Well-Defined Condensation Polymers with Narrow Polydispersity via

Article pubs.acs.org/Macromolecules

Well-Defined Condensation Polymers with Narrow Polydispersity via Unsymmetrical Ladderphanes by Sequential Polymerization Yuan-Zhen Ke,†,‡,§ Ren-Jie Ji,†,‡,§ Te-Chung Wei,†,§ Shern-Long Lee,† Shou-Ling Huang,† Min-Jie Huang,† Chun-hsien Chen,† and Tien-Yau Luh*,† †

Department of Chemistry, National Taiwan University, Taipei, Taiwan 106 Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai, China 200032

‡

S Supporting Information *

ABSTRACT: Sequential polymerizations for the synthesis of three different kinds of functional polymers with well-defined degree of polymerization and narrow polydispersity are described. Both conjugated and nonconjugated functional polymers are obtained selectively. Monomers are designed to have an ester function connecting the first polymerizable norbornene moiety and the second polymerizable module. ROMPs of the norbornene moiety with the first generation Grubbs catalyst yield the template polymers where the molecular weights and their distributions are well-controlled. The presence of Narylpyrrolidene ring fused at 5,6-endo positions of the norbornene moiety is essential to direct the stereoselectivity of the polymerization. The second polymerizable groups attached to such aryl pendants via an ester linkage then proceed further polymerization leading to unsymmetrical double stranded ladderphanes. Hydrolytic cleavages of these ester-linked ladderphanes yield the corresponding daughter polymers that can be easily separated from the polynorbornene template containing carboxylic acid functions. Thus, cross metathesis of divinylarene pendants provides a convenient route for the synthesis of m-arylenevinylene copolymers with narrow polydispersity and well-defined chain lengths. Moreover, the hydroxyl groups in each of the monomeric units in the daughter polymer would offer possibility for further modifications. Accordingly, highly fluorescent conjugated benzofuranylene-ethynylene copolymers are obtained. Unsymmetrical ladderphanes containing polynorbornene as one strand and butadiynylene-para-phenylene as the other are isolated. Hydrolytic cleavage of these ladderphanes yields orthohydroxy-substituted phenylene-butadiynylene intermediate, which would undergo intramolecular annulation to afford benzofuranylene-ethynylene copolymers. Its STM image offers a direct evidence for the formation of unsymmetrical double stranded ladderphanes. In addition, Claisen condensation reaction of arylene-bisacetate has been used to construct nonconjugated alt-poly(acetonylene-m-arylene)s in a controlled manner.

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INTRODUCTION Unlike biosynthesis of proteins and nucleic acids, where the sequence is well-defined and single polydispersity is obtained, chemical synthesis of polymers of similar caliber remains to be one of the most challenging subjects in polymer chemistry. Living polymerizations,1−6 such as anionic,2 cationic,3 radical4 polymerizations, ring-opening polymerizations of heterocycles,5 and metathesis polymerization (ROMP) of strained cyclic alkenes,6 as well as chain growth polymerization7−13 with specially designed substrates or reactions offer useful entries for the construction of narrowly dispersed polymers. The use of template-assisted polymer synthesis can furnish a versatile entry for the design and synthesis of complementary daughter polymers by replication.14−19 Hydrogen bonding14−18 has been used to fasten template and monomers. Specially designed donor and acceptor moieties for hydrogen bonding, however, are necessary for appropriate interactions between templates and substrates. An alternative approach uses a covalent bond such as an ester linkage to connect the polymeric template with small molecules for polymerization.19 The daughter polymer can be obtained selectively by hydrolytic cleavage of the ester groups. The two polymers (template and daughter) can be © XXXX American Chemical Society

easily separated because one contains carboxylic acid functions and the other is neutral. To illustrate this, daughter polynorbornene having same degree of polymerization and polydispersity is obtained from the replication of a polynorbornene template.19a The important aspect of this template directed synthesis requires the accessibility of the template polymer with well-defined sequence, degree of polymerization, and polydispersity. The template polymer could be generated by a living polymerization. It is therefore envisaged that a monomeric species 1 containing two different polymerizable groups connected by an appropriate liner could undergo sequential polymerizations under two different conditions to afford 2 and then an unsymmetrical double stranded ladderphane 3 (Scheme 1). If the first polymerization reaction is a living polymerization, the chain length and polydispersity of this single stranded polymer 2 could be controlled. The second polymerization can thus occur to give 3 in a controlled manner. After cleavage, two strands of 4 and 5 could be separated, and Received: June 15, 2013 Revised: August 6, 2013

