 ion-exchange resin, and 5.0g of isopropanol. The reaction mixture was stirred and heated at 150°C for 12 hours under pressure. After the solution was filtered and passed though a silica gel column using ethyl acetate as the eluent, the solvent was removed in vacuum to give a viscous liquid oligomer V m b (8.4g). 'H-NMR (CDC1 ) ô(ppm) 0-0.1, CH -Si; 0.5, CH -Si; 1.2-2.2, aliphatic protons; 3.1, epoxy CH; 3.4-3.8, OCH . 3
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Results and Discussion
Synthesis of Epoxy-Siloxane Oligomers In Scheme 1 is described the synthesis and photopolymerization of siliconeepoxide resins that we have previously prepared. This method requires epoxy compounds bearing trialkoxysilane groups as precursors for the sol-gel condensation. There is a dearth of such compounds availablefromcommercial sources and also few compounds of this type have been reported in the literature. Accordingly, we have been seeking alternative techniques for the synthesis of epoxy functional sol-gel resins. A novel series of silicon-containing epoxy resins has been prepared by a streamlined three-step process as depicted in equations 3-5 of Scheme 2 that makes use of readily available and inexpensive substrates. The discovery that it is possible to carry out a regioselective monohydrosilation addition of a vinyl epoxide at only one of the two Si-H groups an α,ω-Si-H terminated polydimethylsiloxane (n = 1,2,3) and methylphenylsilane was made in this laboratory (6, 7). This reaction was employed as shown in equation 3 to prepare monoadducts with the general structure IV and also V shown below.
258 In the present case, these monoadducts were not isolated, but were directly carried forward into the next step of the reaction sequence (eq. 4) wherein vinyltrimethoxysilane was added and subjected to further hydrosilation with the remaining Si-Η groups to yield V i a and V i l a respectively (8). Thus, in a simplified, two-step, one-pot process these and other such compounds bearing both trialkoxysilyl and epoxy groups in the same molecule can be readily prepared in high yields. Scheme 2 ÇH H-Si-
(Ph P) RhCi 3
un ι 3
l m
3
j
3
3
^
n
IV H
i(0CH ) 3
3
(H C0) Si— (CH )2— f CH 3
3
3
2
3
Via ion-exchange resin
OCH
9fo
CH
eq. 3
3
eq. 4
3
H2O (ÇH2)2
VIb
eq. 5
The sol-gel condensations of ambifunctional adducts la and VIIa-Xa (eq. 5) proceed smoothly in the presence of water and a weakly acidic ion-exchange resin. Typically, the condensations were carried out in solution in either hot isopropanol or n-propanol. The reactions were usually terminated when greater than 85% of the methoxy groups (3.55 ppm) had undergone reaction as determined by H-NMR. It was found that as the siloxane chain of the spacer is lengthened, increasingly more vigorous conditions for the sol-gel reaction were required. Thus, while la undergoes sol-gel condensation in 6 h in isopropanol at 82 °C, Xa, required 12 h heating at 150 °C under pressure in a sealed vessel. The desired silicone-epoxy oligomers were obtained as colorless viscous oils by filtering the reaction mixture to remove the ion-exchange resin and then stripping off the solvent under reduced pressure. Under these conditions ringopening of the epoxy groups was not observed. The resulting silicone-epoxy oligomers were colorless to very pale yellow viscous liquids that displayed high reactivity in photo- and thermally initiated cationic polymerization. Both H !
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NMR and Si-NMR spectra of VHIb suggest a structure analogous to the proposed structure for oligomer Π in which various degrees of hydrolysis of the trialkoxysilyl groups has taken place. Collected in Table 1 are the structures of the oligomers together with the conditions of their sol-gel condensations as well as the molecular weight and viscosity data. Again, it should be noted that the structures of the oligomers shown in Table 1 are idealized and that, in fact, the actual structure is much more complex with both cyclic and linear siloxane repeat units in the backbone. The approximate degree of polymerization (DP) is also given in Table 1. The oligomers range from hexamers to undecamers implying that they correspondingly possess on the average from six to eleven reactive epoxycyclohexyl groups per molecule. It is interesting to note that the viscosity of the oligomers appears to be closely related to the structure of the oligomers. Oligomer VQb with aromatic groups has an anomalously high viscosity although the molecular weight is comparatively low. As one increases the length of the dimethylsiloxane spacer in the resin, the viscosity decreases.
