The Gilch Synthesis of Poly(p -phenylene vinylenes):Mechanistic Knowledge in the Service of Advanced Materials
Thorsten Schwalm,Jens Wiesecke,Stefan Immel,Matthias Rehahn *
1.Introduction
Much effort has been spent over the decades on the development of powerful synthetic access procedures and on the establishment of the structure–property relation-ships of p -conjugated organic polymers.[1–5]Milestones of this research were the discovery of the electrical con-ductivity of ‘doped’polyacetylene in 1976,[6–8]the discovery of the concept of flexible side chains as ‘chemically bound solvent’for solubilizing the predomi-nantly semi-rigid or even rod-like conducting polymers in the early 1980s,[9]the systematic adaptation of the powerful transition-metal catalyzed condensation reac-tions to the specific requirements of polymer syntheses
Feature
Article
T.Schwalm,J.Wiesecke,M.Rehahn
Ernst-Berl-Institute for Chemical Engineering and
Macromolecular Science,Darmstadt University of Technology,Darmstadt 64287,Germany.
Fax þ49/(0)6151164670;E-mail:mrehahn@dki.tu-darmstadt.de S.Immel
Clemens-Scho ¨pf-Institute for Organic Chemistry and
Biochemistry,Darmstadt University of Technology,Darmstadt 64287,Germany M.Rehahn
German Institute for Polymers (DKI),Darmstadt 64289,Germany
A consistent picture is presented of the mechanistic details and intermediates of the Gilch polymerization leading to poly(p -phenylene vinylenes)(PPVs).In-situ generated p -quinodi-methanes are shown to be the real monomers,and spontaneous formation of the initiating radicals is effected by dimerization of some of these monomers to dimer diradicals,the latter also being the reason why signifi
cant amounts of [2.2]paracyclophanes are formed as side-products.Chain propagation predominantly proceeds by radical chain growth,occasionally interrupted by polyrecombination events between the growing a ,v -macro-diradicals.Based on this knowledge,oxygen is identified as a very efficient molar-mass regulating agent,and the temporary gelation of the reaction mixtures is interpreted to be the consequence of a very high entanglement of the polymers immediately after their formation.Last but not least,it is rationalized why the usually considered consti-tutional defects in Gilch PPVs might not be the only and most relevant ones with respect to the efficiency and durability of the organic light emit-ting devices produced thereof,and why cis -con-figurated halide-bearing vinylene moieties should be perceived as being among the most critical candidates.These considerations result in the recommendation of straightforward mea-sures that should lead to clearly improved
PPVs.
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from the late1980s on,[10–14]and thefirst polymer-based electroluminescent devices created in the early1990s.[15,16] Today,knowledge-driven scientific work on conducting polymers is still attractive,but aspects associated with their technical application in(opto)electronic devices in particular are clearly predominant in current research:this is because,despite an abundance of accumulated informa-tion,it remains a challenge to design,synthesize,and process these functional polymers in the quality that is required for ,efficient,durable and cheap, electronic devices such as light-emitting diodes,[1–5,17]photovoltaic cells,[18–20]andfield-effect transistors.[21–25] The reasons for the persisting limitations are manifold,but it seems in particular a result of the lack of insight into the chemical and morphological changes that may happen in operating devices.In this article,which is focu
sed primarily on a specific synthetic route that leads to poly(p-phenylene vinylenes)(PPVs),i.e.,a material that seems especially qualified for applications in organic light-emitting diodes(OLEDs)and solar cells,the background of some of these open questions will be highlighted in more detail.
T.Schwalm,J.Wiesecke,S.Immel,M.
Rehahn Thorsten Schwalm was born in Darmstadt,Germany,in1975.He studied chemistry at the Technische Universita¨t Darmstadt
(TUD).In2002,hefinalized his diploma thesis entitled‘‘Miniemulsion Polymerization of Methyl Methacrylate under ATRP
Conditions’’in Prof.Rehahn’s group at the Ernst-Berl Institut fu¨r Technische und Makromolekulare Chemie,TUD.Afterwards,
he also worked on his PhD thesis under the supervision of Prof.Rehahn.The main focus here was directed toward profound
elucidation of the reaction mechanism of the Gilch synthesis of poly(p-phenylene vinylene)s(PPVs),and in particular of the
reasons why typical constitutional defects appear in the resulting polymers.Moreover,he analyzed the influence of defects
and impurities on the performance and durability of organic light-emitting diodes(OLEDs)prepared from these materials.