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dx.doi.org/10.1021/ma4012363 | Macromolecules XXXX, XXX, XXX−XXX

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be eclipsed to each other but spread like a peacock tail.20 As such, the span for each of the monomeric units in the second strand of a polynorbornene-based unsymmetrical ladderphane or the like could be allowed to be somewhat larger than the 5.5 Å mentioned. alt-Poly(m-phenylene-vinylene)s. The chemistry and photophysical properties of poly(p-phenylene-vinylene)s and related conjugated polymers have laid a foundation for modern optoelectronic materials research.24 Incorporation of m-phenylene-vinylene moieties into polymers has been shown to interrupt π-conjugation so that the wavelength of emission can be tuned,25−28 and poly(m-phenylene-vinylene) is blueemitting.26f In general, the polydispersities of these polymers by condensation polymerization are fairly large.28 We envisioned that sequential polymerization might offer a useful entry for the synthesis of these interesting polymers in a controlled manner. Our strategy starts with monomer 9 containing a norbornene linked with a divinylarene moiety. Since ROMP will be used for the first polymerization leading to polynorbornene 10, the vinyl groups on the aromatic ring in 9, designed for the second polymerization, will be protected with the trimethylsilyl group so that they would remain intact in the first ROMP step. The double stranded ladderphane 11 could be obtained by desilylation followed by cross metathesis. The span for each of the monomeric species in the poly(m-phenylene-vinylene) strand would be around 6 Å, which could fit nicely into the spacing defined by the vinylcyclopentane moiety in polynorbornene scaffold in 11.20 Monomer 929 was treated with different amounts of 7a followed by quenching with 3Z-hexene and afforded polynorbornene 10a−c. It is noteworthy that the use of 3Z-hexene as the terminating agent would give ethylvinyl moiety at the end group because this end group is a disubstituted alkene that may be less reactive than the terminal vinyl moiety used for the second cross metathesis reaction. Desilylation of 10a−c with trifluoroacetic acid followed by cross metathesis with the second generation Grubbs catalyst 7b21b−e furnished the corresponding double stranded ladderphane 11a−c in 46−52% yield. Base promoted hydrolysis of 11a−c gave the poly(m-phenylene-vinylene)s 12a−c in 47− 58% yield (Scheme 2). The results are outlined in Table 1. Polymer 14 (Mn = 6600, PDI = 2.69) was also synthesized by direct cross metathesis of the 13 with 7b for comparison. The signals for olefinic protons in 1H NMR spectra of 12a−c and 14 appear at ca. δ 7.4 ppm, whereas the trans-olefinic protons of stilbene 1529 exhibit resonance at δ 7.47 ppm. These observations suggest that the double bonds in 12a−c and 14 should also be in trans configuration.28 As shown in Table 1, polymers 12 obtained from sequential polymerization protocol were well-controlled, whereas relatively large PDI was found in 14 and related polymers.28 Polymers 11a and 12a exhibit almost identical emission profiles (Figure 1), but the quantum yield for the latter is almost double that for the former. Because the aminobenzoate moiety also absorbs at 313 nm, the absorptivity for 11a is somewhat higher than that of hydrolyzed polymer 12a in the same region (Figure 1). These photophysical properties are comparable with those of related poly(m-phenylene-vinylene)s, besides the half-width of the emission for 12a (62 nm) was somewhat narrower than those of literature spectra (72 nm).28 Presumably, the narrow PDI for 12a might be responsible for decreasing the half-width of the emission profile.

Scheme 1. Strategy for Sequential Polymerization

the degree of polymerization and the distribution of the molecular weights of 4 and 5 could be well-defined. In this paper, we wish to adopt this strategy by a systematic synthesis of different kinds of polymers, conjugated or nonconjugated, with controlled molecular weights and distributions.