Table 1. Epoxy-Fuctional Silicone Resins Notation
Vllb
lb OCH
3
Structure
Vu H
a
c
- r O
Vfflb f
1 3
?
9
Yield (%) Viscosity(cP) Mw (g/mol) MWD DP #
NPA*, 91°C, 24 h 92 33260 2381 1.1 6 f
3
H3C-S-CH
? 3
?
H3C-51-CH3
IPA*, 82°C, 6h 100 2506 2430 1.3 9
(pCH
1
H3C-^i-CH
Reaction Conditions
Xb
Kb
H3C-^-CH
3
0
?
HjC-^CHa
9
IPA, 150°C, 12 h 98 2929 4186 1.3 9 +
Isopropyl alcohol, n-propyl alcohol, under pressure
IPA, 150°C, 12 h 95 648 6440 1.4 11 +
0
IPA, 150°C, 12 h 93 249 8135 1.7 9 f
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Irradiation Time (s) Figure 1. Real-time infrared study of the cationic photopolymerization of silicone-epoxy oligomeric resins using 2.0 mol% ofIOC 10 as photoinitiator. Photo- and Thermally Initiated Cationic Polymerization In previous papers from this laboratory it was reported that monomers bearing epoxycyelohexyl groups in general (P) and silieone-epoxy monomers (10) containing these groups in particular display high reactivity in photoinitiated cationic polymerization. The multifunctional sol-gel siliconeepoxy oligomers prepared in this investigation similarly undergo facile photoand thermally induced cationic ring-opening polymerization in the presence of photosensitive onium salts. In this study we have employed (4-ndecyloxyphenyl)phenyl- iodonium hexafluoroantimonate (IOC 10) as both the thermal and photoinitiator for the cationic ring-opening polymerization of the epoxide groups. This compound was selected for its high solubility in all the silicone-epoxy oligomeric resins and because this onium salt has a high quantum yield (-0.7) and is a very efficient photoinitiator for the cationic ringopening polymerization of epoxides (11). Employing the neat resins containing 2.0 mol% of IOC 10, the photopolymerizations were monitored using real-time infrared spectroscopy (RTIR) (12, 13). Using this technique, the disappearance of the infrared band at 886 cm" due to the epoxy groups was followed as a 1
261 function of time. Shown in Figure 1 are the conversion versus time curves obtained from these studies. Included in this figure for comparison is the kinetic curve for the photopolymerization of oligomer lb that was prepared using the process described in Scheme 1. All the epoxy functional oligomers display excellent reactivity in cationic epoxide ring-opening photopolymerization. Some structurally dependant trends can be distinguished in this study. Ib shows high reactivity as indicated by the initial portion of the kinetic curve, however, the conversion of epoxy groups reached after 200 seconds irradiation was only approximately 60%. In contrast, the photopolymerizations of all the other resins proceed to nearly quantitative conversions. It may be noted that Ib has the shortest and stiffest spacer linking the epoxycyclohexyl group with the siloxane main chain of all the resins. As a consequence, Ib would be expected to give the most highly crosslinked polymer. Diffusion and mobility of the bulky epoxycyclohexyl groups in this polymer would be expected to be restricted. This results in vitrification of the polymer during polymerization at low conversions. In contrast, the high mobility of the other resins with flexible siloxane groups in the spacers gives rise to much higher conversions due to suppression of the onset of vitrification. 70
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Temperature ( °C) Figure2. DSC scan of the thermally initiated cationic polymerization of silicone-epoxy oligomer Vlllb in the presence of 2.0 mol%IOC10 as initiator.
In Figure 2 is shown the differential scanning calorimetry (DSC) curve for the thermal polymerization of oligomer VDIb in the presence of 2.0 mol% IOC10. The onset of thermal decomposition of IOC10 takes place at 148 °C. Polymerization of this oligomer is very rapid as indicated by the sharpness of the exothermic peak in the DSC curve.
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150 250 350 450 550 650 750 850 Temperature f C)
Figure 3. Thermogravimetric analysis study of crosslinked silicone-epoxy resins carried out at a heating rate of 20 °C/min in nitrogen.
