Application of a moderate external magnetic field increases the efficiency of triplet photoinitiators of polymerization. The reasons for this observation are briefly outlined. Such well-documented magnetic and spin effects as the magnetic isotope effect (MIE) and chemically induced dynamic nuclear and electron polarization (CIDNP and CIDEP) have been observed under the photolysis of photoinitiators. CIDEP results are emphasized. The study of CIDEP under the photolysis of initiators allows the detection and identification of free radicals of initiators and the spin multiplicity of the reacting photoinitiator (triplet or singlet). The transfer of the spin polarization of the initiator free radical to monomers is a promising technique for following radical reaction pathways. The results of a CIDEP study of the sensitized decomposition of phosphine oxide photoinitiators are presented

Arsu, Nergis

Jockusch, Steffen

Khudyakov, Igor V.

Turro, Nicholas J.

Columbia University Academic Commons

DesignedMonomers and Polymers, Vol. 6, No. 1, pp. 91–101 (2003)Ó VSP 2003.Also available online - www.vsppub.comMagnetic and spin effects in the photoinitiationof polymerizationIGOR V. KHUDYAKOV 1 , NERGIS ARSU 2, STEFFEN JOCKUSCH 2and NICHOLAS J. TURRO 2;¤1 Crompton Corporation, Middlebury, CT 06749, USA2Department of Chemistry, Columbia University, New York, NY 10027, USAAbstract—Application of a moderate external magnetic  eld increases the ef ciency of tripletphotoinitiators of polymerization. The reasons for this observation are brie y outlined. Suchwell-documented magnetic and spin effects as the magnetic isotope effect (MIE) and chemicallyinduced dynamic nuclear and electron polarization (CIDNP and CIDEP) have been observed underthe photolysis of photoinitiators. CIDEP results are emphasized. The study of CIDEP under thephotolysis of initiators allows the detection and identi cation of free radicals of initiators and the spinmultiplicityof the reactingphotoinitiator(triplet or singlet). The transfer of the spin polarizationof theinitiator free radical to monomers is a promising technique for following radical reaction pathways.The results of a CIDEP study of the sensitized decomposition of phosphine oxide photoinitiators arepresented.Keywords: Polymerization; radical pairs; magnetic  eld; isotope effect.1. INTRODUCTIONFree radical photopolymerization is accompanied by the formation of transientswith a non-zero spin angular momentum: free radicals, radical pairs, and the tripletstates of photoinitiators. A stable molecule in a triplet state, such as molecular oxy-gen, plays an important role in polymerization. One can expect the manifestationof a magnetic and spin effect in free-radical photopolymerization. The magneticand spin effect has been known in physical chemistry and photochemistry for threedecades [1–5]. A number of mechanisms for the magnetic  eld effects (MFEs) inchemical reactions have been established, well documented, and have received aproper theoretical analysis. However, applications of MFEs to photopolymerizationare rather limited and deserve further exploration.¤To whom correspondence should be addressed.92 I. V. Khudyakov et al.The primary event of the photoinitiation of free-radical polymerization is theformation of a radical pair R., R. being the result of a bond cleavage, or hydrogenatom or electron abstraction by a photoinitiator. Radical pairs (RPs) are importantintermediates and their self-reactivity (cage effect) in uences the ef ciency ofpolymerization. The outcome of a photochemical reaction essentially depends onthe spin evolution of RP intermediates. The reactivity of a RP is sensitive to theapplication of an external moderate magnetic  eld. This report is a brief reviewof the experimental manifestation of magnetic and spin effects in the reactions ofphotoinitiators and in a photopolymerization. We have used experimental resultsobtained in our laboratories as an illustration. The role of alternating magnetic  eldsand microwaves on photoinduced and free radical reactions is beyond the scope ofthis paper.2. MAGNETIC FIELD EFFECTS IN PHOTOPOLYMERIZATIONFig. 