synthesis and thermal and spectroscopic properties of macrocyclic vinyl aromatic polymers

Synthesis and Thermal and Spectroscopic Properties ofMacrocyclic Vinyl Aromatic Polymers

RONG CHEN, GENNADI G. NOSSAREV, THIEO E. HOGEN-ESCH

Loker Hydrocarbon Institute and Department of Chemistry, University of Southern California, Los Angeles,California 90089-1661

Received 14 June 2004; accepted 12 July 2004DOI: 10.1002/pola.20413Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Well-defined and narrow molecular weight distribution macrocyclic poly(2-vinylnaphthalene) (P2VN) and poly(2-vinyl-9,9-dimethylfluorene) (PDMVF) containinga single 1,4-benzylidene or 9,10-anthracenylidene unit have been synthesized via thepotassium naphthalide initiated polymerization of the monomers followed by the end-to-end coupling of the resulting P2VN dianions under high-dilution conditions with1,4-bis(bromomethyl)benzene or 9,10-bis(chloromethyl)anthracene. Molecular charac-terization has been carried out by size exclusion chromatography, nuclear magneticresonance, differential scanning calorimetry, ultraviolet–visible spectroscopy, and ma-trix-assisted laser desorption/ionization time-of-flight mass spectrometry. The thermalproperties show distinct differences between the cyclic and matching linear polymers,with the macrocycles showing much higher glass-transition and decomposition temper-atures. The absorption bands are both hyperchromic and hypochromic with respect tothe model aromatic compounds, and this is consistent with intensity borrowing. © 2004Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5488–5503, 2004Keywords: anionic polymerization; anthracene; emission; macrocyclic polymers;poly(vinyl fluorene); poly(vinyl naphthalene); UV absorption; structure-property rela-tions; termal properties

INTRODUCTION

A number of articles have appeared recently on thesynthesis and characterization of macrocyclic poly-mers by anionic, cationic, and other techniques.1–19

We have reported extensively on the synthesis andcharacterization of matching linear and macrocyclicvinyl aromatic polymers and block copolymers byend-to-end cyclization of the corresponding dian-ions with bifunctional electrophiles, such as 1,4-bis-(bromomethyl)benzene (DBX) and 9,10-bis(chloro-methyl)anthracene (BCMA), in highly dilute solu-tions (10�4–10�6 M).4,12–19

Vinyl aromatic polymers containing phenyl,2-naphthyl, or 2-fluorenyl pendent groups haveinteresting photoluminescent properties and goodchemical and thermal stabilities.12,14,17–19 For in-stance, the molar absorptivities of naphthaleneand fluorene in the near-UV region are approxi-mately 5 � 103 and 104, respectively. The fluores-cence quantum efficiency of fluorene is relativelyhigh (ca. 0.80).20 Thus, polymers or copolymers offluorene are interesting in energy transfer inlight-harvesting polymers,21–25 particularly cyclicvinyl aromatic polymers, such as polystyrene(PS)11 and poly(9,9-dimethyl-2-vinylfluorene)(PDMVF),19(b) which show more intense emis-sions than their matching linear analogues.

Here we describe the potassium naphthalide(K-Naph) mediated electron-transfer-initiated po-

Correspondence to: T. E. Hogen-Esch (E-mail: [email protected])Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 5488–5503 (2004)© 2004 Wiley Periodicals, Inc.

5488

lymerization of 2-vinylnaphthalene (2VN) and2-vinyl-9,9-dimethylfluorene and the intramolec-ular cyclization and coupling of the resulting poly-mer dianion precursors with DBX or BCMA togive the corresponding macrocyclic polymers. Theincorporation of anthracene in such polymers isinteresting as it is a useful fluorescent acceptorprobe in the study of energy migration.25,26

EXPERIMENTAL

Materials

2VN (5 g, 95%; Alfa Aesar) was dissolved in tolu-ene (10 mL) and stirred under a high vacuum oversolid LiAlH4 for 5 h at room temperature. Toluenewas then distilled into a side flask, and the re-maining 2VN sublimed at 60 °C into this flask.For the removal of residual LiAlH4, toluene wasdistilled into the trap, and the monomer was sub-limed into a collection flask. Purified tetrahydro-furan (THF;40 mL) was added into this flask froman ampule. The resulting monomer solution wasdistributed into ampules equipped with break-seals. THF was purified by distillation from sodi-um/potassium benzophenone, and this was fol-lowed by distillation from the stable carbanion1,4-dipotassio-1,1,4,4-tetraphenylbutane. Naph-thalene (�99%; Aldrich) and 1,1,4,4,7,7,10,10-octamethyl-1,2,3,4,7,8,9,10-octahydronaphtha-cene (OMOHN)27 were recrystallized from meth-anol, dried in vacuo, and dissolved in purifiedTHF. K-Naph and the corresponding salt ofOMOHN were prepared by the stirring of naph-thalene or OMOHN in THF over a potassiummirror for 15–20 min at 0 °C. The initiators wereused immediately after preparation. The couplingreagents DBX (�98%; TCI America) and BCMA(98%; TCI America) were recrystallized twicefrom chloroform, dried on a vacuum line for 24 h,and dissolved in purified THF to the needed con-centrations.

