High-Energy-Radiation-Induced Cationic ... - ACS Publications

Chapter 32

Downloaded by UNIV OF NORTH CAROLINA on July 12, 2016 | http://pubs.acs.org Publication Date: December 28, 1990 | doi: 10.1021/bk-1990-0417.ch032

High-Energy-Radiation-Induced Cationic Polymerization of Vinyl Ethers in the Presence of Onium Salt Initiators 1

2

1

Per-Erik Sundell , Sonny Jönsson , and Anders Hult 1

Department of Polymer Technology, The Royal Institute of Technology, S-100 44 Stockholm, Sweden AB Wilhelm Becker, Box 2041, S-19502, Märsta, Sweden 2

Cationic polymerization of diethyleneglycol divinyl ether and butanediol divinyl ether i n the presence of onium salts was i n d u c e d by γ-irradiation. T h e m e c h a n i s m for the i n i t i a t i o n process involves the reduction of o n i u m salts either by organic free r a d i c a l s or solvated electrons depending on the reduction potentials of the o n i u m salts. The reduction potentials of sulfonium salts was determined by polarography at the dropping mercury electrode. O n l y solvated electrons were capable of reducing the salts w i t h reduction potentials lower t h a n approximately -100 kJ/mol. T h e redox process liberates the non-nucleophilic a n i o n from the reduced onium salt and leads to the formation of a Brönsted acid or a stabilized carbenium ion. These species are the true initiators of cationic polymerization i n this system. T h e γ-induced decomposition of o n i u m salts i n 2ethoxyethyl ether was also followed by m e a s u r i n g the formation of protons. An ESR study of the structure of the radicals formed i n the γ-radiolysis of butanediol d i v i n y l ether showed that only α-ether radicals were formed.

D u r i n g the last decade there has been a growing interest i n industrial applications of radiation induced polymerizations. Photoimaging (1), photocuring of coatings (2.) a n d p r i n t i n g i n k s (3.) are examples of industrial applications depending on photo-crosslinking of polymers. These processes are based on the photogeneration of r a d i c a l or cationic species w h i c h initiate p o l y m e r i z a t i o n a n d c r o s s l i n k i n g . Photoinitiated free radical polymerization of organic coatings is a well established technology i n the coating industry today. Although thereare 0097-6156/90/0417-0459$06.00/0 © 1990 American Chemical Society Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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RADIATION CURING OF POLYMERIC MATERIALS


a few i n d u s t r i a l processes based on photoinitiated cationic polymerization, this field is i n an early state of development. Only acrylate systems are used for industrial electron beam curing today. No initiator is required for electron beam curing since free radicals are formed directly upon electron beam irradiation of the coating. The cationic photoinitiators that are most effective for coating applications are various onium salts. The existence of these onium salts has been known for nearly a century (4. 5). However, the utility of these substances as cationic initiators was not realized until seventy years after their discovery. Now, a wide range of thermally stable photoinitiators based on diaryliodonium (6-8) and triarylsulfonium (9. 10) salts of non-nucleophilic, complex metal halides has been synthesized and evaluated. UV-irradiation of these compounds results i n cleavage of a carbon-iodine ( H ) or carbonsulfur (12) bond to generate a reactive cation radical which abstracts hydrogen from the surrounding medium to ultimately form a stable, long-lived, strong Bronsted acid. This photogenerated acid can be employed to initiate a variety of cationic polymerization processes as well as other acid catalyzed reactions. Diaryliodonium and some sulfonium salts can also produce active cationic species via a redox reaction with a photosensitizer (IS). A n electron is transferred from the excited photosensitizer to the onium salt resulting i n generation of a photosensitizer cation radical capable of initiating polymerization. The reduced onium salt decomposes to a radical, aryliodide or a sulfide, and the counterion is liberated. Electron transfer to onium salts also occurs from electron donating organic free radicals (14). which are oxidized to highly reactive carbenium ions. The radical may be generated thermally (14). photochemically (15). or by ionizing radiation (16. 17). Solvated electrons are also capable of reducing diphenyliodonium (18) and triphenylsulfonium (12.) salts. This process has been found to promote high energy radiation-induced cationic polymerization of tetrahydrofuran (20). In these cases, reduction of the onium salts does not directly generate cationic species. Instead, the effect is due to the scavenging of solvated electrons by the onium salt and stabilization of radiolytically produced cations by the non-nucleophilic counterion which is liberated when the onium salt decomposes. The main reason for the limited industrial application of radiation-induced cationic polymerization is that oligomers that offer high cure rates are not commercially available. So far, due to their excellent combination of chemical, physical and electrical properties, only multifunctional epoxy oligomers have been used, but these have poor cure speeds compared to acrylate based systems. This is a serious limitation. Interesting alternatives to the epoxys are oligomers based on highly reactive vinyl ethers (21. 22) and the recently developed distyrene ethers (2â), which are as reactive as vinyl