1 demonstrates an idea of the MFE as the photodissociation of an initiatorof polymerization in a triplet state. In most cases, triplet RPs formed under thephotodissociation of an initiator in a triplet state cannot recombine. Thus, it is notsurprising that most of the commercially available photoinitiators of polymerizationdissociate exclusively in a triplet state. A certain fraction of RPs undergo inter-system crossing into the singlet state and recombine inside a cage. The applicationof a moderate external magnetic  eld (B > Aeff) decreases the ef ciency of inter-system crossing. Aeff, the effective hyper ne coupling (HFC) constant of RP, formany pairs with HFC to protons is ca. 50 G. Thus, the application of a moderatemagnetic  eld can increase the exit of radicals from a solvent cage, which increasesthe initiation rate of polymerization. This is the essence of the MFE occurringaccording to the HFC mechanism. The relative wideness of the arrows in Fig. 1indicates the relative rates or ef ciencies of the processes; the wider the arrow, thefaster the process.In practice, such S–T evolution occurs relatively slowly for RPs with a speci edAeff, and a reduction in the mobility of a RP is required for the manifestationof a MFE. A reduction in mobility can be achieved in different ways: by usinginclusion a RP in a micelle; by an increase in solvent viscosity, etc. Free radicalpolymerization accompanied by a viscosity increase is a promising process for themanifestation of MFEs.MFEs were observed for the  rst time in the emulsion polymerization of styrenephotoinitiated by dibenzyl ketone (see Fig. 2) [6, 7].Application of a moderate magnetic  eld of 50–1000 G signi cantly acceleratedthe polymerization and resulted in an increase of the molecular weight (MW) ofpolystyrene formed in the emulsion polymerization, due to the HFC mechanismof MFE [6, 7]. Experiments on the photopolymerization of acrylamide initiatedby the dye Methylene Blue in a viscous water–glycerol solution and on the pho-topolymerization of N ,N ,N ,N-dimethyldiallyl ammonium chloride in an aqueousMagnetic and spin effects in the photoinitiationof polymerization 93Figure 1. Schematic representation of the hyper ne coupling mechanism of a MFE.solution initiated by uranyl salt resulted in acceleration of the polymerization and areduction in the MW of the formed polymer [5, 8]. Fig. 3 demonstrates the MFE inthe polymerization of acrylamide.The magnitudes of the MFEs observed so far in homogeneous solutions havebeen rather small. Fig. 4 presents recent data on the polymerization of neatmethyl methacrylate (MMA) and two other monomers initiated by 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropanone-1, known as Irgacure 907 of CibaAdditives.Some data on MFEs in polymerization are compiled in review articles [9, 10].94 I. V. Khudyakov et al.Figure 2. Diagram of a SDS micelle containing the photoinitiatordibenzyl ketone and styrene.Figure 3. Time-dependent rate of the polymerization of acrylamide in air-saturatedwater–glycerolmixture containing Methylene Blue and triethanolamine. q stands for the conversion of a monomer.The period of application of a magnetic  eld of 1000 G is marked on the plot [8].3. MAGNETIC ISOTOPE EFFECT UNDER THE PHOTOLYSIS OF INITIATORSMagnetic isotope effect (MIE) separation of magnetic (with non-zero spin angularnuclear momentum) and non-magnetic isotopes has been observed under theMagnetic and spin effects in the photoinitiationof polymerization 95Figure 4. Photoinduced radical polymerizationof vinylacrylate(3.2M) (a, b), n-butylacrylate(4.7M)(c, d), and MMA (6.2 M) (e, f) in an air-saturated methylene chloride solution (2 : 1, v/v) initiatedby the irradiation of Irgacure 907 (cw lamp ¸ 366 nm; »62 mW/cm2) in the presence (a, c, e) andabsence (b, d, f) of an external magnetic  eld (2 kG and 0.3 G, respectively;N. Arsu and S. Jokusch,unpublished results).photolysis of such photoinitiators as dibenzyl ketone and uranyl salts [1–5].Figure 5 presents a scheme of the MIE. Photolysis of an initiator A occurs via atriplet state. A has a