The 9,9-dimethyl-2-vinylflourene (DMVF) mono-mer was prepared by the bismethylation of 2-vi-nylfluorene (2VF) at the 9-position with a modifiedprocedure.28 To a solution of 3.95 g of 2VF (20.6mmol) in 80 mL of dimethyl sulfoxide was added 40mL of 50 wt % aqueous NaOH and 0.37 g (1.0 mmol)of tetrabutylammonium iodide under argon. Themixture turned pink, and upon the dropwise addi-tion of 7 mL (50 mmol) of CH3I, the pink colorturned dark purple. After vigorous stirring at roomtemperature for 16 h, 100 mL of water and 100 mL

of ether were added, and the mixture was stirred for15 min and then separated. The aqueous layer wasextracted with two portions of 30 mL of ether, andthe combined ether extracts were washed with20 mL of 5% HCl (twice) and then with water untilthe aqueous solution was neutral. The ether ex-tracts were dried with anhydrous MgSO4, and thesolvent was removed; this left an orange solid. Pu-rification with column chromatography (silica gel,60–200 mesh; J. T. Baker) with hexane as an eluentproduced a white crystalline solid in a 91% yield(mp � 81.9–82.4 °C). DMVF was then dissolved inpurified THF, stirred over freshly crushed CaH2,purified with crosslinked PS–fluorene beads con-taining pendent fluorenyllithium anions,19 and dis-tributed into ampules equipped with break-seals.

1H NMR (�, ppm): 7.80–7.30 (m, 7H), 6.90–6.70 (dd, 1H), 5.90–5.75 (dd, 1H), 5.30–5.20 (dd,1H), 1.50 (s, 6H).

Polymerization/Cyclization Reactions

These were carried out in THF at �78 °C underhigh-vacuum conditions (10�5–10�6 Torr) as re-ported elsewhere.11 The needed amount of THFwas distilled in, and the entire apparatus wasrinsed with a triphenylmethyl carbanion solution.The precursor dianion was prepared by a one-time addition of a precooled monomer/THF solu-tion (0.45 mmol in 10 mL) to an approximately20-mL solution of K-Naph (0.50 mmol) in THF at�78 °C. After 5 min, about two-thirds of the poly-mer dianion solution and a DBX or BCMA solu-tion were simultaneously added dropwise into acyclization reactor containing approximately250 mL of precooled and vigorously stirred THF.The addition rates were adjusted to allow a faintcolor of the anion to persist. The remaining one-third of the polymer dianion solutions were ter-minated with degassed methanol to give thematching linear polymers.

After the evaporation of THF, the residueswere dissolved in 5–10 mL of CH3Cl and washedwith water to remove inorganic salts. The poly-mers were then precipitated in excess (ca.200 mL) methanol. Fractionation was carried outby the incremental addition of methanol into thepolymer–THF solutions (0.70–1.0 g dissolved in5 mL of THF). As soon as precipitation was justvisible (the addition of 3–5 mL of methanol), theprecipitate was filtered off, and the solution wasanalyzed by size exclusion chromatography(SEC). This methanol addition filtration sequencewas repeated until the shape and maximum of the

MACROCYCLIC VINYL AROMATIC POLYMERS 5489

SEC curve of the cyclic polymer were identical tothose in the crude product. Then, the polymerswere isolated by solvent evaporation.

Characterization

SEC was carried out at room temperature withTHF as the carrier solvent at a flow rate of 1 mL/min with a Waters model 510 pump, a model 410differential refractometer, and two Ultrastyragelcolumns (500 and 104 Å) calibrated with PS stan-dards (Polysciences). All the polymers were ana-lyzed by SEC before and after precipitation and/orfractionation. 1H NMR was run on a Bruker AM-250 FT instrument operating at 250 MHz inCDCl3. The anthracenylidene-containing macro-cycles were subjected to SEC (UV) analysis at 405and 295 nm.

Ultraviolet–visible (UV–vis) absorption on thepoly(2-vinylnaphthalene) (P2VN) samples wasrun on a Varian Cary 50 spectrometer with abaseline correction with 50 mg/L solutions in cy-clohexane/THF (90/10 v/v) and 1-mm quartz cells.UV–vis measurements of the PDMVF sampleswere carried out in cyclohexane at 5 mg/L on anHP 8453 spectrophotometer or a Varian Cary 50spectrometer with a 1-cm quartz cell with a base-line set with a solvent. Fluorescence experimentswere run in cyclohexane at 5 mg/L in a 1-cmfluorescence cell on a PTI Quanta Master TMC-60SE spectrofluorimeter with a 3-nm bandpass.