Hoyle and Kinstle; Radiation Curing of Polymeric Materials ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

32.

SUNDELL ET

Cationic Polymerization of Vinyl Ethers

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ethers. Multifunctional vinyl ethers i n the presence of onium salts have also been shown to be an alternative to acrylate systems for electron beam curing (17. 22). Three major advantages of vinyl ethers compared to acrylates are 1) the low toxicity (24), 2) the low dosage required for E B curing and 3) reduced susceptibility to hydrolysis of the cured coating since there are no ester groups i n crosslinks. The present paper reports a study of the initiation mechanism for high energy radiation-induced cationic polymerization of divinyl ethers i n the presence of various onium salts. Although there is a great difference i n the dose rates of γ-radiation and electron beam, the radiation chemistry is essentially the same. Experimental Section M a t e r i a l s . Diethylene glycol divinyl ether, D E G D V E (GAF), butanediol divinyl ether, B D D V E (GAF) and 2-ethoxyethyl ether (Aldrich) were distilled from calcium hydride prior to gamma experiments. A l l starting materials for the synthesis of onium salts were obtained from Aldrich and used without further purification. Synthesis. Diphenyliodonium hexafluorophosphate, PI12IPF6, and triphenylsulfonium hexafluoroantimonate, Ph3SSbF6, were synthesized using l i t e r a t u r e procedures (25). Tetra-nbutylammonium hexafluorophosphate, TBAPF6, was obtained by direct metathesis between tetra-n-butylammonium bromide (99%) and potassium hexafluorophosphate (tech.), KPF6. The following general procedure has been employed for the synthesis of sulfonium salts listed below. The synthesis is accomplished by the condensation of a halide and a sulfide followed by metathesis to give the desired anion. The yields ranged from 35 to 70%. Equivalent amounts (0.04 mol) of the halide and sulfide were stirred i n 15 ml acetonitrile (99%) at room temperature. After five days the solvent was evaporated and the residue was dissolved i n 30 ml water. Sodium hexafluoroantimonate (tech.), NaSbF6 (s) (0.04 mol), or KPF6 (0.04 mol), i n 15 ml water was added to give the corresponding SbF6" and PF6" salts. The salts usually precipitated immediately and were filtered, washed with cold diethyl ether and dried. A l l salts were recrystallized twice from ethanol (99.5%). Phenacyltetramethylenesulfonium salts, P T S P F and PTSSbF , were synthesized from 2-chloroacetophenone (99%), tetrahydrothiophene (99%) and K P F g or NaSbFg. Benzyltetramethylenesulfonium hexafluoroantimonate, BTSSbF6, was synthesized from benzyl bromide (97%), tetrahydrothiophene and NaSbF. 4-Methoxybenzyltetramethylenesulfonium hexafluoroantimonate, MBTSSbFô, 6


6


462

was synthesized from 4-methoxybenzylchloride (97%), tetrahydrothiophene and NaSbF. Di-n-butylmethylsulfonium hexafluoro phosphate, BMSPF6 , was synthesized from di-n-butylsulfide (96%), iodomethane (99%) and KPF6. Satisfactory analytical values were found for all compounds, Table I.