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RESULTS AND DISCUSSION Strategy. Metal-carbene-catalyzed ROMP of norbornene derivatives are well-known to be living polymerization.6 Polynorbornene skeletons are relatively rigid and could serve as a supporting framework for the pending groups coherently aligned toward the same direction.20 In particular, the presence of endo-fused N-arylpyrrolidene moieties in polymers 6 is crucial for the stereoselective formation of the nearly twodimensional comb-like polymers with isotactic stereochemistry and all double bonds in trans configuration, when the first generation Grubbs catalyst 7a21a is used.20 Double stranded ladderphanes, 8, are thus obtained conveniently.22,23 It is therefore obvious that a norbornene fused with endo-Narylpyrrolidene moiety will be chosen as one of the polymerizable groups for the first polymerization reaction. The aryl pendants in these polynorbornenes offer a convenient access connecting to the second polymerizable functionality. The second polymerization can be any kind of condensation reactions of difunctional moieties, as long as the span for each of the monomeric unit in the second strand can fit into the spacing (ca. 5.5 Å) occupied by each of vinylcyclopentane moiety in the polynorbornene strand.20 It is noteworthy that the polymeric backbone and the ester linkages in 6 and 8 can be, to a certain extent, flexible, and the aryl pendants in this single stranded polynorbornene can rotate along the C−N and C−CO single bonds. The pendants in these polymers may not B


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Scheme 2. Sequential Polymerization of 9 Followed by Hydrolysis Giving alt-Poly(m-phenylene-vinylene)s 12

Figure 1. Absorption (dashed) and emission (solid) of 11 (red, 4 × 10−5 M), 12 (blue, 6 × 10−6 M) in THF.

It is envisaged that the corresponding para analogue 1829 might also proceed via a similar reaction sequence. The phenolic ester 18 was chosen not only because of synthetic convenience but also because the possible phenoxide moieties in intermediate 21, which may be formed from the hydrolysis of the unsymmetrical ladderphane 20, could undergo annulations to in situ generate the benzofuran-incorporated conjugated polymer 22.30 Treatment of 18 with 10 mol % of 7a in DCM gave 90% yield of 19a (Mn = 7400, PDI = 1.18). In a similar manner, 19b (Mn = 10 700, PDI = 1.29) was obtained in 92% yield, when 6 mol % of 7a was used (Scheme 3). The results are summarized in Table 2. Both 19a and 19b exhibit 13C NMR signals at δ 0.1 ppm due to the trimethylsilyl group and at δ 100 and 101 ppm attributed to the silyl-subsituted alkynyl carbons. In addition, the signal for the olefinic carbons shifts from δ 135 ppm in 18 to δ 132 ppm in 19. Unsymmetrical ladderphanes 20a (Mn = 5000, PDI = 1.28) and 20b (Mn = 9100, PDI = 1.33) were obtained in 85 and 82% yield from 19a and 19b, respectively, upon treatment with excess of Cu(OAc)2 in the presence of TBAF (Table 2). The disappearance of the trimethylsilyl substituents in 19a in couple with the shifts of alkynyl carbons from around δ 100 ppm to three broad peaks at δ79, 83, and 84 ppm suggests the formation of possible nonequivalent conjugated diyne moieties in 20a.19b,31 Hydrolysis of 20 with 10% aqueous NaOH in THF in the presence of 1.2 equiv of TBAF gave the corresponding polyphenoxide intermediate 21, which would undergo intramolecular nucleophilic addition to the ortho triple bond leading to benzofuran-containing polymer 22. The 1H NMR spectrum of 22a is shown in Figure 2a. The complicated signals in the

a

Reaction conditions: 7a, DCM, then EtOCHCH2, 90−92%. bi. TFA, DCM; ii. 7b, DCM, then EtOCHCH2, 58−65%. cBu4NBr, NaOH, DCM/MeOH, 47−58%.