Since2008,when hefinalized his PhD thesis,he has been active as a project leader in thefield of organic electronics in
Prof.Rehahn’s group.His major focus is on the synthesis and characterization of novel semiconductor materials based on
oligoacenes,and on testing these materials in OFET and OLED
devices.
Jens Wiesecke,born in Karlsruhe,Germany,in1974,studied chemistry at the Universita¨t Karlsruhe.In1999,hefinalized his
reaction to a book or an articlediploma thesis dealing with the crystallographic characterization of cyclodextrin inclusion compounds in the group of Prof.
Wenz at the Polymer-Institut,Universita¨t Karlsruhe.Afterwards,he moved to Darmstadt and carried out his PhD thesis at
the Ernst-Berl Institut fu¨r Technische und Makromolekulare Chemie under the supervision of Prof.Rehahn.Here,he focused
on the analysis of the reaction mechanism of the Gilch polymerization leading to PPVs.He was thefirst who identified
[2.2]paracyclophanes with specific substitution patterns as the dominant side products of this PPV synthesis,and to take
advantage of this observation for further deepening the mechanistic understanding of the whole reaction cascade.Also,
he showed that diradicals,formed by spontaneous dimerization of two p-quinodimethane monomer molecules,are the
initiators of the chain growth process.Since2003,he has been active in the polyethylene product development at Basell
Polyolefines,now Lyondell Basell Industries,where he is responsible for the development of new products from gas phase
technology.
Stefan Immel was born in Darmstadt,Germany,in1965.He studied chemistry at the Technische Universita¨t Darmstadt
(TUD)and obtained his PhD under the supervision of Prof.Lichtenthaler in1995on‘‘Computer Simulations of Chemical and
Biological Properties of Saccharides:Sucrose,Fructose,Cyclodextrins,and Starch’’.After continuing his scientific career at
the TUD and a postdoctoral stay at the Scripps Research Institute,San Diego,California,in the group of Prof.Sharpless,he
returned to the TUD where he completed his habilitation in organic chemistry in2004on the chemistry of‘‘Rigid and Flexible
Cyclodextrin Derivatives’’.At the beginning of2007,he took on a guest professorship at the Ruprecht-Karls-Universita¨t
Heidelberg,and since the end of2007he has been working as a guest professor at the Universita¨t Leipzig.His scientific
interest is focused on applied theoretical organic chemistry.Computational methods are applied to current problems and
unsolved questions in reaction mechanisms of polymerization processes,asymmetric catalysis,and organo-catalysis.His
research projects are characterized by interdisciplinary collaboration with research groups active in thefield of synthetic and
experimental organic
chemistry.
Matthias Rehahn,born in Frankfurt/Main,Germany,in1961,studied chemistry at the Johannes Gutenberg Universita¨t
Mainz.His PhD thesis was carried out under the supervision of Prof.A.-D.Schlu¨ter at the Max-Planck-Institut fu¨r
Polymerforschung,Mainz,in Prof.Wegner’s department.Discovery andfirst publications on the palladium-catalyzed
polycondensation of halide-and boronic acid-functionalized aromatic monomers based on the Suzuki-Miyaura cross-
coupling methodology,nowadays called‘‘Suzuki polycondensation’’,was one of the key results.After a post-doctoral stay at
the ETH Zu¨rich in Prof.Suter’s group,he completed his habilitation at the Universita¨t Karlsruhe in Prof.Ballauff’s Polymer-
Institut.Main topics were the synthesis and characterization of rod-like polyelectrolytes and transition-metal coordination
polymers.Since1999,he is full professor at the Technische Universita¨t Darmstadt and simultaneously head of the German
Institute for Polymers(DKI).Currentfields of research include living anionic and cationic polymerization methodologies,
transition-metal catalyzed polycondensation reactions,semi-conducting oligomers and polymers for optoelectronics,and
polyelectrolytes with special focus on basic understanding and fuel-cell applications.
