Macromolecules 1999, 32, 6989; c) R. Schmidt, T. Zhao, J.-B.Green, D. J. Dyer, Langmuir 2002, 18, 1281.

[17] O. Prucker, J. Habicht, I.-J. Park, J. R¸he, Mater. Sci. Eng. C1999, 8±9, 291.

[18] A. Gˆlzh‰user, W. Eck, W. Geyer, V. Stadler, Th. Weimann, P.Hinze, M. Grunze, Adv. Mater. 2001, 13, 806.

[19] W. Geyer, V. Stadler, W. Eck, A. Gˆlzh‰user, M. Grunze, M.Sauer, T. Weimann, P. Hinze, J. Vac. Sci. Technol. B 2001, 19,2732.

[20] W. Eck, V. Stadler, W. Geyer, M. Zharnikov, A. Gˆlzh‰user, M.Grunze, Adv. Mater. 2000, 12, 805.

[21] A. Gˆlzh‰user, W. Geyer, V. Stadler, W. Eck, M. Grunze, K.Edinger, T. Weimann, P. Hinze, J. Vac. Sci. Technol. B 2000, 18,3414.

[22] M. J. Lercel, H. G. Craighead, A. N. Parikh, K. Seshadri, D. L.Allara, Appl. Phys. Lett. 1996, 68, 1504.

[23] O. Nuyken, R. Weidner, Adv. Polym. Sci. 1986, 73±74, 147, andreferences therein.

[24] N. Fery, R. Hoene, K. Hamann, Angew. Chem. 1972, 84, 359;Angew. Chem. Int. Ed. 1972, 11, 337.

[25] Another strategy to obtain this is to immobilize both termini ofthe azo compound: a) G. Boven, M. L. C. M. Oosterling, G.Challa, A. J. Schouten, Polymer 1990, 31, 2377; b) E. Carlier, A.Guyot, A. Revillon, M.-F. Llauro-Darricades, R. Petiaud, React.Polym. 1991, 16, 41.

[26] It is suspected that the resonance-stabilized methylmalonodini-trile radical acts as a reversible termination agent in thepolymerization (P. C. Wieland, O. Nuyken, M. Schmidt, K.Fischer, Macromol. Rapid Commun. 2001, 22, 1255). Relatedstudies are currently underway in our laboratories.

[27] The external reflection FTIR spectra of the purified surfaces(Soxhlet extraction, toluene, 12 h) confirmed that in all cases alayer of covalently attached polystyrene was formed (typicalabsorption bands: (n in cm�1): CH aromatic stretching: 3102,3018, 3060, 3026, 2999; CH aliphatic stretching: 2922, 2848;overtones of CH out-of-plane fundamentals for monosubstitutedaromatic rings: 1945, 1867, 1795; ring breathing: 1602, 1493,1452, CH deformation modes (out-of-plane): 758, 698.

[28] Quantitative analysis of the film thickness as a function of thepolymerization time and irradiation conditions are currentlyunderway. However, the film thickness steadily increases withthe time of polymerization, although not as rapidly as reportedin reference [17] for the photoinitiated polymerization in thepresence of a similar initiator.

[29] The stencil mask was obtained from Quantifoil Micro Tools,Jena, Germany.

[30] Georg Albert PVD-Coatings, Heidelberg, Germany (GeorgAl-bert@web.de).

[31] Methylmalonodinitrile was obtained by ammonolysis (R.Meyer,P. Bock, Justus Liebigs Ann. Chem. 1906, 347, 98) andsubsequent dehydration (S. Starck, Ber. Dtsch. Chem. Ges. A1934, 67, 42) of methylmalonic acid diethyl ester.

