Wolfram Schnabel

Photosensitized cationic polymerization of cyclohexene oxide: A mechanistic study concerning the use of pyridinium-type salts

Abstract The photoinitiation of the polymerization of bulk cyclohexene oxide (CHO) containing N -ethoxy-2-methyl pyridinium hexafluorophosphate (EMP + PF 6 − ) and either anthracene or thioxanthone (TX) at λ inc> 340 nm was studied. Regarding the action of anthracene it is notable that upon u.v. irradiation of a CHO solution of poly(tetrahydrofuran) bearing terminal anthryl groups a block copolymer, poly(tetrahydrofuran- block -cyclohexane oxide), is formed. The optical absorption spectrum of the block copolymer does not possess bands characteristic for anthracene. Therefore, the following mechanism is postulated: electron transfer from singlet excited anthracene molecules to EMP + ions results in the formation of anthracene radical cations that react with ethoxyl radicals stemming from the decomposition of EMP• radicals. 9-Ethoxy-9,10-dihydroanthryl ions generated in this way react with CHO thus initiating its polymerization. Regarding the action of TX it seems that the polymerization of CHO is essentially initiated by protons. The generation of protons has been evidenced. In propylene carbonate solution protons are generated with Φ(H + = 0.27 whereas Φ(−TX) = 0.028. The postulated mechanism is based on the reaction of triplets, 3 TX * , with both CHO ( k RH= 3 × 10 4 1 mol −1 s −1 ) and EMP + ions ( k ET= 4 × 10 7 1 mol −1 s −1 ). At low concentration of EMP + PF 6 − (6.8 × 10 −4 mol −1 l −1 ) 3 TX * molecules react almost exclusively (93%) with CHO and it appears that ketyl radicals thus formed react with EMP + ions, a process eventually resulting in the formation of protons and the regeneration of TX. At relatively high concentration of EMP + PF 6 − (6.8 × 10 −3 mol l −1 ), thioxanthone triplets are largely (47%) deactivated by electron transfer to EMP + ions. The importance of this reaction with respect to its contribution to the initiation of the polymerization of CHO has not yet been revealed.

Photosensitized cationic polymerization using N-ethoxy-2-methylpyridinium hexafluorophosphate

Abstract The cationic polymerization of cyclohexene oxide is initiated at room temperature upon irradiation at λinc > 350 nm of CH2Cl2 solutions containing N-ethoxy-2-methylpyridinium hexafluorophosphate (EMP+PF6−) and one of the following compounds: thioxanthone, anthracene, perylene or phenothiazine. Poly(cyclohexene oxide) of molar mass exceeding 105 is produced. In the absence of EMP+PF6−, which is transparent at λinc > 350 nm, these sensitizers are ineffective. Acetophenone and benzophenone do not act as sensitizers. An initiation mechanism involving electron transfer from the excited sensitizers to EMP+ ions is proposed. In flash photolysis experiments, electron transfer was evidenced by the formation of radical cations in the cases of anthracene, perylene or phenothiazine.

Initiation of cationic polymerization via oxidation of free radicals using pyridinium salts

Abstract Pyridinium ions of appropriate reduction potential E red 1 2 are capable of oxidizing carbon-centred free radicals to carbocations that can initiate the polymerization of various compounds. 1-Ethoxy-2-methyl pyridinium ions of E red 1 2 = −0.7 V were found to react with free radicals generated by (a) photolysis or (b) thermolysis of various compounds. Radical generation was achieved in case (a) with benzoinmethylether, diphenyl-2,4,6-trimethylbenzoylphosphine oxide or benzophenone/tetrahydrofuran (BP/THF) and in case (b) with phenylazotriphenylmethane, BP/THF, benzoylperoxide/THF or 2,2-azobisisobutyronitrile/THF. The following monomers were polymerized: 1,2-epoxycyclohexane (cyclohexene oxide) and n-butylvinyl ether.