Glass-transition temperatures (Tg’s) were ac-quired with 5–10-mg samples with a PerkinElmerDSC-7 instrument at a scanning rate of 10 °/minin two consecutive runs for each polymer sample.The Tg values from the second run are reported as

the maxima on the differential scanning calorim-etry curves.

RESULTS AND DISCUSSION

Synthesis

P2VN Macrocycles

Although the anionic polymerization of 2VN hasbeen reported,29 the control of the molecularweight (MW) and the formation of a narrow MWdistribution P2VN [1.09 � Mw/Mn (PDI) � 1.26]have only recently been achieved and are due to amore rigorous monomer purification with LiAlH4,which removes the persistent 2-acetylnaphtha-lene impurity.30,31

Thus, macrocyclic P2VN samples with num-ber-average degrees of polymerization (DPn’s) of7–120 could be synthesized in THF at �78 °Cthrough potassium naphthalide initiated poly-merization and coupling in the presence of potas-sium ions by end-to-end coupling at anion concen-trations of 10�6–10�4 M with DBX or BCMA, asdescribed elesewhere.14 The matching linear an-alogues (1.11 � PDI � 1.34) were prepared by theprotonation of the same P2VN dianions.

The use of DBX or BCMA, designated as a cou-pling reagent (EX2; Scheme 1), adds a small unit (E)into the macrocycle backbone. The ring constrainthas been shown to affect some properties such asthe hydrodynamic size,4,12,13 Tg,

4,12–14 and emis-sion.11 Although the presence of a 1,4-benzylideneunit in macrocyclic PS could affect its spectroscopicproperties,11 this should not be the case for P2VN asthe absorption and emission of 1,4-benzylidene are

Scheme 1. Synthesis of cyclic and matching linear vinyl aromatic polymers.

5490 CHEN, NOSSAREV, AND HOGEN-ESCH

negligible in comparison with those of P2VN orPDMVF.14,19(b) The use of coupling reagents, suchas dibromomethane, is not desirable, as side reac-tions, such as metal–halogen exchange and hydrideelimination, will lead to the formation of spectro-scopically active vinyl groups.11,31

Two experiments (Table 1, runs 1 and 2) wererun at relatively high polymer anion concen-

trations (2.2 � 10�3 and 3.2 � 10�3 M, respec-tively) to determine the efficiency ofthe DBX coupling reaction. The SEC chromato-gram unexpectedly shows three new peaks [Fig.1(a)]. As expected, the cyclization product at amuch lower concentration (10�4–10�6 M) pro-duces a much better cyclization yield[Fig. 1(b)].

Table 1. Formation of Macrocyclic P2VN by the Coupling of P2VN-K2 and DBX or BCMA in THF at �78 °Ca

P2VNlin P2VNcyclb

Mn � 10�3

(g/mol)dMp � 10�3

(g/mol)Mn � 10�3

(g/mol) PDIMp � 10�3

(g/mol)Mn � 10�3

(g/mol) PDI �G�

1 0.55 0.95 0.72 1.34 0.91 0.79 (2.65) 0.952 1.10 1.25 1.25 1.13 1.07 1.00 1.13 0.863 1.50 1.80 1.75 1.13 1.53 1.58 1.12 0.854 1.60 1.85 1.60 1.14 1.50 1.75 1.26 0.825 3.40 4.40 4.05 1.10 3.35 2.90 1.11 0.766 5.70 6.50 5.60 1.12 5.24 11.5 (3.86) 0.807 5.70 7.00 6.00 1.12 5.45 11.7 (5.22) 0.788 11.0 14.8 12.3 1.16 10.6 11.1 1.12 0.719c 0.85 1.10 1.00 1.18 0.98 1.15 (2.70) 0.89

10c 3.70 4.43 4.00 1.11 3.48 3.15 1.13 0.79

a The coupling agent was DBX, except as indicated, and MWs were determined by SEC with PS standards.b The values in parentheses are PDIs of unfractionated P2VN.c BCMA was used as the coupling reagent.d Calculated MWs.

Figure 1. Normalized SEC (RI) traces of (a) the P2VN linear precursor (I) and thereaction product of high-concentration coupling with DBX (II; Table 1, run 7) and (b) theP2VN precursor (I), the product of coupling with DBX under high dilution (II), and thefractionated macrocyclic P2VN (III; Table 1, run 3).


The broad peak in Figure 1(a) centered at12 mL corresponds to the expected polycondensa-tion product with a high MW [apparent peak mo-lecular weight (Mp) � 112,000].32 The narrowpeak at 15.5 mL represents the intended P2VNmacrocycle with an apparent peak molecularweight (Mp) of about 5500, which is clearly lowerthan that of the matching linear P2VN precursorwith an Mp value of 7000. Mp corresponding to thepeak at 14.5 mL (Mp � 11,200) is almost exactlydouble that of the target macrocycle, but not thatof the linear P2VN precursor (Table 1, run 2), andthis is attributed to the macrocyclic dimer formedby one intermolecular coupling and one intramo-lecular coupling.