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Among the huge variety of polymers that have been tested as active layers in optoelectronic devices,PPVs seem particularly attractive,and it is not surprising that essentially all promising synthetic access procedures have been evaluated carefully with respect to their benefits and drawbacks in the context of PPV preparation.Examples are the Wessling,[26]sulfinyl,[27–34]sulfonyl,[35,36]xanthate,[37–39]dithiocarbamate,[40]Gilch,[41–68]Heck,[69–73]Suzuki,[74,75]McMurry,[76]ring-opening metathesis,[77,78]acyclic diene metathesis,[79–81]Wittig,[82–84]Horner–Emmons,[85]Knoe-venagel,[86]chemical vapor deposition,[87–91]and electro-polymerization [92]methods.By assessing the pros and cons of all these approaches,the Gilch route stands out as one of the most powerful,versatile,and
economically efficient approaches:simply by the treatment of a ,a 0-dihalogenated p -xylene derivatives 1with potassium tert -butoxide (KO t Bu),high-molecular-weight PPVs 2can be obtained in good to excellent yields (Scheme 1).
Moreover,the Gilch process is not only convenient from a practical point of view,it also tolerates a variety of lateral substituents R which may be attached for electronic,solubility,and/or reactivity reasons.On the other hand,simple synthetic procedures and convenient fine-tuning of properties are only one side of the picture.The negative sides are the characteristic constitutional and configura-tional defects which obviously reside intrinsically in all
Gilch PPVs.[93–101]Usually specified examples are cis -configurated vinylene moieties B in the predominantly trans -configurated chains A ,halide-containing repeating units C and chain termini D ,and chain segments composed of a rod-like phenylene ethynylene subunit followed by a non-conjugated phenylene ethylene bridge (the so-called ‘tolane-bis-benzyl’(TBB)defect,E ,see Figure 1).In addition to these,further still unidentified defects are likely,caused by initiation,termination,transfer,oxidation,hydrolysis,or cross-linking events.
Also,some experimental evidence has been published that supports the conclusion that at least some
of these defects are critical with respect to efficiency,ageing,and fatigue of the OLED devices prepared from Gilch PPVs.Therefore,it is a consensus in the community that striking measures are required that provide access to materials of extreme purity and constitutional perfection.Moreover,uncontrollable molar masses,large polydispersities,and gelation of the reaction mixtures represent further challenging features of Gilch PPVs.
All these crucial issues can only be solved through a really profound understanding of the reaction mechanism of the Gilch process.However,this understanding is still full of gaps because the reaction,although seemingly simple when inspected as globally as shown in Scheme 1,comprises a sequence of fast and coupled partial steps.
The Gilch Synthesis of Poly(p -phenylene vinylenes):
...
Scheme 1.Global representation of the Gilch
reaction.
Figure 1.Regular repeating units of PPV 2(A )and some defects (B –E )residing in Gilch PPVs;R ¼lateral substituent,X ¼halogen.
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Therefore,we started a research program dedicated explicitly to the elucidation of all open mechanistic facets of the Gilch synthesis and their implications on i)the control of molar masses,ii)the prevention of gel formation during polymer synthesis,and iii)the minimization of critical defects and chain termini in the resulting PPVs.[62–68]Thefirst step in this program was the careful review and evaluation of all available mechanistic information not only for the Gilch reaction but also for all‘Gilch-related’PPV syntheses.a Guided by this ground-work,a step-by-step analysis was carried out of the intermediates,products,and side-products,and how the involved reactions proceed when syntheses are carried out either under the standard conditions or affected by specific modifications and additives.These studies performed in our labs were supported significantly by very helpful complementary work done in other research teams,and primarily in Vanderzande’s group at Hasselt University, Belgium.[102–108]Still today,it is not possible to ultimately answer all possible questions ab
out distinct details of the Gilch process,but compared to what was known just a few years ago,the recently collected information provides a much deeper and much more consistent picture.Also,a simple and very efficient procedure was derived for molar-mass control,and a consolidated explanation can now be offered for the mysterious reversible gelation,which has often been observed during Gilch reactions.Last but not least,a well-founded re-evaluation of the origin and the constitution of the most relevant defects in Gilch PPVs has become feasible.We assume that these insights will have a considerable impact on the future scientific work in this field,and may also lead to technological improvements. It is this article’s primary intention,therefore,to offer a survey of how the mentioned results were generated,why specific conclusions were drawn,and what their implica-tions might be concerning a‘best synthetic procedure’. Nevertheless,let us start with a short summary of the formerly established status of discussion on the mechan-ism of Gilch and Gilch-related PPV syntheses.