natural or a special level of a magnetic isotope; such A withmagnetic isotopes is denoted as A#. Photolysis of A# occurs with the formationof a radical pair which has an additional HFC on a magnetic isotope, compared tothe photolysis of A. Such triplet RPs with a magnetic isotope undergo faster inter-system crossing into singlet RPs with the formation of a cage product enriched witha magnetic isotope. Prolonged photolysis of A/A# in the earth’s magnetic  led orin a moderate external magnetic  eld results in a measurable separation of isotopes.A detailed analysis of the MIE can be found in Refs. [1] and [4].4. CHEMICALLY INDUCED DYNAMIC NUCLEAR POLARIZATION (CIDNP)UNDER THE PHOTOLYSIS OF INITIATORSPhotolysis and thermolysis of the initiators of polymerization in the cavity of anNMR spectrometer is accompanied by CIDNP, i.e. by the formation of productswith a non-Boltzmann population of nuclear Zeeman levels. CIDNPmanifests itself96 I. V. Khudyakov et al.Figure 5. Scheme explaining the action of MIE (see text).in the enhanced absorption or emission of certain components in the NMR spectraof the products of free-radical reactions. Study of CIDNP allows an understandingof the origin of the diamagnetic products of free radical reactions. A multitudeof CIDNP experiments have been performed with 1H-NMR. 31P-CIDNP under thephotolysis of phosphine oxide photoinitiators is important for a detailed study ofthese very ef cient photoinitiators. 1H-CIDNP was studied under the photolysis ofuranyl salts and uranyl complexes in Refs. [11–13] (see Fig. 6).5. CHEMICALLY INDUCED DYNAMIC ELECTRON POLARIZATION (CIDEP)UNDER THE PHOTOINITIATION OF POLYMERIZATIONFlash photolysis of the initiators of polymerization in the cavity of an ESRspectrometer is often accompanied by CIDEP, i.e. by the formation of radicals witha non-Boltzmann population of electron Zeeman levels. CIDEP manifests itselfin the enhanced absorption or emission of all or certain components in the ESRspectra of photogenerated free-radicals. The main mechanisms leading to CIDEPin photoinduced reactions are well established [14, 15].Study of the CIDEP pattern of time-resolved (TR) ESP spectra allows theestablishment of solid conclusions on the spin multiplicity of molecular precursorsof polarized free radicals (singlet or triplet excited molecules) and the tracking offast reactions of polarized radicals leading to secondary radicals. An importantreaction of this type is the addition of a free radical of a photoinitiator (R.) to amonomer (M):R. CM! R M.Magnetic and spin effects in the photoinitiationof polymerization 97Figure 6. Top: 1H-NMR of 2,6-di-t -butylphenol and uranyl nitrate in methanol-d4 (a) before and (b)during irradiation. Bottom: 1H-NMR of acidi ed methanol-d4 solution of benzylic acid and uranylnitrate (a) before and (b) during irradiation. In these experiments, uranyl nitrate absorbed light [13].As an example, we will consider direct and sensitized photoinitiation with phos-phine oxides in the polymerization of isobornyl acrylate [16]. Chemical structuresand abbreviations of the names of the compounds studied are presented below:98 I. V. Khudyakov et al.Figure 7. TR ESR spectra of TMDPO in ethyl acetate taken at different times and computer-simulatedspectra. A solution of TMDPO was subjected to pulse laser excitation [16].The direct photolysis of phosphine oxides results in a well-documented initialstrong absorptive (A) pattern of ESR spectra (see Fig. 7). TX- sensitized or ITXphotolysis of phosphine oxides evidently leads to the same radicals, but the initialpolarization pattern is quite different, namely an emission / absorption (E/A) patternMagnetic and spin effects in the photoinitiationof polymerization 99Figure 8. TR ESR spectra in ethyl acetate of: (a) TMDPO in the presence of TX; (b) a mixture ofTMDPO and BAPO in the presence of ITX. Solutions were subjected to pulse laser excitation, lightwas absorbed mainly by triplet sensitizers [16].Figure 9. TR EPR spectra in ethyl acetate of: (a) TMDPO in the presence of IBOA; (b) TMDPO inthe presence of TX and IBOA. Computer-simulated spectra are also shown below the experimentalspectra [16]. Solutions were subjected to pulse laser excitation. See text for discussion.100 I. V. Khudyakov et al.