The presence of macrocyclic P2VN, the verysimilar shapes for the linear and macrocyclicP2VN, and the presence of a significant fraction ofthe macrocyclic polymer dimer show an interest-ing tendency, consistent with predictions, to formmacrocyclic P2VN even at relatively high anionconcentrations (2 � 10�3–3 � 10�3 M).33,34 Thenumber-average molecular weight (Mn) of thehighest MW fraction (Mn � 101,600) indicates ahigh efficiency of coupling as this MW is 17 timesthat of the linear precursor (Table 1).

Following the simplified Carothers expres-sion32 for DPn of this step polymerization, wehave

DPn � 2/�2 � pfav) (1)

DPn � 2/�2 � fav) (2)

fav � �Nif/�Ni (3)

where p and fav denote the extent of reaction andthe average functionality, respectively, and Ni andfi are the numbers of the monomers and their func-tionalities, respectively. p is taken as unity as allthe anions have reacted, and so eq 1 reduces to eq 2.

The quantity of fav then denotes an empiricalapparent number-average functionality of the tworeagents. This includes the loss of functionalityresulting from the presence of impurities in eitherreagent, inadvertent protonation (e.g., due to thereaction of the carbanions with the solvent), orthe occurrence of side reactions.

The Mn value of the highest MW fraction in Fig-ure 1(a) indicates a high coupling efficiency (17times that of the linear precursor), giving an effec-tive DPn of 34 if the coupling agent does not con-tribute significantly to the hydrodynamic volume.

The corresponding apparent average monomerfunctionality calculated from eq 2 is equal to 1.94,indicating that approximately 3% of the functionalgroups have been terminated. If we assume randomtermination, the expected fraction of P2VN chainsterminated at both chain ends under these condi-tions is thus negligible [0.0009 � (0.03)2], and so thefraction of the linear precursor in the cyclizationproduct should be also.

The most plausible cause for the slightly lowerapparent functionality of the P2VN precursor isthe occurrence of side reactions, such as the base-mediated intramolecular elimination reactions of1-bromo-1-(2-naphthyl) alkyl polymer end-groupimpurities generated by metal–halogen exchangeduring coupling.31,35 These produce vinyl endgroups, the fluorescence of which has been de-tected in the formation of macrocyclic PS by sim-ilar coupling reactions.

As shown in Table 1 (runs 3–8), the �G values,representing the ratio of Mp of the macrocycle inthe unfractionated products to that of the linearprecursor, steadily decrease from 0.95 to 0.71 asDPn increases from 7 to 125 (Table 1, runs 3–8).The origin of this trend may be severe conforma-tional restraints as the number of monomer unitsdecreases. We have observed such trends forother macrocyclic vinyl aromatic polymers.4,11–19

The existence of this trend provides additionalsupport for the formation of cycles. To the best ofour knowledge, the existence of this systematictrend has not been reported elsewhere.

The proton spectrum of a P2VN macrocyclecoupled with DBX and its matching linear poly-mer (Table 1, run 5) provides additional evi-dence for the presence of the benzylidene unit inthe cycle (Fig. 2). Thus, a new broad downfieldresonance is visible at 3.40 ppm, correspondingto four benzylic methylene protons. The broad-ness of this resonance is due in part to thegeminal coupling of the methylene protons,which are diastereotopic because of the pres-ence of the nearby asymmetric center, and tovicinal coupling with the 2-naphthyl methineproton, which is also visible at 2.50 ppm. Theresonance at 2.30 ppm for the linear P2VN isdue to the 2-naphthyl methylene end group anddiffers from the 2-naphthyl methine proton inthe cycle observed at 2.50 ppm.

9,10-Anthracenylidene P2VN Macrocycles

The incorporation of larger aromatic chromo-phores, such as anthracene, pyrene, or perylene,


into polymer rings through the aforementionedcoupling reactions is interesting for intramolecu-lar-energy-transfer studies. However, the forma-tion of such rings is complicated by side reactionsarising from halogen exchange and the low oxida-tion-reduction potentials of these groups.31 Thus,the coupling of the PS anion by BCMA and simi-lar derivatives leads to incomplete or multipleanthracene incorporation.31

The coupling reactions of linear poly(2-vinyl-naphthalene)dianion potassium (P2VN-K2) withBCMA at high anion concentrations (10�3 M) in-crease Mp only by a factor of 5, which is consider-ably lower than that of the DBX coupling productsunder similar conditions (as discussed previously).Furthermore, the SEC [refractive index (RI)] tracesshow the presence of 30% macrocyclic polymer and10–20% linear precursor in addition to the polycon-densation product. In addition, SEC (UV) moni-tored at 405 nm, at which point only anthraceneabsorbs, shows excessive incorporation of the an-

thracenylidene group. Similar reactions of poly-styryl lithium (PSLi) and BCMA in THF at �78 °Calso produce multiple anthracene labeling consis-tent with lithium–halogen exchange.31

Surprisingly, the formation of macrocyclicP2VN containing a single 9,10-anthracenylideneunit proceeds well under high-dilution conditions,as indicated by a symmetrical SEC peak and aclear shift to a higher elution volume and a highSEC yield (70%) of a macrocyclic P2VN [Table 1,run 10, and Fig. 3(a)]. In addition, the �G valuesof the BCMA-coupled polymers follow the sametrends obtained for the DBX cycles, being slightlyhigher for the same MWs, plausibly on account ofthe larger 9,10-anthracenylidene spacer.