Table 1. Properties of Polynorbornenes 10, Unsymmetrical Ladderphanes 11 and Alt-poly(m-phenylene-vinylene)s 12 substrate

Mna

PDIa

DPb

10a 11a 12a 10b 11b 12b 10c 11c 12c

8300 6600 3400 11 000 7800 4600 18 700 14 200 7300

1.17 1.33 1.22 1.16 1.35 1.17 1.12 1.30 1.22

11 12 11 15 14 15 26 26 23

λmax nm 251, 310, 308, 251, 311, 308, 251, 312, 308,

313 341(sh) 356(sh) 313 342(sh) 356(sh) 313 342(sh) 357(sh)

λem nm

Φc

376 406 409 376 406 410 377 407 409

0.06 0.21 0.41 0.06 0.23 0.42 0.08 0.28 0.48

a

Obtained by gel permeation chromatography (GPC). bDegree of polymerization by GPC. cMeasured in EtOAc using coumarin I as the reference (Φ = 0.99).

alt-Poly(benzofuranylene-ethynylene)s. The successful results described are concerned with the synthesis of polymers containing interruption of conjugations via meta-linkages of aromatic moieties. In this section, conjugated polymers by this sequential polymerization protocol are described. In our preliminary communication, alt-poly(m-phenylenebutadiynylene)s 17 with well-defined chain lengths and polydispersity are obtained by sequential ROMP and Glaser oxidation of 16 followed by hydrolysis (eq 1).19b C


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signals around δ 6.7 and 3.3 ppm in 1H NMR spectrum may be attributed to the benzofuran and terminal alkyne end groups, respectively. The results are also tabulated in Table 2. The degrees of polymerization of 22a and 22b based on GPC were 10 and 15, respectively, which were consistent with those of the corresponding original template polymers 19a and 19b and the double-stranded ladderphanes 20a and 20b. These results indicated that the chain lengths of daughter polymers 22 could again be well controlled through template process. The PDI’s of 22 were comparable with those of corresponding 19 and 20. The two ethynyl groups on the aryl pendants in 18 are nonequivalent. They are designated as h for the alkynyl moiety ortho to the phenolic ester function and t for that meta to the same ester group. Glaser oxidative polymerization might proceed unselectively between the two alkynyl groups after in situ desilylation. As shown in Scheme 2, 20 might contain a combination of three possible regiochemically distinct diads along the daughter strand: 23a (h−h), 23b (t−t), and 23c (h− t). Upon hydrolysis and annulation, they would give three different types of alkynylbenzofuran fragments 24a−c (Scheme 4). This may account for different chemical shifts of the alkynyl carbons in 20 mentioned and for the complex 1H NMR signals in the aromatic region in 22a. In order to establish that polymer 22 may contain fragments 24a−c, polymers 25 and 26 having structural fragments similar to those of 24a, 24b, and 24c, respectively, were synthesized.29 In addition, random copolymer 29 (Mn = 12,100, PDI = 2.42) was synthesized from 27 via 28 (Scheme 5).29 The 1H NMR spectra of 25, 26, and 29 are also shown in Figure 2b−d for comparison. It is interesting to note that the summation of the spectra of 25 and 26 appears to be similar to those of 22a and 29. The 13C NMR spectra of 22a, 25, 26, and 29 are shown in Figure 3. The internal alkynyl carbons in 25 exhibit 13C NMR signals at δ 79 and 80 ppm, whereas the internal alkynyl carbons in 26 exhibit 13C NMR signals at δ 84 and 94 ppm. The presence of signals at δ 79, 80, 83, and 94 ppm in both 22a and 29 again indicated a combination of the benzofuran fragments 24a, 24b, and 24c in the polymeric chain. These results support the assumption that the desilylative Glaser oxidation of 19 may lead to three different kinds of regiochemically distinct diads 23a−c in the daughter strand in 20. The formation of diyne moiety 23c is somewhat striking. The two alkynyl substituents (h and t) on two adjacent aryl pendants are directing toward different directions. The diyne moiety in 23c would be relatively more strained than the other analogues 23a and 23b. The relative ratio of such h−t linkage to h−h and t−t couplings was estimated by a detailed analysis of the 1H NMR spectrum of 22. As shown in Figure 2b, the protons on the furan rings in 26 would appear characteristically at δ 6.9 ppm, while the other aromatic protons in 26 exhibit signals at δ 7.0−7.6 ppm. On the other hand, all aromatic protons including those on furan rings in 25 show peaks at lower field than δ 7.0 ppm (Figure 2c). It seems likely that the signal around δ 6.9 in 22a and 29 may arise from the presence of the fragment such as 26 (Figure 2a and d). Accordingly, by integration of 1H NMR signals at δ6.9 ppm versus all aromatic protons, the ratios of fragments 24c versus 24a and 24b were estimated to be around 33% for 22a and 50% for 29. These results suggest that, for every three Glaser reaction of 19, one of the coupling fragments may proceed the h to t linkage, while the rest may undergo h to h and t to t couplings, whereas the