Radical Anion Complexes

Coordinated o-Dithio- and o-Iminothiobenzo-semiquinonate(1�) p Radicals in[MII(bpy)(LC)](PF6) Complexes**

Prasanta Ghosh, Ameerunisha Begum, Diran Herebian,Eberhard Bothe, Knut Hildenbrand,Thomas Weyherm¸ller, and Karl Wieghardt*

Dedicated to Professor Gottfried Huttneron the occasion of his 65th birthday

o-Benzosemiquinonate(1�) p-radical anions are archetypicalopen-shell, bidentate ligands in the coordination chemistry oftransition-metal ions[1] but for their o-dithio derivatives thesituation is not clear. The existence of S,S-coordinatedo-dithiobenzosemiquinonate(1�) radical anions (rather thantheir closed-shell, aromatic o-dithiolate dianions) has occa-sionally been suggested[2] to occur in some complexes but–attimes–has also been explicitly denied.[3] Clear experimentalevidence is lacking to date. Similarly, recently it has beenshown indirectly that o-iminobenzosemiquinonate(1�) p

radical ions[4] and their sulfur analogues[5] as well aso-diiminobenzosemiquinonate(1�)[6] p radical anions(Scheme 1) are coordinated in diamagnetic, square-planarcomplexes [NiII(XC)2]. Density functional theoretical (DFT)calculations have shown that the electronic structure of thesespecies is in accord with the description as singlet diradicals.[7]

Here we report the synthesis and characterization ofdiamagnetic square-planar neutral complexes of PdII and PtII

containing a single aromatic (L)2� ligand and a 2,2’-bipyridineligand (Scheme 1): [MII(bpy)(X)]0 (1±5). We show thatcomplexes 1±5 undergo a reversible, ligand-centered one-electron oxidation yielding paramagnetic complexes[MII(bpy)(XC)](PF6) (1a±5a) with a S¼ 1=2 ground state anda coordinated p radical anionic ligand (LC)�.

The following complexes have been isolated as solidmaterials: [PdII(bpy)(A)] (1), [PdII(bpy)(AC)](PF6) (1a),[PdIIbpy)(BC)](PF6) (2a), [Pd(bpy)(C)] (3), [PdII(bpy)(D)](4), [PdII(bpy)(DC)](PF6) (4a), [PtII(bpy)(E)] (5), and[PtII(bpy)(EC)](PF6) (5a).

The reaction of [M(bpy)Cl2] (M¼Pd, Pt) and therespective ligand H2[A], ºH2[E] (1:1) in CH3CN or THF inthe presence of two equivalents of a base (NaOCH3, BH4

�, orNEt3) under argon led to the formation of the neutral species1, 3, and 5, which were isolated as dark blue needles (1) ordark green crystals (3 and 5). The oxidized species 1a (dark

[*] Prof. Dr. K. Wieghardt, Dr. P. Ghosh, Dr. A. Begum, Dr. D. Herebian,Dr. E. Bothe, Dr. K. Hildenbrand, Dr. T. Weyherm¸llerMax-Planck-Institut f¸r StrahlenchemieStiftstrasse 34-36, 45470 M¸lheim an der Ruhr (Germany)Fax: (þ49)208-306-3952E-mail: wieghardt@mpi-muelheim.mpg.de

[**] This work was supported by the Fonds der Chemischen Industrie.P. Ghosh thanks the Alexander von Humboldt Foundation for astipend. MII¼Pd, Pt; bpy¼2,2’-bipyridine.

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green) and 5a (orange) were prepared by oxidation of theneutral complexes 1 and 5 in CH2Cl2 with one equivalent offerrocenium hexafluorophosphate; 2a was obtained by oxi-dation with air of the reaction mixture of 2 (after addition of[N(nBu)4](PF6)). Complexes 4 and 4a were described pre-viously.[8] Owing to its extreme O2-sensitivity it was notpossible to prepare a pure sample of 2; 3a was generatedelectrochemically at 20 8C in CH2Cl2 solution and frozenimmediately with liquid nitrogen.