The uniform incorporation of a single anthra-cene into macrocyclic P2VN was confirmed byidentical SEC (RI) and SEC (UV) traces [Fig. 3(b)]of the fractionated macrocycles run at both 405and 295 nm, the wavelengths at which only naph-thalene absorbs. The anthracene content in thefractionated macrocyclic P2VN calculated fromthe UV spectrum (Fig. 4) with DPn from the linearprecursor and the absorption at 405 nm with theextinction coefficient (�) for 9,10-dimethylanthra-cene20 (10,000) has been calculated to be 90%, andthis confirm a high degree of anthracene incorpo-ration. The spectrum does not show the presenceof chromophores other than naphthalene and an-thracene (346, 365, 385, and 406 nm). Studies onthe photophysics of these labeled macrocycles arebeing conducted.36

PDMVF Macrocycles

The synthesis of macrocyclic PDMVF with appar-ent Mn’s ranging from 2600 to 31,200 was carriedout in THF at �78 °C via a DBX- or BCMA-mediated end-to-end cyclization at 10�4–10�6 M.(Table 2). The MWs directly determined fromSEC with PS standards were corrected by themultiplication of this MW by a factor of 2.12(mass of DMVF/mass of styrene � 220/104).18 Thefractionated macrocyclic polymers were obtainedby the incremental addition of methanol into aTHF solution of the crude cyclization product,which precipitated the undesirable polycondensa-tion product (see the Experimental section). Asfor macrocyclic P2VN, this process was monitoredby the SEC analysis of the supernatant solutions.

Just as for macrocyclic P2VN, one experiment(Table 2, run 3) was carried out at a relativelyhigh concentration (ca. 10�2 M) of the polymerdianion to determine the efficiency of the inter-

Figure 2. 1H NMR spectra (250 MHz) in CDCl3 of (a)the linear protonated P2VN precursor and (b) thematching DBX-coupled fractionated macrocyclic P2VN(Table 1, run 5).


Figure 3. Normalized SEC (RI) traces of (a) the linear precursor (I) and the unfrac-tionated product of high-dilution coupling of P2VN-K2 with BCMA (II; Table 1, run 10)and (b) the corresponding fractionated linear P2VN (I) and macrocyclic P2VN acquiredwith an RI detector (II) and a UV detector at 405 nm (III).

Figure 4. Absorption spectrum of fractionated macrocyclic P2VN (Table 1, run 10)containing 9,10-anthracenylidene groups (54 mg/L in 80/20 v/v cyclohexane/THF,0.1-cm cell). The insert is a 10� magnification.


molecular coupling reactions. In this case, an ap-preciable SEC macrocycle yield was also obtained(50%, data not shown). A small peak (Mp � 9300),double that of the macrocycle, and a broad andhigh MW (Mp � 42,100) band are attributed tothe macrocyclic dimer resulting from one inter-molecular coupling reaction and one intramolec-ular coupling reaction and to a presumably linearstep polymerization product.

As the MW of the high-MW fraction has anMn value roughly 5 times that of the polymerprecursor, the apparent number-average degreeof coupling is 10, and so the apparent averagefunctionality is 1.80, corresponding to 10%overall termination; therefore, the fraction ofpolymer terminated at both chain ends is esti-mated to be about 1%. Thus, the extent of con-tamination of the macrocycle with the matchinglinear polymer should be negligible here aswell.

As for P2VN, the macrocyclic PDMVF hasapparent peak MWs 9 –29% lower than those ofthe protonated linear precursor, and this iscomparable to that observed for P2VN. The SECcyclization yields are also similar (38 –72%) andlikewise decrease with increasing DPn as thelarger end-to-end distance between the PDMVFanion and electrophile site reduces the rate ofintramolecular coupling (Table 2).34 Like theP2VN cycles, the ratios of the apparent peakMWs of the unfractionated cyclic polymers andthe matching linear polymers, given as the �G

values, increase from 0.71 to 0.91 (Table 2, runs1 and 7) as the MWs decrease.