Scheme 3. Sequential Polymerization of 18 Followed by Hydrolysis Giving alt-Poly(benzofuranylene-ethynylene)s 22

a

Reaction conditions: 7a, DCM, then EtOCHCH2, 90−92%. Cu(OAc)2, pyridine/DCM, TBAF, 82−85%. cNaOH, TBAF, DCM/MeOH, 67−74%.

b

Table 2. Properties of 19, 20, 21, and Related Polymers substrate

Mna

PDIa

DPb

DP′b

19a 20a 22a 19b 20b 22b 25 26

7400 5000 3400 10 700 9100 5000 7200 5000

1.18 1.28 1.22 1.29 1.33 1.37 2.25 1.71

10 9 10 15 16 15 22 15

9 9 15 15 22 14

a

1

Obtained by GPC. bDegree of polymerization (DP, by GPC; DP′, by H NMR).

aromatic region (δ 6.9−7.7 ppm) may suggest that the environment around the aromatic rings in 22a might not be equivalent. In other words, there would be more than one kind of aromatic rings in 22a. As mentioned, different diyne moieties might be incorporated in 20. Upon annulation, different kinds of benzofuran heterocycles would be expected. The small D


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Figure 2. 1H NMR spectra of (a) 22a, (b) 26, (c) 25, and (d) 29.

Scheme 4. Possible Fragments of Glaser Oxidation of 19 and Their Annulation Products

Scheme 5. Synthesis of 28 and 29

a

Reaction conditions: Cu(OAc)2, pyridine, 90%. bNaOH, 83%.

would be much larger than the space occupied by each of the vinylcyclopentane monomeric unit (ca. 5.5 Å) in polynorbornene scaffold. In order to fit into the double stranded nature of 20, it seems likely that the daughter strand might adopt a zigzag conformation. In this regard, h to t coupling may become feasible. Interestingly, the STM image of 20a (see below) appears to be consistent with this point. The alkynyl carbons in 28 exhibit 13C NMR signals around δ 78−80 ppm. It is noteworthy that 13C NMR chemical shifts of twisted alkynyl carbons, in general, appears at few ppm downfiled than those of the unstrained ones.31,32 The presence of signals at δ 83 and 84 ppm in 20a may imply that certain alkynyl carbons in the daughter strand could be somewhat

random polymerization of 27 gave a one to one ratio of h to t versus h to h and t to t couplings. The span for each of arylbutadiyne moiety in the poly(butadiynylene-p-arylene) daughter strand (ca. 8.5 Å) E


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Figure 3. 13C NMR spectra of (a) 22a, (b) 25, (c) 26, and (d) 29.

from the HOPG surface, and it turns out that these fragments are arranged in a regular manner. The STM image seems to be consistent with the NMR results described in the previous paragraphs. Photophysical Properties. Furan-containing oligoaryls are known to be highly fluorescent and good hole transporting materials.33 Small molecules and conjugated polymers based on benzofuran chromophores have been used for electroluminescent devices,34 solar cell applications,35 and field effect transistors.36 As shown in Figure 5 and Table 3, alt-

strained. This suggestion seems to be coherent with possible zigzag conformation for this daughter strand, as proposed above. STM Image. The structure of double-stranded polymer 20a was studied by scanning tunneling microscopy (STM). As shown in Figure 4, 20a exhibited well-aligned self-organization

Figure 4. STM image of 20a on highly oriented pyrolytic graphite (HOPG). Conditions: Ebias, 0.70 V; itunneling, 40 pA; image size, 22 × 22 nm2. Figure 5. Absorption (dashed) and emission (solid) of 22a (blue), 25 (red), and 26 (black) in CH2Cl2 (2 × 10−5 M). The molar absorptivities were based on the molecular weight of the corresponding monomeric unit.