Table I. Characteristics and elemental analysis data of synthesized onium salts. Elemental analysis shows found and (calculated) values %S

%Sb

2.7 (3.0)

7.3 (6.4)

25.3±1.3 (24.4)

Onium salt M W g/mol

mp °C

%C

%H

%P

426.1

143

33.6 (33.8)

2.0 (2.4)

7.3 (7.3)

499.1

177

42.5 (43.3)

Ph IPF 2

6

Ph SSbF 3

6

PTSSbF

6

443.1

186

32.3 (32.5)

3.1 (3.4)

7.3 (7.2)

29.3±1.5 (27.5)

BTSSbF

6

415.0

122

31.8 (31.8)

3.4 (3.6)

7.9 (7.7)

30.7±1.5 (29.3)

429.1

62

32.2 (33.6)

3.7 (4.0)

7.0 (7.5)

28.311.4 (28.4)

306.3

120

35.2 (35.3)

6.9 (6.9)

10.4 10.5 (10.5) (10.5)

MBTSSbF BMSPF

6

6

Procedures. Polymerizations were carried out i n polypropylene tubes (1 cm diameter). Solutions (2 ml) of oligomers with an initiator concentration of Ι Ο M were bubbled with either argon, A r , nitrous oxide, N2O, or oxygen, O2, for 20 minutes and then the tubes were sealed. The reactions were induced by exposure to gamma, γ, rays from an A E C L 2 2 0 C 0 γ-cell giving a dose rate of 1500 Gy/h. Samples were checked at intervals of 1 minute (25 Gy) until 12 minutes (300 Gy) had passed and thereafter at 2 minute intervals (50 Gy). The dosage required for inducing polymerization was taken as the gelation point of the sample. The γ-radiation-induced proton formation from ΙΟ" M onium salt i n 2-ethoxyethyl ether was followed spectroscopically at 540 nm using α-naphtyl red as indicator. A calibration curve with methanesulfonic acid i n 2-ethoxyethyl ether was used to determine the H concentration of irradiated samples. U V measurements were conducted on a Hewlett Packard 8451A Diode Array Spectrophoto-3

6 0

3

+


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SUNDELL ET AL.

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Cationic Polymerization oj Vinyl Ethers

meter. B D D V E and 2-methyl-2-nitrosopropane as a spin trap were used for E S R experiments. E S R samples were placed i n 3 mm Suprasil quartz ESR tubes, evacuated with the freeze-thaw method, sealed and exposed to a dose of 1500 Gy. The E S R instrument used was a Bruker E P R 420. The polaro-graphic apparatus was a Princeton Applied Research Model 174A Polarographic Analyzer. The aqueous p H 7.6 buffer for polarography was a solution of 0.042 M potassium dihydrogen phosphate (p.a.) and 0.015 M sodium tetraborate decahydrate (p.a.). The buffer solution containing Ι Ο M onium salt was bubbled with nitrogen for 15 minutes to remove oxygen before measurement and the half wave potential, Ε1/2, of the onium salt was determined at the dropping mercury cathode with a saturated calomel electrode as anode. A three second drop time was employed. Gelatin (0.002 %) was used as a maximum supressor.


-3

Results and Discussion When an organic coating is exposed to γ-rays, fast electrons are generated i n the sample by essentially three processes: photoelectric absorption, Compton scattering and pair production. Fast electrons dissipate most of their energy i n matter causing ionization, Reaction 1 and excitation of molecules, Reaction 2. When a molecule is excited above its ionization potential (10-12 eV for most organic molecules) it may lose energy by ionization or dissociation, Reaction 5, 6. The yield of ions i n a liquid depends on the distance the ejected electron travels before reaching thermal energy levels (about 0.025 eV at room temperature). This distance depends to a large extent on the electron density of the medium. For liquids with low dielectric constants, ε, the G-value for free ion production very low, indicating that most electrons rapidly recombine with their geminate positive ions, Reaction 3. The recombination produces an excited molecule with an energy excess lower than the ionization potential but often high enough to cause radical fragmentation. As ionized and excited species undergo further reactions, neutral free radicals are produced by several processes, Reaction 4-10. The yield of free radicals is much higher than that of free ions. For example, i n the radiolysis of liquid hydrocarbons, ε«2, the stationary state concentration of free radicals is about two orders of magnitude higher than that of free ions (2β) due to lower G-value (about 0.15 (27)) and higher recombination rates for ions relative to radicals (G =5.5 (28)). In ethers, having ε«4, the yield of ions is higher, Gi(diethyl ether)=0.35 (27). reportedly due to the possibility of electrons becoming solvated (29). Whether electrons really are solvated or not i n D E G D V E and B D D V E is not clear but electrons will be considered to be solvated i n the following discussion. Thus the main product from radiolysis of organic liquids is free radicals, which, are consequently responsible for a majority of the subsequent chemical reactions. r


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High-Energy-Radiation-Induced Cationic ... - ACS Publications

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