The 1H NMR spectrum of the smallest PDMVFcycle with a degree of polymerization (DP) of 12(Table 1, run 1) shows a resonance around 2.3corresponding to the two methine protons of thePDMVF end units (data not shown). A broad res-onance at about 2.6–3.4 and 7.0 ppm correspondsto the four benzylic protons and the four equiva-lent aromatic protons of the 1,4-benzylidene cou-pling unit, which is not present in the linear PD-MVF. The broad resonance of the geminal meth-ylene protons is consistent with the presence ofthe nearby asymmetric centers, which render theprotons nonequivalent.

Matrix-assisted laser desorption/ionizationtime-of-flight (MALDI-TOF) measurements ofthe fractionated macrocyclic PDMVF and thematching linear polymer of the same mass (Ta-ble 1, run 1) show further and clear evidence forring formation (Fig. 5). The interval betweenthe peaks is 220 Da, corresponding to the molarmass of the DMVF unit. The experimental andcalculated masses are in good agreement, asshown in Table 3. The formation of the ringstructure is supported by the 102 Da mass dif-ference between the macrocycle and matchinglinear precursor. For instance, the masses ofthe cyclic PDMVF with a DPn value of 11 and itslinear precursor are 2635 and 2533 Da, respec-tively, which agree well with the calculated val-ues of 2635.5 and 2533.3 Da.

Table 2. Cyclization of DMVF Coupled with DBX or BCMA at �78 °C in THFa

Mncalcd

(g/mol)

Linear PDMVF Cyclic PDMVF

SECYieldd �G�

Mn

(g/mol)bMp

(g/mol)b PDIMn

(g/mol)bMp

(g/mol)b,c PDIb

1 2,200 2,620 2,920 1.12 2,560 2,600 1.16 72% 0.912 3,300 3,930 4,420 1.12 3,470 3,930 1.09 65% 0.893e 3,530 4,210 4,970 1.13 4,420 50% 0.894 6,180 6,560 7,810 1.12 5,860 6,600 1.14 50% 0.845f 8,820 9,710 11,400 1.10 8,080 8,930 1.12 66% 0.776 9,240 10,300 12,000 1.09 7,910 8,820 1.09 70% 0.737 26,400 31,200 34,400 1.07 20,800 24,700 1.08 38% 0.71

a The monomer concentration was 0.15 M. Cyclization was carried out over about 20 min at an anion concentration ofapproximately 10�5–10�6 M.

b Determined by SEC with PS standards.c A SEC maxima of the unfractionated cyclic PDMVF.d Estimated from SEC.e Cyclization was carried out at an anion concentration of 10�2 M.f BCMA was used as the coupling reagent.


Figure 5. MALDI-TOF spectra of macrocyclic PDMVF (top) and the matching linearpolymer (bottom; Table 2, run 1).

Table 3. Calculated and Observed Molecular Weight of Macrocyclic PDMVF andthe Matching Linear Precursor (Table 1, 1) by MALDI-TOF

DPn

MW of Linear PDMVFMW of Fractionated Cyclic

PDMVF

Calculateda Experimental Calculatedb Experimental

6 1431.8 1432 1533.9 15337 1652.1 1652 1754.2 17548 1872.4 1873 1974.5 19749 2092.7 2092 2194.9 2195

10 2313.0 2312 2415.2 241511 2533.3 2533 2635.5 263512 2753.7 2753 2855.8 285513 2974.0 2973 3076.1 307514 3194.3 3192 3296.4 329615 3414.6 3415 3516.7 351716 3634.9 3636 3737.1 3737

a Calculated mass of the linear PDMVF precursor terminated by two protons plus the silvermatrix: MW � DPn � 220.315 2 � 1.008 107.87.

b Calculated mass of the macrocyclic PDMVF with a 1,4-benzylidene linking unit plus the silvermatrix: MW � DPn � 220.315 104.152 107.87.


PDMVF Anthracenylidene Macrocycles

A macrocyclic PDMVF containing a 9,10-anthra-cenylidene linking unit was successfully preparedwith the same methods (Table 2, run 5). Thesample had a �G value of 0.77, which was consis-tent with the DP value. The anthracene incorpo-ration, determined by UV–vis absorption at406 nm with the � value of 9,10-dimethylanthra-cene (10,000)35 and the DPn of the matching lin-ear PDMVF precursor, in this case was about95%. Detailed photophysical studies on thesepolymers are underway.

Thermal Properties

The fractionated macrocyclic P2VN and PDMVFpolymers and the corresponding matching linearprecursors have been used to study the depen-dence of Tg on DP. For linear P2VN, the expectedTg decrease can be observed with decreasing DP(Fig. 6) and is similar to values reported in theliterature.37–39 The reported value34 of Tg � 95� 5 °C for linear P2VN with DPn � 10 fits thistrend.40 For a very high MW linear P2VN (DPn� 8120) prepared by an inadvertent LiAlH4-initi-ated polymerization in THF, we have observedwhat appears to be a limiting Tg value of 152 °C.

