Photoinitiator Activity, Electrochemistry, and Spectroscopy of Cationic

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Downloaded by LOUISIANA STATE UNIV on May 8, 2015 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0238.ch021

Photoinitiator Activity, Electrochemistry, and Spectroscopy of Cationic Organometallic Compounds W. A. Hendrickson and M. C. Palazzotto Corporate Research Laboratories, 3M Company, St. Paul, M N 55144 Of particular interest as photoinitiators of cationic polymenzation are the (n -cyclopentadienyl)(n -arene)iron(+) compounds. In an effort to produce compounds with longer wavelength sensitivity, we prepared a series of (n -fluorenyl)(η -cyclopentadienyl)iron(+) compounds along with the correspondingfreefluorenylligands. The iron complexes of these ligands have transitions that are much more intense than the parent simple arene derivatives and electrochemistry more characteristic of the free ligand A com parison of the electrochemical and spectroscopic data for metal complex and free ligand showed that the electronic transitions in the metal complex are intraligand in origin, yet still giveriseto metal-centered photochemistry. The metal complexes of the fluorenyl ligands provide a route to easily prepare variablewavelength photoinitiators. 5

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V ^ A T O N I C O R G A N O M E T A L L I C COMPOUNDS O F M A N Y TYPES are efficient photoinitiators of cationic polymerization. The activity of these compounds is primarily dependent on the nature of the counterion and to a lesser degree on the identity of the ancillary ligands remaining i n the coordination sphere of the metal. The (^-cyclopentadienyl) (n arene)iron(+) compounds are attractive for industrial applications because of their ease of preparation, efficiency of initiating polymeriza tion, and visible light sensitivity. One of the most important industrial applications is i n the solvent and coatings industry. 6

0065-2393/93/0238-041l$06.00/0 © 1993 American Chemical Society

In Photosensitive Metal—Organic Systems; Kutal, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1993.

412

PHOTOSENSITIVE M E T A L - O R G A N I C SYSTEMS

Solvents and Coatings The coatings industry is a major user of solvents (consuming 33% of an estimated 12 billion tons in 1987) (2). Related industries consume an additional 15% of the U.S. solvent market. The trend is toward decreas ing the use of solvent-borne systems (see Figure 1), but solvents still will represent a major fraction in the year 2010 (1). 9 Downloaded by LOUISIANA STATE UNIV on May 8, 2015 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/ba-1993-0238.ch021

Weight 10 η

of S h i p m e n t s in 10 lb.

1987

1992

1997

2010

Year ϋϋ

Solventborne

H

Radiation Curable

H H Waterborne

J i l l Powder

High Solids-Liquid

Figure 1. Trends in shipments of industrial coatings in the United States. Continued use of solvents in the coatings and related industries (sealants, printing inks, electronics, and magnetic media) has led to some critical energy and environmental concerns. First, solvent-based manufac turing is energy intensive because of the high energy content of petroleum-based solvents and the energy required for processing and sol vent emission control. Second, lower emission limits for volatile organic compounds (VOC) are forcing the coatings industry to invest in expensive solvent emissions control equipment with the added cost of disposal of recovered solvents. In addition to these important economic and environmental issues, the coatings and related industries must meet these demands while main taining or improving product standards. The ideal solution is to convert coating processes to 100% reactive systems. This conversion would elim inate both the energy and emission control requirements of solvent-based systems, but this must be done without sacrificing product quality. Solventless coatings can be produced in many ways, but the focus of the work presented here is to develop methods based on radiation pro-


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HENDRICKSON & PALAZZOTTO

Cationic Organometallic Compounds 4 1 3

cessing. This work is built around a new series of photocatalysts based on cationic organometallic compounds. The photochemical-thermal behav ior of these catalysts combined with resultant physical properties of the cured compositions should make the production of 100% reactive systems more likely.


Description of the Photocatalyst Systems For photoinitiated cationic polymerization, the technology that has received the most attention in recent years has been based on the onium salt systems (diazonium, iodonium, and sulfonium). These onium salts, when exposed to light, will decompose to generate both protonic acid and free radicals. A general mechanism for iodonium salt photochemistry is shown in Scheme I (2).

[hn P

2

[Ph f]V

χ"

2

[Ph l V

Ph-I +

Ph-

+

Ph-i~H

+

:

2

Ph-!

!