on the highly oriented pyrolytic graphite (HOPG). The stripes have a uniform width of around 3.1 nm, consistent with the estimated structural feature of 20a. The STM image of 20a shows that bright short rods are arranged regularly on the left side of each stripe. The average span of these bright rods plus the immediate adjacent dimmed region was about 1.6 nm. The span for each of vinylcyclopentane monomeric units is known to be around 5.5 Å.20 Accordingly, each repeat (bright rod plus dimmed region) on the STM image may contain three such spacings or four arylene-butadiynylene moieties. It is particularly noteworthy that each bright rod appears to consist of three parallel distinct bright spots, presumably, due to three tilted arylene moieties on the daughter strand in 20a. The butadiynlyene modules could, however, be away from the substrate plane. The fourth arylene module may also be away from the HOPG surface, resulting in dimmed region on the STM image. As mentioned, the substituted poly(p-arylenebutadinylene) strand in 20a might have zigzag conformation in order to accommodate into the double stranded structure. In addition, there would be one h to t linkage in every three diyne diads. Hence, there might be fragments that would be away

Table 3. Photophysical Properties of altPoly(benzofuranylene-ethynylene)s and Related Polymers substrate 19a 20a 22a 19b 20b 22b 25 26 29

λmax, nm

λem, nm

285, 320, 427 285, 322, 426 423 420 420

361, 447, 470, 361, 450, 471, 472, 465, 454

323 397 323 400

446 479 497 446 486 498 502 493

Φa 0.04b 0.10b 0.77 0.03b 0.12b 0.78 0.78 0.76 0.44

a

Quantum yield (Φ) was measured in DCM using coumarin 153 as the reference (Φ = 0.53 in EtOH). bUsing coumarin I as the reference (Φ = 0.99 in EtOAc). F


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solubility of the polymers formed by the sequential polymerization protocol. ROMP of 33 with different amounts of 7a afforded the corresponding polynorbornenes 34a−c in excellent yields. The results are summarized in Table 4.

poly(benzofuranylene-ethynylene)s 22, 25, and 26 are also highly fluorescent. It is interesting to note that the photophysical properties of these polymers are similar, no matter how benzofuran moieties are connected to carbon−carbon triple bonds. alt-Poly(acetonylene-m-arylene)s. Incorporation of ketone moieties in a polymeric backbone has received much attention, because the carbonyl groups can readily be transformed into other functionalities so that polymers of different fascinating structural varieties can be accessed.37,38 Most of these carbonyl-containing polymers (e.g., 30) are obtained by transition metal-catalyzed copolymerization of αolefins and carbon monoxide.37 Incidentally, 30 can be considered as a substituted polyacetonylene. To the best of our knowledge, the incorporation of isolated acetonylene moieties in the backbone of an alternating polymer has not been explored, not to mention the controlled synthesis of these polymers. 1,3-Diarylacetones 31 have been shown to be useful precursors for the synthesis of graphene-like polynulcear aromatic compounds and polymers.39 Apparently, oligomers or polymers derived from acetonylene-arylenes would be useful for the construction of oligoaryls in a selective manner. Diaryl acetones 31 can be readily obtained from the Claisen condensation of two arylacetates followed by hydrolytic decarboxylation,40 although such condensation protocol has not been used for polymer synthesis. Our initial effort was to prepare arylene diacetate 32a, which was used for the synthesis of monomer 33 (Scheme 6).29 The presence of dodecyloxy substituent on the aromatic ring in 33 would enhance the

Table 4. Properties of Polynorbornenes 34, Unsymmetrical Ladderphanes 35 and alt-Poly(acetonylene-arylene)s 36

a

polymer

Mna

PDIa

DPb

34a 35a 36a 34b 35b 36b 34c 35c 36c

4900 4100 2400 8300 7400 3800 10 400 9400 4700

1.11 1.33 1.33 1.12 1.27 1.42 1.18 1.36 1.41

7 7 7 12 12 11 15 15 13

Obtained by GPC. bDegree of polymerization by GPC.