N-Alkoxy pyridinium ion terminated polytetrahydrofurans. Synthesis and their use in photoinitiated block copolymerization

Abstract Living polytetrahydrofuran was terminated with pyridinium or isoquinolinium N-oxide to yield polymers with the corresponding end groups. Direct and sensitized irradiation of these photoactive polytetrahydrofurans produced alkoxyl radicals at both chain ends capable of initiating the radical polymerization of methyl methacrylate. In this way, triblock copolymers were formed. The block copolymer composition was determined with the aid of g.p.c. and optical and 1H n.m.r. spectroscopy.

Charge-transfer complexes of pyridinium ions and methyl- and methoxy-substituted benzenes as photoinitiators for the cationic polymerization of cyclohexene oxide and related compounds

Abstract The cationic photopolymerization of cyclohexene oxide and 4-vinylcyclohexene dioxide was achieved by using charge-transfer (CT) complexes of pyridinium salts and aromatic electron donors (hexamethylbenzene and 1,2,4-trimethoxybenzene) as initiators. Irradiation of the CT complexes with light of relatively long wavelength produces radical cations of the aromatic electron donors capable of initiating cationic polymerization. N-Vinylcarbazole and n-butyl vinyl ether are spontaneously polymerized in CH2Cl2 solution in the dark at room temperature upon addition of the CT complexes mentioned above. The molar extinction coefficients ect and equilibrium constants Kct of the CT complexes have been determined.

Long wavelength photoinitiated cationic polymerization using diphenyliodonium salt and catena-poly (phenyl-4-phenylphenylsilicon)

Abstract The photolysis of catena-poly (phenyl-4-phenylphenylsilicon) at λinc = 365–400 nm leads to products (probably free radicals) which are oxidized by diphenyliodonium ions. The resulting ionic species is capable of readily initiating the cationic polymerization of tetrahydrofuran, n-butyl vinyl ether, cyclohexene oxide and N-vinylcarbazol.

Photochemical cationic polymerization of cyclohexene oxide in solution containing pyridinium salt and polysilane

Abstract The cationic polymerization of cyclohexene oxide (CHO) is initiated upon u.v. irradiation (λinc > 350 nm) of dichloromethane solutions containing N-ethoxy-2-methyl pyridinium hexafluorophosphate (EMP+PF6−) and poly(methyl phenyl silane) or poly(dimethyl diphenyl silane). A feasible initiation mechanism involves the photogeneration of silylene biradicals and silyl radicals by extrusion and main-chain scission of the polysilanes, respectively. Oxidation of these radicals by EMP+ ions yields reactive cations capable of initiating the polymerization of CHO.

Formation and decay of nitronic acid in the photorearrangement of o-nitrobenzyl esters

Abstract o-Nitrobenzyl benzoate (oNBB), α-methyl-o-nitrobenzyl benzoate (αMoNBB), α-phenyl-o-nitrobenzyl benzoate (αPoNBB) and copolymers of methyl methacrylate and of o-nitrobenzyl, methyl-o-nitrobenzyl and phenyl-o-nitrobenzyl acrylates were flash photolysed or continuously irradiated with UV light in dilute solution. The quantum yield φ(PR) of bond cleavage in the photorearrangement of the o-nitrobenzyl esters does not depend on the chemical nature of the ester group but is significantly influenced by the substituent in the α position φ(PR) = 0.22 ± 0.03 (αMoNBB, αPoNBB) and φ = 0.10 ± 0.02 (oNBB). This was revealed from the yields of carboxylic acid and o-nitrobenzyl groups. The differences in φ(PR) correspond to the differences in the yields of nitronic acid formed as an intermediate. In aqueous systems the kinetics of the decay of the nitronic acid were found to be determined by the formation of rather stable nitronate ions according to the equilibrium The relatively long lifetime of the nitronate ion formed in the case of oNBB (longer than 100 ms) is assumed to be caused by a rather strong intramolecular interaction of the hydrogen at the α-carbon with the nitronate group and the carbonyl group. In acetonitrile solution, where nitronate ions were not formed, the non-substituted compound (oNBB) also behaved differently from the substituted compounds (αMoNBB and αPoNBB): two transients differing in lifetime (6 × 10−4 and 5.8 × 10−3 s) were observed with oNBB which were ascribed to different isomers of the nitronic acid. Addition of H2SO4 to acetonitrile solutions accelerated the rearrangement of the nitronic acid. The acceleration is assumed to be caused by protonation of the oxygen of the carbonyl group. The carbocation formed this way strongly interacts intramolecularly with oxygens of the nitronic acid group, thus inducing the rearrangement.