Like the PS,12 poly(2-vinylpyridine) (P2VP),4

and poly(d-methylstyrene) (PAMS)13 cycles, Tg’sof the P2VN macrocycles (150 °C) are indepen-dent of MW above a DP of about 20.14 The reasonfor this is not completely clear but must be due inpart to the lower flexibility of the conformation-ally more encumbered P2VN. There are Tg in-creases of as much as 35 °C in comparison withthe Tg values of the matching linear polymers(Fig. 6). These differences for macrocyclic and lin-ear PS in this DPn range are less than 20°.12 Thesharp decreases in Tg below DPn’s of around 20are not well understood.

As shown in Figure 7, the Tg values of linearPDMVF, ranging from 126 (DPn � 12, Table 2, run1) to 178 °C (DPn � 142, Table 2, run 7), are com-parable to those reported.18 However, Tg’s of themacrocyclic PDMVF do not follow the trend forP2VN14 and for other vinyl aromatic macro-cycles,12–16 for which the Tg values of all macro-cycles with a DPn larger than 10–30 are identicaland equal to those of high-MW linear polymers.Thus, Tg’s increase with decreasing DPn’s from 164(DPn � 142) to about 171 °C (DPn � 12–18). Fur-thermore, the Tg values of the linear and cyclicPDMVFs at high MWs do not converge at the high-est MW, the Tg value of the highest DP cycle being14° lower than that of the matching linear polymer,

Figure 6. Relationship between Tg and DPn of (a) linear and (b) macrocyclic P2VN(the data point of linear P2VN ofDPn � 10 is taken from ref. 14).


and there is no sharp drop in Tg at low DPs. Thisbehavior requires further study.

Thermal Decomposition

Interesting differences between linear and cyclicP2VN and PDMVF can also be seen in their ther-

mal decomposition under nonoxidative condi-tions. A typical thermogravimetric analysis(TGA) curve for a low-MW macrocyclic P2VN(Table 1, run 4) shows a loss of 50% mass at atemperature that is 12° higher than that for thematching linear polymer [Fig. 8(a)].

Figure 8. Differences in the thermal nonoxidative decomposition (TGA) for (—) linearand ( � � � ) macrocyclic P2VN with DPn values of (a) 18 (Table 1, run 4) and (b) 125(Table 2, run 8).

Figure 7. Relationship between Tg and DPn of macrocyclic and linear PDMVF.


A similar trend can be observed for cyclic PS-b-poly(dimethylsiloxane) block copolymers, inwhich some macrocycles have a 50% mass loss attemperatures 34 °C higher than that of thematching linear block copolymer.15 These differ-ences are probably attributable to the absence ofchain ends in the macrocycles, as the chain endstypically are involved in the initial cleavage reac-tions. The difference in the decomposition tem-peratures becomes less pronounced for higherMW P2VN [Fig. 8(b)] as the fraction of polymerchain ends decreases.

The PDMVF macrocycles and matching linearchains show thermal stability profiles very simi-lar to those of the corresponding P2VN polymers(data not shown).19(a) Thus, the temperatures ofthe smaller cycles at which comparable weightlosses are observed are roughly 20 °C lower thanthose of the matching linear chains. Although thelargest macrocycle (Table 2, run 7) has a lower Tgthan the matching linear polymer, it seems tohave about the same thermal stability (50%weight loss temperature � 425 °C) as the match-ing linear polymer (50% weight loss temperature� 424 °C). The better heat resistance of macrocy-clic polymers is again attributable to the absenceof chain ends.41

Spectroscopy

The UV spectrum of PDMVF (Fig. 9) resemblesthat of free fluorene (except for a 6-nm redshiftobserved for the substituted fluorene), for which arelatively complete assignment has been pro-posed.42 In comparison with the model compound,both linear and cyclic polymers have lower molarabsorptivities, with some bands being affectedmore than others. Furthermore, at equal fluore-nyl concentrations, the cycles have lower UV ab-sorptivities than the matching linear polymers atalmost all wavelengths, except at 229 and235 nm, at which the absorptivities are higher.

As shown more clearly in the figure, the ratio ofthe � values of cyclic and linear PDMVF (�c/�l)indicates a clearly structured progression ofbands that correspond to distinct fluorene transi-tions,42 with similar �c/�l spectra being observedfor each MW. The intensity increases at 229 and235 nm for the rings, with respect to the linearpolymers, are due to borrowing from the othertransitions.43

As DPn’s increase from 12 to 142, the UV ab-sorptivities of the linear and cyclic polymers at307 nm decrease by 15 and 30%, respectively, theabsorptivities of the linear polymers being 4–22%

Figure 9. (top) Ratio of � of cyclic PDMVF to � of the linear polymer and (bottom) UVabsorption spectra of (- - -) the DMEF model compound and ( � � � ) cyclic and (—) linearPDMVF (DPn � 30). The concentration was 5 mg/L in cyclohexane; there was correctionfor mass differences due to the presence of linking units in the macrocycles.


higher than those of the matching macrocycles,depending on MW (Fig. 10).19(b) The absorptivityof the lowest MW linear PDMVF (DPn � 12, �� 13,900) is nearly the same as that of the 9,9-dimethyl-2-ethylfluorene (DMEF) model com-pound (� � 13,800).