Treatment of 34 with an excess amount isopropyl Grignard reagent gave the corresponding Claisen condensation products 35a−c.40b The completeness of the reaction was monitored by the relative integration of the total methylene groups of ethoxycarbonyl moieties (δ 4.8−5.0 ppm) versus the benzylic methine proton (δ 4.0−4.4 ppm) after condensation. Depending on the chain length of the polymer, or more precisely, the degree of polymerization of the polynorbornene backbone formed by ROMP, the theoretical ratios for complete condensation were calculated. Repetitive Claisen condensation reactions were occasionally necessary to ensure complete coupling between two acetate moieties in the adjacent monomeric units in 34 until the ratio was within 5% of the theoretical ratio.29 The results are also outlined in Table 4. It is interesting to note that the spacing occupied by a phenylacetonyl moiety (equivalent to a monomeric unit in the daughter strand of 35) would be larger than 5.5 Å. Rotation of arylene rings and carbon−carbon σ-bonds in acetonylene moieties in 35 may fit into the space defined by the vinylcyclopentane monomeric repeat in 35. Treatment of 35a−c with sodium hydroxide in the presence of Bu4NBr in THF, MeOH, and DCM mixed solvent under refluxing conditions for five days afforded a yellowish oily product which was taken up in 6 N hydrochloric acid and acetic acid (1:1) under reflux for 12 h to give 36a−c in 48−66% yield. 1 H NMR spectra of 36a−c show a broad signal at δ 4.60−4.70 ppm attributed to the hydroxymethyl group and a broad peak at δ 3.70−3.90 ppm due to the methylene group from acetonylene moieties in the backbone and α-protons in dodecyloxy substituents. The results are also tabulated in Table 4. It is noteworthy that the degrees of polymerization for 36a−c were comparable to those of respective unsymmetrical double stranded ladderphanes 35a−c and to those of single stranded polynorbornenes 34a−c. Direct condensation of 32b in the presence of isopropyl Grignard reagent followed by hydrolytic decarboxylation gave 36d. Gel permeation chromatographic (GPC) analysis (Mn = 2800) suggests the 36d would be an octamer with relatively large polydispersity (PDI = 2.16). Attempts to get higher molecular weights of polymer 36d from this direct condensation, however, were unsuccessful. These results suggest that the sequential polymerization protocol offers a useful route for the selective synthesis of alt-poly(acetonylene-

Scheme 6. Sequential Polymerization of 33 Followed by Hydrolysis Giving alt-Poly(acetonylene-m-arylene) 36

a

Reaction conditions: 7a, DCM, then EtOCHCH2, 87−92%. PrMgBr, THF, 22−32%. ci. Bu4NBr, NaOH, DCM/THF, ii. HCl (6 N), HOAc, 48−66%.