S-H

Ph-I-H*

>

+

Ph-! +

H

S +

Scheme I. General mechanism for onium salt photochemistry. The protonic acid generated can be used to initiate the cationic poly merization of monomers such as epoxies. The counterion X"" plays a role in the propagation of the polymerization reaction, and for epoxies, the trend for the relative rate of polymerization is B F ~ < PF "~ < A s F ~ < SbF ~ (J). The free radicals generated in these systems can be used to ini tiate the free radical polymerization of acrylates, for example. Cationic organometallic compounds are capable of generating a species that can also initiate cationic polymerization (4). The compounds can be selected from a wide variety of complexes (Figure 2), a number of which are relatively easy to synthesize and are air-stable. These last two factors are important when considering the application of such catalysts to industrial processes. The focus of this chapter will be the iron-containing compounds. Some of the characteristics of the compounds themselves will be described along with some of their curing chemistry, with a focus on epoxy compositions. A major portion of the chapter will be devoted to describ4

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414



ing a particular series of iron-containing cationic organometallics and the effort expended to modify their absorption properties and understand the electronic and electrochemical behavior.

0

Figure 2. Cationic organometallic compounds useful as photoinitiators of cationic polymerization.


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HENDRICKSON & PALAZZOTTO

Cationic Organometallic Compounds

415


Photochemistry of the Catalyst Systems In order to appreciate the capabilities of these organometallic-based pho tocatalysts, it is important to have some understanding of the photochemi cal reactions involved in the generation of the active catalysts. The action of light upon these compounds generates a Lewis acid. No evidence has been found for the initial generation of protonic acid. This property dif ferentiates these compounds from the onium salts described. The Lewis acid can be used to initiate cationic polymerization. The Lewis acid is a thermally active catalyst. Cationic polymerization is generally slower than free radical, but it can continue after irradiation has ceased. The two general types of cationic organometallic compounds that are active as photoinitiators of cationic polymerization are 1. arene-ring-only metal complexes 2. carbonyl-containing metal complexes Some discussion of the photochemistry and curing chemistry of a representative example of each type of complex will be presented. The basic photochemistry of the type 1 compounds can be under stood by examining the activity of iron(+) (CpFeArene ) upon exposure to light. A n absorption spectrum characteristic of this compound is shown in Figure 3. The spectrum shows the long wavelength and low extinction coefficient ligand field absorption bands typical of 3d transition metal complexes. +

300

350

400 Wavelength (nm)

450

500

Figure 3. Absorption spectrum of a typical cyclopentadienylareneiron cationic organometallic compound.


416


Whether the compound is irradiated in the ultraviolet or visible region, the first action upon absorption of a photon is loss of the arene ligand (Scheme II) (5). This loss generates a coordinatively unsaturated metal center with potentially three sites available to bond ligands or reac tive monomers. This initially generated species still has the cyclopenta dienyl moiety attached (this fact has been verified by ligand-exchange stu dies). Subsequent thermal or photochemical steps may generate a free F e species. The actual active Lewis acid may be the C p F e or the F e species. Free radicals are generated from the photochemistry of these compounds, although not very efficiently. The source of the free radicals is most likely a product of thermal reactions of the Cp~ moiety. If no suitable ligand is present, the intermediates go on to form ferrocene and Fe .


2 +

+

2 +

2 +

Scheme IL Irradiation of CpFeArene*. 4

These CpFeArene " catalysts can be used to photoinitiate the cure of epoxy-containing monomers. The compositions can be made to cure quite rapidly in air, and the speed of polymerization can be controlled by the identity of the counterion, as the following table shows: Counterion

Time to Cure (s)

SbF -

30

AsF -

45

PF "

120

BF "

>1800

6

6

6

4


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H E N D R I C K S O N & PALAZZOTTO

Cationic Organometallic Compounds

417

This order of counterion dependence follows that of the onium salts. fj-Cyclopentadienyl)dicarbonyltriphenylphosphineiron(+) is a typical example of compounds of type 2. The absorption spectrum of CpFe(CO) L PF "~ is shown in Figure 4. When this compound is exposed to light, it loses a carbonyl (Scheme III). This is the typical reac tion of a metal carbonyl-containing complex. This process has been veri fied by infrared studies, because the loss of the carbonyl group is easily detected. The coordinatively unsaturated species generated in this step is the active Lewis acid. This series of carbonyl-containing compounds allows for subtle con trol of reactivity because of the ease of which ancillary ligands can be introduced into the coordination sphere of the metal (Table I). The same counterion dependence is seen in these compounds, but there is also a +


2

300

6

350

400 Wavelength (nm)

450

500

Figure 4. Absorption spectrum of a typical cyclopentadienyldicarbonyliron ligand cationic organometallic compound L is triphenylphosphine and X~ isPFf.

CO

Scheme III Irradiation of CpFe(CO) L+PFf. 2


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Table I. Effect of Counterion and Ligand on Epoxy Cure Rate for CpFe(CO) L Compounds +

2

Counterion

3

3

SbF "

45

Photoinitiator Activity, Electrochemistry, and Spectroscopy of Cationic

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