The crystal structures of 1, 5 (Figure 1), and 2a (Figure 2)were determined by X-ray structure analysis; those of 4, 4a,[8]

and [PdII(bpy)(C6H4S2)][9] as an analogue of 3 are known.Table 1 summarizes the bond lengths in the coordinateddianions L2� and in their oxidized p-radical anions(LC)�. It isclearly established, that the same geometric changes occurupon oxidation of L2� to (LC)�: 1) In the dianions L2� the sixC�C bonds of the ring are equidistant within 3s but in thecorresponding radicals these rings adopt a quinoid-typestructure with two alternating short C�C distances (ca.

1.37 ä) and four longer C�C distances (ca. 1.41 ä). This isalso true for the o-aminothiophenolates (EC)� and, as shownhere for the first time, for the o-dithiobenzosemiquinonates(CC)�. 2) The C�X and C�Y bonds are always ~ 0.04 ä longerin X,Y-coordinated dianions (single bonds) than in the radicalanions (double-bond character).

Scheme 1. Structures and redox scheme of the ligands L2� employed.

Table 1: Bond lengths [ä] of X,Y-coordinated dianions L2� and of their radical anions (LC)�.

Complex C�X C�Y C(1)�C(2) C(2)�C(3) C(3)�C(4) C(4)�C(5) C(5)�C(6) C(1)�C(6)

1¥CH3CN 1.351(6) 1.371(6) 1.423(7) 1.404(7) 1.392(7) 1.403(7) 1.390(7) 1.411(7)[Pd(AC)2] [16] 1.307(4) 1.302(5) 1.435(5) 1.420(5) 1.368(5) 1.436(5) 1.366(5) 1.407(5)[W(CO)3(HNC6H4NH)]2� [17] 1.372(6) 1.418(6) 1.420(7) 1.432(7) 1.396(8) 1.382(8) 1.390(8) 1.390(8)2a 1.326(2) 1.348(2) 1.453(2) 1.422(2) 1.374(2) 1.423(2) 1.373(2) 1.420(2)[Pd(bpy)(C6H4S2)][9] 1.756(2) 1.759(2) 1,396(5) 1.399(5) 1.378(5) 1.394(5) 1.375(5) 1.399(5)[Ni(CC)2][3b] 1.723(2) 1.731(2) 1.419(3) 1.409(3) 1.373(3) 1.422(3) 1.375(3) 1.429(3)4[8] 1.348(2) 1.387(2) 1.421(3) 1.406(2) 1.405(3) 1.398(3) 1.402(2) 1.395(2)4a[8] 1.309(2) 1.346(2) 1.450(2) 1,428(3) 1.382(3) 1.437(2) 1.366(3) 1.420(3)5 1.379(4) 1.772(4) 1.418(4) 1.406(5) 1.386(5) 1.408(5) 1.396(5) 1.419(5)[Ni(EC)2][5] 1.348(4) 1.724(3) 1.418(4) 1.411(4) 1.364(4) 1.424(4) 1.384(4) 1.431(4)

Figure 1. Structures of the neutral molecules in crystals of 1¥CH3CN(top) and 5 (bottom). Selected bond lengths [ä] in 1: Pd-O(1)1.973(4), Pd-O(2) 1.959(4), Pd-N(1) 2.001(5), Pd-N(2) 1.999(5) and in5 : Pt-N(1) 1.950(3), Pt-N(21) 2.013(3), Pt-N(32) 2.048(3), Pt-S(1)2.2566(8).

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The structure of 2a is interesting because two ion pairs of[Pd(bpy)(BC)]PF6 form a ™dimer∫ through p-stacking of two(BC)� radical anions (Figure 2). The centroid of the six-membered ring of one (BC)� ligand is only 2.923 ä above thecarbon atom C(1) of the second ring. This accounts for theobserved diamagnetism of 2a in the solid state.[10] In contrast,in CH2Cl2 solution 2a is paramagnetic (see below).