(see Fig. 8). Thus, observation of the pattern of TR ESR spectra allows thedetermination of the reaction pathway leading to the same radicals. In our case,it is direct vs. sensitized photolysis.TR ESR spectra obtained after the addition of IBOA to solutions under investi-gation are presented in Fig. 9. The observed spectra consist mainly of polarizedsignals of spin adducts of phosphinoyl radical and acrylate:The spectra presented in Figs 7–9 demonstrate that the polarization pattern ispreserved during the fast addition of radicals to a monomer. Absorptive (A)polarization observed under the direct photolysis of photoinitiators (Fig. 7) results inan A pattern of the spectrum of a spin adduct (Fig. 9a) and the E/A pattern observedunder sensitized photolysis (Fig. 8) results in an E/A pattern of the spectrum of aspin adduct (Fig. 9b). Thus, TR ESR study allows the establishment of not only theorigin of free radicals, but also their reactions.In some special cases, it is possible to trace the polarized tertiary radicals formedin a reaction:R M. CM! R M.2A tertiary polarized radical was observed in particular under the photolysis ofTMPDO in the presence of t-butyl acrylate [17].6. CONCLUDING REMARKSMagnetic and spin effects in photoinduced chemical reactions have been extensivelystudied experimentally and theoretically. Application of a magnetic  eld affects therates and molecular weights of polymers formed in free radical polymerization. Theuse of a magnetic  eld in free radical polymerization has not yet found a practicalapplication, to the best of our knowledge. We can speculate that a magnetic  eldcan be used in cases where MFEs are signi cant, or when a non-intrusive in uenceon the rate of polymerization is required, or when there is a need to increase theef ciency of a costly photoinitiator.TR ESR study of photoinitiators and their reactions is very promising for anunderstanding of chain initiation in free radical polymerization. The CIDEP patternallows the origin of free radicals and their fast subsequent reactions to be traced.The most important such reaction is addition to a monomer.Magnetic and spin effects in the photoinitiationof polymerization 101REFERENCES1. S. Nagakura, H. Hayashi and T. Azumi (Eds),Dynamic Spin Chemistry. Kodansha-Wiley,Tokyo(1998).2. N. J. Turro and B. Kraeutler,Acc. Chem. Res. 13, 369 (1980).3. I. V. Khudyakov, Yu. A. Serebrennikov and N. J. Turro, Chem. Rev. 93, 537 (1993).4. A. L. Buchachenko,Pure Appl. Chem. 72, 2243 (2000).5. A. L. Buchachenko and I. V. Khudyakov, Acc. Chem. Res. 24, 177 (1991).6. N. J. Turro, M.-F. Chow, C.-J. Chung and C.-H. Tung, J. Am. Chem. Soc. 102, 7391 (1980).7. N. J. Turro, M.-F. Chow, C.-J. Chung and C.-H. Tung, J. Am. Chem. Soc. 105, 1572 (1983).8. A. A. Vedeneev, I. V. Khudyakov and N. A. Golubkova, J. Chem. Soc. Faraday Trans. 86, 3545(1990).9. A. P. Chiriac and C. I. Simonescu, Prog. Polym. Sci. 25, 219 (2000).10. V. B. Ivanov, Visokomol. Soedin., Ser. A 33, 1811 (1991).11. S. V. Rykov, I. V. Khudyakov and E. D. Skakovski, J. Photochem. Photobiol. A 66, 127 (1992).12. S. V. Rykov, I. V. Khudyakov, E. D. Skakovski, H. D. Burrows, S. J. Formosinho and M. G. M.Miguel, J. Chem. Soc. Perkin Trans. 2 8314 (1991).13. A. L. Buchchenko, I. V. Khudyakov and E. S. Klimchuk, J. Photochem. Photobiol. A 46, 281(1989).14. N. J. Turro and I. V. Khudyakov, Res. Chem. Intermed. 25, 505 (1999).15. N. J. Turro, M. H. Kleinman and E. Karatekin, Angew. Chem. Int. Ed. Engl. 39, 4436 (2000).16. R. M. Williams, I. V. Khudyakov, M. B. Purvis, B. J. Overton and N. J. Turro, J. Phys. Chem. B104, 10437 (2000).17. M. Weber and N. J. Turro, J. Phys. Chem. A 106, 1938 (2002).

Magnetic and spin effects in the photoinitiation of polymerization

https://academiccommons.columbia.edu/doi/10.7916/D8QF903S/download

Magnetic and spin effects in the photoinitiation of polymerization

Abstract

Similar works

Full text

Available Versions

Columbia University Academic Commons