2.Gilch and Gilch-Related Syntheses–The Former State of Knowledge
Research on the mechanism of the Gilch synthesis was always inspired and supported by complementary studies carried out on related PPV syntheses.Therefore,a quick insight will be provided into the current state of knowl-edge on the most relevant‘Gilch-type’PPV syntheses as published in the literature.2.1.The Wessling Route
In the Wessling synthesis,bis-cationic starting materials I are treated with sodium hydroxide as the base.[26,109,110] Ho¨rhold et al.showed that activation of I proceeds step-wise by deprotonation that leads to ylide II,followed by elimination of dimethyl sulfide(Scheme2).[111]Hatch[112] and Lahti et al.[113]confirmed step-wise monomer activa-tion by hydrogen-deuterium exchange studies.Also,they observed the resulting p-quinodimethane III experimen-tally using low-temperature in-situ UV-vis and NMR spectroscopy.Being the active monomer of the Wessling route,intermediate III polymerizes by1,6-type chain polymerization,and the formed precursor polymer IV converts into PPV V by afinal macromolecular elimination cascade.
Nofinal decision was possible on whether the chains IV grow by radicals or anions.Wessling postulated a radical mechanism,possibly initiated by dimeric diradicals VI (Scheme2).[26]In contrast,Lahti et al.,[113]Garay and Lenz,[114]and Hsieh[115]reported results that pointed toward anionic chain growth,while Hall and co-work-ers[116]and,later,Lahti and co-workers[117,118]realized that polymerization also proceeds in acidic media but is suppressed when2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)is added as a scavenger.Also Cho et al.,who first favored anionic chain growth,[119]revised his earlier standpoint and has favored radical chain growth since then.[120–122]To summarize,the majority of information available supports1,6-type radical polymerization of the p-quino
dimethane-type active monomer III as the key step of the Wessling route,and some evidence was reported in favor of spontaneous formation of diradicals VI as the initiators of subsequent radical chain growth.
2.2.The Sulfinyl and Sulfonyl Routes
In contrast to the Wessling route,uncharged starting materials VII are used in the sulfinyl and sulfonyl routes, which are non-symmetrically substituted in their a and a0 positions:the halogen substituent X acts as a leaving group while the sulfinyl or sulfonyl group P is to polarize the benzylic CÀH bond(Scheme3).
Vanderzande and co-workers showed that activation of starting material VII proceeds by1,6-elimination of HX, and they could also provide experimental evidence of radical chain propagation of the assumed p-quinodi-methane-type active monomer IX.[107,108]Dimerization of a small number of species IX,which leads to dimeric diradicals XI,was assumed to be the dominant initiation event.[106]In1999,moreover,Vanderzande and co-workers showed that high-molecular-weight polymers form in the polar-protic solvent N-methylformamide whereas a bimo-dal molar mass distribution appears in the polar-aprotic
T.Schwalm,J.Wiesecke,S.Immel,M.Rehahn
a The latter syntheses include all preparation methods where a,a0-difunctionalized p-xylene derivatives are converted into PPVs by the treatment with stoichiometric amounts of base.
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solvent N -methylpyrrolidone (NMP).[105]This implies that anionic and radical chain growth occur in parallel in NMP,an interpretation that was further supported by experi-ments where either water or TEMPO was added to the entries in NMP:in the case of water,the low-molecular-weight part of the bimodal distribution curve was missing,
whereas it was the high-molecular-weight part with TEMPO.Thus the sulfonyl and sulfinyl routes seem to proceed as radical chain growth processes under most synthetic conditions,but specific media exist where radical and anionic chain growth can occur simulta-neously.
The Gilch Synthesis of Poly(p -phenylene vinylenes):
...
Scheme 3.Key steps in the sulfinyl and sulfonyl routes (top),and proposed mechanism of in-situ formation of initiating radicals (bottom);X ¼Cl,Br (‘leaving group’),P ¼SOBu or SO 2Bu (‘polarizing
group’).
Scheme 2.Key steps of the Wessling route (top),and proposed mechanism of in-situ formation of initiating radicals (bottom);lateral substituents R,which are usually attached to the PPVs to ensure their solubility and to tailor their properties,are omitted here as well as in many of the following schemes for reasons of clarity.
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