Figure 3 displays the cyclic voltammograms of 1, 2a, 3,and 5 in CH2Cl2 (0.1m [(nBu)4N](PF6)); that of 4 was reported

previously.[8] Table 2 gives the redox potentials versus theferrocenium/ferrocene couple (Fcþ/Fc). All complexes exhib-it at least two, ligand-centered reversible one-electron trans-fer waves, E1

1=2 and E21=2, and a similar third wave, E3

1=2, which isirreversible for 1 and 3. These waves are assigned to processesshown in Scheme 2, in which (bpyC)� is the radical anion of2,2’-bipyridine (bpy) and (LBQ)0 is the neutral benzoquinoneform of the ligand L2�.

Interestingly, E11=2 is nearly constant throughout the series

at ~�1.87 V which is also observed for the reduction of[PtII(bpy)Cl2] to EPR-active [PtII(bpyC)Cl2]� .[11] E2

1=2 and E31=2

involve the successive oxidations of the aromatic dianions L2�

(Scheme 1) to the semiquinonate radical anions (LC)�, andfurther to the quinones (LBQ)0. Interestingly, the o-dithiolatespecies 3 is the most difficult neutral species to oxidize,[12]

followed by the catecholate complex 1, whereas the o-diiminospecies 2 is the easiest to oxidize.

The electronic spectra of the neutral complexes 1±5(Figure 4) are dominated by a charge-transfer band in thevisible which was assigned to a ™mixed metal±ligand (M(L))to ligand (bpy)∫ transition,[13] LLCT. Accordingly, the HO-MO involves contributions from the metal ion and theC6H4XY2� system, while the LUMO is localized on the bpyligand (p* orbital). Therefore, the energy of the LUMOshould be approximately constant throughout the series. Ifthis is the case, the redox potential E2

1=2 for the couple [neutralcomplex]/[monocation]þ should correlate with the energy ofthis LLCT transition as shown in Figure 5.

Temperature-dependent (3±298 K) magnetic susceptibil-ity measurements (SQUID magnetometer, 1 T) establishedthat solid samples of 1a, 4a,[8] and 5a are paramagnetic withan effective magnetic moment (meff) of 1.6±1.8 mB at 298 Kcorresponding to one unpaired electron per formula unit. Incontrast, solid 2a is diamagnetic (meff¼ 0.2 mB (10 K) and0.8 mB per Pd center at 298 K corresponding to temperature-independent paramagnetism, cTIP) because the N,N-coordi-nated ligands (BC)� form dimers through p-stacking in thesolid state.

Figure 2. Structure of 2a in the crystal showing an ion pair dimer{[Pd(bpy)(BC)]þ(PF6

�)}2. Selected bond lengths [ä]: Pd-Nbpy 2.054(1),2.025(1), Pd-NBC 1.976(1), 2.012(1). The dotted line represents theshortest distance between the atom C(1) of the first (BC)� ligand andthe centroid of the six-membered ring of the second (BC)� ligand.

Table 2: Redox potentials of complexes 1, 2a, 3±5, and [PtII(bpy)Cl2] .[a]

Complex E11=2 [V] E21

1=2 [V] E31=2 [V]

1 �1.80 (r) �0.18 (r) þ0.70 (irr)2a �1.91 (r) �0.79 (r) �0.01 (r)3 �1.87 (r) 0.04 (r) þ0.90 (irr)4[8] �1.90 (r) �0.58 (r) þ0.42 (r)5 �1.86 (r) �0.46 (r) þ0.49 (r)[PtII(bpy)Cl2][11] �1.61 (in DMF)

[a] Measured in CH2Cl2 (0.1m [(nBu)4N](PF6)) versus Fcþ/Fc at 20 8C andand a scan rate of 200 mVs�1. r¼ reversible, irr¼ irreversible.