Thermal Stability of Epoxy Functional Organic-Inorganic Hybrid Polymers The thermogravimetric analysis (TGA) curves for thermally cured samples of the silicone-epoxy oligomers are given in Figure 3. The TGA study was carried out in nitrogen at a heating rate of 20°C/min. The polymers display excellent stability with the onset of thermal decomposition at approximately 350-400°C. In contrast, most common epoxy resins decompose below 200°C and this severely limits their utility in many applications. Mechanical Properties of Epoxy Functional Organic-Inorganic Hybrid Polymers The mechanical properties of the organic-inorganic hybrid polymers produced by thermal polymerization of the corresponding epoxy functional solgel oligomers were determined using dynamic mechanical analysis (DMA). The results are shown in Figures 4 and 5. Again, the polymer derivedfromIb was included in this study for comparison. The mechanical properties of the resins are directly related to the length and type of spacer group separating the epoxycyclohexyl groups from the siloxane main chain. Accordingly, Ib would be expected to be the stiffest and highest glass transition polymer. In Figure 4 it can be seen that this polymer exhibits no decrease in the storage modulus at temperatures up to 180°C. In fact, there is no decrease in the modulus or evidence of a glass transition at temperatures below 300°C. As the length of the spacer is increased and flexible siloxane linkages are introduced, one sees a
263 progressive decrease in the temperature of the fall off of the storage modulus that corresponds to a decrease in the glass transition temperature (Figure 5). Polymer Xb displays the lowest glass transition temperature (-6.5°C) while the TgS for K b and VHIb are respectively, 33°C and 80°C. A similar observation was made for photo-polymerized thin films of these oligomers. While the film derivedfromIb was brittle and could not be handled, thefilmsfromVlllb, IXb and Xb were increasingly moreflexibleand tough. 1.E4-10 ι
I
l.E+07 A
1
1
1
1
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»
Temperature ( °C) Figure 4. Comparison of the storage moduli as determined by dynamic mechanical testingfor thermally crosslinked silicone-epoxy resins Temperature scan rate 10 °C/min in nitrogen.
Conclusions A convenient and high yield method for the synthesis of novel siliconeepoxy resins has been developed. This method takes advantage of several new reactions discovered in this laboratory. First, the regioselective hydrosilation addition of a vinyl epoxide at only one end of an α,ω-Si-H difunctional siloxane or silane provides an intermediate bearing one epoxy and one Si-Η group. Further hydrosilation with vinyltrimethoxysilane leads to ambifunctional molecules bearing both trimethoxysilyl and epoxy groups that may be employed as substrates in the sol-gel condensation. In this work, we have utilized acidic ion-exchange catalysts to catalyze this latter reaction. The resulting siliconeepoxide oligomers can be readily converted by either photo- or thermally induced cationic ring-opening polymerization to crosslinked organic-inorganic hybrid resins. The properties of the crosslinked resins depend on the length of the spacer group between the siloxane backbone and the pendant epoxy groups. Short spacer groups lead to highly rigid, glass-like matrices, while longer spacer groups produce more flexible materials with considerable elongation. Many novel applications for these novel resins are anticipated including
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Temperature ( ° C )
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160
Figure 5. Comparison of the tan delta plots for thermally crosslinked siliconeepoxy resins as determined by dynamic mechanical testing. Temperature scan rate 10 °C/min in nitrogen.
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265 composites, optical adhesives and coatings, wave guides and electronic potting and encapsulating resins.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Brinker, C.J.; Scherer, G.W. Sol-Gel Science, Academic Press, Inc, New York, 1990. Wenzel, J. J. Noncryst. Solids 1985, 73, 693. Schubert, U.; Hüsing, N.; Lorenz, A. Chem. Mater. 1995, 7, 2010. Crivello, J.V.; Mao, Z. Chem. Mater. 1997, 9, 1554. Crivello, J.V.; Lee, J.L. J. Polym. Sci. Part A: Polym. Chem. 1989, 27, 3951. Crivello, J.V.; Bi, D. J. Polym. Sci., Polym. Chem. Ed. 1993, 31, 2563. Crivello, J.V.; Bi, D. J. Polym. Sci., Polym. Chem. Ed. 1993, 31, 2729. Crivello, J.V.; Bi, D. J. Polym. Sci., Polym. Chem. Ed. 1993, 31, 3121. Crivello, J.V.; Linzer, V. Polimery 1998, 68, 661. Crivello, J.V.; Lee, J.L. J. Polym. Sci., Polym. Chem. Ed. 1990, 28, 479. Crivello, J.V. Ring-Opening Polymerization, Brunelle, D.J. editor, Hanser Pub., Munich, 1993, p. 157. Decker, C.; Moussa, Κ. J. Polymer Sci.: Part A: Polym. Chem. 1990, 28, 3429. Decker, C.; Moussa, Κ. Makromol. Chem. 1990, 191, 963.