Modification of poly(methyl phenylsilane) by the attachment of π-conjugated substituents. Synthesis and photochemical studies

Abstract Poly(methyl phenlylsilane), PMPSi, has been partly chloromethylated and the chloromethylated PMPSi has been formylated via the pyridinium salt and the nitrone (Kroehnke transformation). Subsequently, the aldehyde groups have been converted by condensation with aniline, 4-aminoazobenzene and 4-nitroaniline to the corresponding Schiff bases. Also, the aldehyde groups have been converted to 2,4-dinitrophenylhydrazone groups. The extension of the π-conjugated system of the phenyl groups by attachment of chromophoric groups does not cause a detectable shift of the u.v. absorption band of the δ-conjugated backbone. Apart from the formylated derivative, all substituted polysilanes are much less photolabile than the original PMPSi and the partly chloromethylated PMPSi. Emission studies have revealed that the fluorescence of PMPSi is strongly quenched by the attached chromophores.

Photoinitiation of cationic polymerization. IV. Direct and sensitized photolysis of aryl iodonium and sulfonium salts

Abstract Aryl iodonium and sulfonium salts are thermally stable photoinitiators for cationic polymerization. Laser flash photolysis of diphenyliodonium and diphenyl-4-thiophenoxyphenylsulfonium hexafluoroarsenates provides direct evidence for homolytic Ar-I and Ar-S bond cleavage to yield phenyliodinium (PhI +. ) and, primarily, diphenylsulfinium (Ph 2 S +. ) ion radicals, respectively. The radical ions were generated independently by flash-induced electron transfer from iodobenzene and diphenylsulfide to a phenanthrolinium salt. The radical ions are highly reactive with nucleophiles, including iodobenzene and cyclohexene oxide, in the case of PhI +. . Apparent secondorder rate constants were determined for the reaction of the transients with several nucleophiles. Quantum yields of acid formation from stationary photolysis of diphenyliodonium and triphenylsulfonium hexafluoroarsenates were found to be significantly higher than yields of iodobenzene and diphenylsulfide, respectively. These results may be explained by facile reaction of PhI +. (or Ph 2 S +. ) with PhI (or Ph 2 S) to yield new onium salts together with a proton. The high reactivity of PhI +. with cyclohexene oxide suggests that the transient may directly initiate cationic polymerization of epoxides. Photoinitiated cationic polymerization by photosensitization of diphenyliodonium and triphenylsulfonium salts is shown to proceed by two distinct electron transfer processes: (1) direct electron transfer from excited-state photosensitizers, and (2) indirect electron transfer from photogenerated radicals. The efficiency of the former process is attributed to instability of the reduction products (from diphenyliodonium and triphenylsulfonium salts) which dissociate in competition with undergoing energy-wastage reverse electron transfer. Amplification of photons in the production of protons (or other reactive cations) is postulated to account for the high quantum yields observed in the latter process. Potential advantages of utilizing the indirect redox process in the design of UV curable hybrid systems, which contain functionality for both radical and cationic polymerization, are noted. The sensitization results also provide evidence against the importance of triplet states of the onium salts in photoinitiator activity.