Figure 3. Cyclic voltammograms of 1, 2a, 3, and 5 in CH2Cl2 (0.10m[(nBu)4N](PF6)) at a glassy carbon working electrode at a scan rate of200 mVs�1.

Scheme 2. Electron transfer processes of the complexes investigatedby cyclic voltammometry.

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CH2Cl2 solutions of 1a, 2a, 4a, and 5a as well as anelectrochemically generated solution of 3a display typical p-radical signals at g ~ 2.0 in the X-band EPR spectra. Theobserved g values and hyperfine coupling constants aresummarized in Table 3. The spectrum of 3a could only berecorded in frozen solution below 60 K;[14] a rhombic signal

with very small g anisotropy and without detectable hyperfinesplitting is observed. This p-radical anion is predominantly S-centered,[15] whereas in (AC)�, (BC)�, (DC)�, and (EC)� theunpaired electron is delocalized over the six-membered ring(Scheme 3).

In summary, we have presented spectroscopic evidencefor the existence of the S,S- and N,S-coordinated p-radicalso-dithiobenzosemiquinonates(1�) and o-iminothiobenzo-semiquinonates(1�), respectively, in [M(bpy)(LC)]þ com-plexes. These results necessitate a reformulation of oxidationstates and ligand oxidation levels of a number of o-dithiolatotransition-metal ion complexes.

Experimental SectionDetailed procedures for the synthesis of complexes and theircharacterizations using mass spectrometry and 1H NMR spectroscopywill be reported later in a full paper. All new compounds 1, 1a, 2a, 3,5, and 5a gave satisfactory elemental analyses (C, H, N, S).

Crystal structure analysis data for 1: C24H28N2O2Pd¥CH3CN,Mr¼523.94, monoclinic, P21/c, a¼ 6.7891(6), b¼ 14.053(1), c¼25.259(3) ä, b¼ 91.57(1)8, V¼ 2409.0(4) ä3, Z¼ 4, 1calcd¼1.445 Mgm�3, m(MoKa)¼ 0.797 mm�1, F(000)¼ 1080, 12870 reflec-tions collected at 100(2) K, 4212 independent reflections, 290parameters, GOF¼ 1.028, R1¼ 0.066, wR2¼ 0.1074. Crystal structureanalysis data for 2a : C22H18F6N4PPd, Mr¼ 589.77, triclinic, P�11, a¼8.9146(3), b¼ 10.8276(4), c¼ 12.2280(4) ä, a¼ 76.34(1), b¼ 83.24(1),g¼ 66.57(1)8, V¼ 1051.97(6) ä3, Z¼ 2, 1calcd¼ 1.862 Mgm�3,m(MoKa)¼ 1.031 mm�1, F(000)¼ 586, 39386 reflections collected,8052 independent reflections, 310 parameters, GOF¼ 1.032, R1¼0.030, wR2¼ 0.0606. Crystal structure analysis data of 5 : C24H29N3SPt,Mr¼ 586.65, monoclinic, P21/c, a¼ 10.4225(6), b¼ 12.6824(9), c¼16.4985(9) ä, b¼ 91.00(1)8, V¼ 2180.5(2) ä3, Z¼ 4, 1calcd¼1.787 Mgm�3, m(MoKa)¼ 6.546 mm�1, F(000)¼ 1152, 24351 reflec-tions collected, 8271 independent reflections, 271 parameters, GOF¼1.033, R1¼ 0.036, wR2¼ 0.079. CCDC-187891 (1), CCDC-187892(2a), and CCDC-187893 (5) contain the supplementary crystallo-graphic data for this paper. These data can be obtained free of chargevia www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-bridge Crystallographic Data Centre, 12, Union Road, CambridgeCB21EZ, UK; fax: (þ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).

Received: June 21, 2002Revised: October 10, 2002 [Z19586]

[1] C. G. Pierpont, C. W. Lange, Prog. Inorg. Chem. 1994, 41, 331.[2] D. T. Sawyer, G. S. Srivatsa, M. E. Bodini, W. P. Schaefer, R. M.