The absorptivities of linear PDMVF decreaseabout 12% when DPn increases from 12 to 18 butdecrease little at higher DPn’s. The large hypo-chromic effects for PDMVF cycles suggest thepresence of stronger chromophore stacking in themore congested higher MW PDMVF rings.

For example, isotactic PS shows well-knownbut modest hypochromism (ca. 10%).44 Larger hy-pochromic effects (�50%) have been observed es-pecially for high-MW linear vinyl polymers withlarge pendent �-chromophores such as poly(N-vinylcarbazole).45 It has also been found that thehypochromism varies with the solvent polariz-ability and to a lesser extent with the addition ofa nonsolvent, which leads to more � stacking ofpendent chromophores.45

Similar but weaker absorptivity changes havebeen observed for macrocyclic P2VN. Thus, theabsorption spectra of linear and macrocyclic DBX-coupled P2VN in the near-UV closely follow thatof 2-ethylnaphthalene (2EN) (Fig. 11).46 The S2absorption intensity of macrocyclic P2VN of thesame mass, when corrected for the presence of

1,4-benzylidene, is approximately 5% lower thanthat of the linear polymer, whereas the S1 absorp-tivities are slightly higher. These differences pre-sumably are due to intensity borrowing as welland suggest the occurrence of aromatic stacking.

This indicates significant conformationalchanges with MW in the PDMVF and P2VN ringsand, to a lesser extent, in the linear polymers.This is also suggested by the much higher Tg’s ofthe macrocycles compared with those of thematching linear polymers.

Fluorescence

As shown in Figure 12, cyclic and linear PDMVFswith a DPn of 18 (Table 2, run 2) exhibit strongand characteristic fluorene (monomer) emissionsat 311 and 322 nm [excitation wavelength (�ex)� 307 nm]. There is a weaker emission between360 and 370 nm that is likely due to excimerformation, which is consistent with the absence ofa long red tail in the fluorescence spectrum of theDMEF model.47

Like the absorption spectra, the shapes and peakpositions of the monomer emission bands of cyclicand linear PDMVF are the same. However, aftercorrection for absorptivity differences, the emissionintensities of the macrocycles at 311 and 322 nm areincreased by 19 and 16%, respectively, in compari-

Figure 10. � of the DMEF model compound (at 306 nm) and cyclic and linear PDMVF(at 307 nm) as a function of DPn (the trend lines are guides for the reader).


son with the matching linear polymers. The totalemission quantum yield of the DPn � 18 macrocycleis the same as the that of DMEF model, but alllinear polymers have lower emission yields. An en-

hanced monomer emission has also been observedfor cyclic PS11 and cyclic P2VN.46,36

In comparison with the linear polymer, the DP� 18 cycle shows a decrease in the excimer band,

Figure 11. Absorption spectra of (a) linear P2VN (50 mg/L) and (b) macrocyclic P2VN(53 mg/L) with DPn � 12 (Table 1, run 2) and (c) 2EN (50 mg/L) in 90/10 (v/v)cyclohexane/THF (1-cm cell).

Figure 12. Fluorescence emission spectra of (- - -) the DMEF model compound (�ex

� 306 nm) and ( � � � ) cyclic and (—) linear PDMVF (DPn � 18, �ex � 307 nm). Theconcentration was 5 mg/L in cyclohexane. The counts were normalized by the opticaldensity at �ex.


as shown in Figure 12 (inset). Although excimeremissions increase with DPs, this is the case forall cycles. This appears not to be due to aggrega-tion, as the ratios of monomer and excimer bandsare concentration-independent. The decrease inthe monomer emission and the concomitant in-crease in the excimer emission observed for bothcyclic and linear polymers with increasing MWare consistent with the increased number of exci-mer traps available on a given chain or cycle.

The formation of excimer sites on the macro-cycles may be hindered by the bond angles andtorsional strains expected for the low-MW cyclicPDMVF. This is consistent with the much higherTg’s of the cyclic in comparison with those of thematching linear PDMVF. However, low-MW lin-ear polymers also have been shown to havesmaller excimer site densities, presumably be-cause of end-group entropic effects consistentwith their lower Tg’s.48

This work was supported in part by the National Sci-ence Foundation (DMR 9810283 and STC 594-608) andthe Loker Hydrocarbon Research Institute. The au-thors thank W. Weber for the use of the thermogravi-metric analysis and differential scanning calorimetryinstrumentation in his laboratory at the University ofSouthern California. They also acknowledge the MassSpec Facility at the University of California at River-side for its help with the matrix-assisted laser desorp-tion/ionization time-of-flight mass spectrometry.

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synthesis and thermal and spectroscopic properties of macrocyclic vinyl aromatic polymers

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