Wing, J. Am. Chem. Soc. 1986, 108, 936.[3] a) D. Sellmann, M. Geck, F. Knoch, G. Ritter, J. Dengler, J. Am.

Chem. Soc. 1991, 113, 3819; b) D. Sellmann, H. Binder, D.H‰ussinger, F. W. Heinemann, J. Sutter, Inorg. Chim. Acta 2000,300±302, 829.

Figure 4. Electronic spectra of the neutral species 1, 2, 3, and 5 and theirelectrochemically generated one-electron oxidized forms 1a, 2a, 3a, and 5a(0.10m [(nBu)4N)(PF6)).

Table 3: X-band EPR spectra of [M(bpy)(XC)]þ complexes in CH2Cl2solution at 298 K.

Complex giso Hyperfine coupling constant [G]

1a 2.002 aH¼3.5(2H); a105Pd¼2.42a 1.999 aN¼6.2, 5.8, aH¼5.1, 3.23a 2.0057[a] n.o.[b]

4a[8] 2.002 aN¼7.7; aH¼4.6; a105Pd¼3.565a 1.99 a195Pt¼50

[a] Measured in frozen solution (CH2Cl2, 0.10m [(nBu)4N](PF6)) at 60 K;rhombic signal g1¼2.018, g2¼2.006, g3¼1.993. [b] n.o. no hyperfinecoupling observed.

Scheme 3. Resonance structures of the p-radical ligands (AC)�, (BC)�,(CC)�, (DC)�, and (EC)�.

Figure 5. Correlation between the redox potential E21=2 for the couple

[neutral complex]/[monocation]þ and the energy of the LLCT transi-tions of the neutral complexes.

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[4] P. Chaudhuri, C. N. Verani, E. Bill, E. Bothe, T. Weyherm¸ller,K. Wieghardt, J. Am. Chem. Soc. 2001, 123, 2213.

[5] D. Herebian, E. Bothe, E. Bill, T. Weyherm¸ller, K. Wieghardt,J. Am. Chem. Soc. 2001, 123, 10012.

[6] H.-Y. Cheng, C.-C. Lin, B.-C. Tzeng, S.-M. Peng, J. Chin. Chem.Soc. 1994, 41, 775.

[7] V. Bachler, G. Olbrich, F. Neese, K. Wieghardt, Inorg. Chem.2002, 41, 4179.

[8] X. Sun, H. Chun, K. Hildenbrand, E. Bothe, T. Weyherm¸ller, F.Neese, K. Wieghardt, Inorg. Chem. 2002, 41, 4295.

[9] T. M. Cocker, R. E. Bachman, Inorg. Chem. 2001, 40, 1550.[10] For a similar p-stacking of a metalloporphyrin p-radical cation

[Zn(tppC)(OClO3)]2 see: H. Song, N. P. Rath, C. A. Reed, W. R.Scheidt, Inorg. Chem. 1989, 28, 1839.

[11] E. J. L. McInnes, R. D. Farley, S. A. Macgregor, K. J. Taylor, L. J.Yellowlees, C. C. Rowlands, J. Chem. Soc. Faraday Trans. 1998,94, 2985.

[12] In line with this observation it was recently reported that a(thiophenolato)cobalt(iii) and its (phenolato)cobalt(iii) ana-logue are oxidized at 0.55 and 0.36 V versus Fcþ/Fc, respectivelyto generate the coordinated thinyl and phenoxy radicals. S.Kimura, E. Bill, E. Bothe, T. Weyherm¸ller, K. Wieghardt, J.Am. Chem. Soc. 2001, 123, 6025.

[13] a) J. A. Zuleta, J. M. Bevilacqua, D. M. Proserpio, P. D. Harvey,R. Eisenberg, Inorg. Chem. 1992, 31, 2396; b) A. Vogler, H.Kunkely, J. Hlavatsch, Inorg. Chem. 1984, 23, 506; c) A. Vogler,H. Kunkely, J. Am. Chem. Soc. 1981, 103, 1559; d) T. R. Miller,G. Dance, J. Am. Chem. Soc. 1973, 95, 6970.

[14] We rationalize this behavior by a fast dimerization at ambienttemperature of the radical 3a with S�S bond formation;however, 3a is stable in frozen solution at < 77 K.

[15] The EPR spectrum of the 2,4,6-tris(tert-butyl)phenylthiyl radicalis similar; it also lacks observable hyperfine splitting. Z. B.Alfassi in S-Centered Radicals, Wiley, New York, 1999, p. 7.

[16] G. A. Fox, C. G. Pierpont, Inorg. Chem. 1992, 31, 3718.[17] D. Darensbourg, K. K. Klausmeyer, J. H. Reibenspiess, Inorg.

Chem. 1996, 35, 1535.

Polymerization Mechanism Determined

[2.2]Paracyclophanes with Defined SubstitutionPattern–Key Compounds for the MechanisticUnderstanding of the Gilch Reaction to Poly-(p-phenylene vinylene)s**

Jens Wiesecke and Matthias Rehahn*

Since the discovery of (semi)conductivity in organic polymersin 1976,[1,2] and electroluminescence in semiconducting poly-mers in the early 1990s,[3] much effort has been made todetermine the structure±property relationships in conjugatedmacromolecular systems. The knowledge of these correla-tions is essential for further systematic development forapplication in light-emitting diodes (LEDs),[4] photovoltaiccells,[5] and field-effect transistors (FETs).[6] Poly(p-phenylenevinylene) (PPV) 1 is one of the most promising polymers forLED applications. Furthermore, it can be prepared by manysynthetic methods in good to excellent yields.[7] The Gilchroute[8] has a few advantages compared to other routes such asthe Heck,[9] Wessling,[10] and Knoevenagel reactions.[11] Theseadvantages include easily accessible starting materials, mildreaction conditions, and formation of film-forming products.To be suitable for LED applications, the PPVs must be free ofstructural defects.[12] Unfortunately, it is well-known that theGilch process generates a variety of characteristic defectswithin the polymer chains, such as saturated ethylene bridges,rodlike ethynylene subunits, and halogenated chain ends.[13]

Lowering the occurrence of such defects to a minimum level istherefore of crucial importance and requires a profoundunderstanding of the Gilch reaction©s mechanism. If oneignores some rather speculative ideas about mechanismsoccurring via carbene intermediates,[14] just one generalreaction scheme remains (Scheme 1): using KOtBu as thebase, the starting material 2 eliminates one HCl molecule. Theresulting ™real∫ monomer 3 polymerizes, leading to thenonconjugated but chlorinated poly(p-xylylene) (PPX; 4). Itis particularly the mechanism of this chain growth processwhich is presently under controversial discussion, althoughthe mechanisms of chain initiation, chain termination, andcross-linking in this reaction are also unclear. For the finalstep, there is agreement that 4 converts into PPV 1 byelimination of a second equivalent of HCl.

For the polymerization reaction anionic,[15] radical,[16] andsimultaneous anionic and radical chain-growth processes

[*] Prof. Dr. M. Rehahn, Dipl.-Chem. J. WieseckeTechnische Universit‰t DarmstadtInstitut f¸r Makromolekulare ChemiePetersenstrasse 22, 64287 Darmstadt (Germany)Fax: (þ49)6151-16-4670E-mail: mrehahn@dki.tu-darmstadt.de

[**] We thank the Fonds der Chemischen Industrie e.V. (FCI) and theVereinigung der Freunde der TU Darmstadt e.V. for financialsupport.

Supporting Information (experimental and analytical details) forthis article available on the WWW under http://www.angewand-te.org or from the author.

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