bi

G


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46.4, 49.8, 61.1, 64.0, 74.5, 110.9, 116.1, 125.7, 127.9, 128.0, 128.4, 131.1, 136.9, 150.2, 154.4, 167.0, 170.2; GPC (THF) Mn = 4900, PDI = 1.11. 34b: (87%), GPC Mn = 8300, PDI = 1.12. 34c: (92%), GPC Mn = 10400, PDI = 1.18. Double Stranded Ladderphanes 35a−c. A mixture of Mg (0.77g, 32 mmol) and isopropyl bromide (2.9 mL, 3.8 g, 30.9 mmol) in THF (20 mL) was stirred for 2 h under nitrogen to give isopropylmagnesium bromide (1.34 M). Polymer 34 (500 mg, 0.71 mmol) in THF (255 mL) was added to the Grignard reagent (6 mL, 8 mmol) chilled in an ice-bath, and the mixture was stirred overnight under nitrogen. Water was added, and the mixture was extracted with DCM (200 mL). The organic layer was washed with saturated NH4Cl (50 mL), brined (50 mL), dried (MgSO4), and concentrated in vacuo. The residue was taken into DCM, and the solution was added to methanol. Solid was collected and washed with Et2O. This procedure was repeated 3−7 times to afford 35. 35a (143 mg, 32%): IR (KBr) ν 3056, 2981, 2851, 1731, 1606, 1524, 1471, 1433, 1369, 1293, 1180, 1148, 1103, 1033, 975, 909, 827, 767, 705, 584, 465 cm−1; 1H NMR (400 MHz, CDCl3) δ 0.30−2.00 (br, 31 H), 2.50−3.30 (br, 8 H), 3.40−3.90 (br, 5 H), 3.90−4.40 (br, 3 H), 4.70−4.90 (br, 1 H), 5.00−5.60 (br, 4 H), 6.30−6.70 (br, 2 H), 7.30−7.60 (br, 2 H), 7.70−8.10 (br, 2 H); 13C NMR (100 MHz, CDCl3) δ 14.57, 14.58, 23.1, 26.6, 27.0, 29.80, 29.88, 30.1, 30.7, 32.2, 36.1, 37.9, 41.8, 45.0, 46.9, 50.2, 58.2, 61.08, 61.14, 64.2, 74.9, 111.6, 117.3, 125.5, 127.7, 129.9, 130.1, 131.3, 138.0, 150.2, 154.3, 167.1, 170.2, 200.0; GPC (THF) Mn = 4100, PDI = 1.33. 35b (23%): GPC Mn = 7350, PDI = 1.27. 35c (22%): GPC Mn = 9400, PDI = 1.36. alt-Poly(acetonylene-m-phenylene)s 36a−c. To a mixtue of 35a (100 mg) and n-Bu4NBr (1.6 mg, 0.005 mmol) in THF (20 mL) and DCM (5 mL) was added 10% NaOH (8 mL) and methanol (5 mL). The mixture was refluxed for 5 days under nitrogen and then cooled to room temperature (rt). Water was added, the aqueous layer was acidified with 6 N HCl, and the mixture was extracted with EtOAc 5 times. The organic layer was dried (MgSO4), and the solvent was removed in vacuo to give the residue as a light oil, which was then added to 6 N HCl solution (10 mL) and acetic acid (10 mL). The mixture was refluxed overnight under nitrogen, cooled to rt, and extracted with DCM (100 mL × 3). The organic layer was washed with water (20 mL x 3), dried (MgSO4) and concentrated in vacuo to give the residue, which was redissolved in Et2O, and precipitated by adding to pentane to afford 36a (38 mg, 66%): IR (KBr) ν 3220, 2914, 2850, 1709, 1536, 1470, 1442, 1391, 1346, 1246, 1193, 1061, 985, 947, 911, 889, 845, 790, 704, 632, 548, 511 cm−1; 1H NMR (400 MHz, CDCl3) δ 0.80−1.00 (br, 3 H), 1.10−1.40 (br, 16 H), 1.40−1.50 (br, 2 H), 1.70−1.90 (br, 2 H), 3.60−3.90 (br, 6 H), 4.50−4.70 (br, 2 H), 7.30−7.40 (br, 2 H); 13C NMR (100 MHz, CDCl3) δ 14.4, 23.0, 26.2, 29.6, 29.77, 29.87, 29.90, 29.93, 30.4, 32.2, 35.2, 43.7, 63.0, 74.4, 127.9, 128.5, 137.0, 154.5, 177.2, 205.3; GPC Mn = 2350, PDI = 1.33. 36b (48%): GPC Mn = 3800, PDI = 1.42. 36c (52%): GPC Mn = 4,700, PDI = 1.41. alt-Poly(acetonylene-m-phenylene)s 36d. In a manner similar to that described for 35, under nitrogen, a THF solution (50 mL) of 32b (0.5 g, 0.86 mmol) was added to isopropylmagnesium bromide (1.34 M, 6.6 mL, 9 mmol) at 0 °C, and the mixture was stirred at rt overnight. Water was added, followed by workup to give a crude polymer that was treated with 6 N HCl solution (50 mL) and acetic acid (50 mL) to afford 36d (0.26 g, 0.72 mmol, 84%): IR (KBr) ν 3215, 2915, 2850, 1709, 1536, 1470, 1442, 1391, 1347, 1246, 1193, 1061, 985, 947, 911, 890, 845, 780, 704, 632, 548, 511; 1H NMR (400 MHz, CDCl3) δ 0.80−1.00 (br, 3 H), 1.10−1.50 (br, 18 H), 1.70− 1.90 (br, 2 H), 3.70−3.90 (br, 6 H), 4.60−4.70 (br, 2 H), 7.30−7.40 (br, 2 H); 13C NMR (100 MHz, CDCl3) δ 14.4, 23.0, 26.2, 29.8, 29.86, 29.90, 29.92, 30.4, 32.2, 35.2, 43.7, 63.0, 74.4, 127.9, 128.5, 137.0, 154.5, 177.3, 205.3; GPC Mn = 2800, PDI = 2.16. Single Stranded Polynorbornene (10). A mixture of 929 (300 mg, 0.4 mmol) and 7a (28 mg for 10a, 22 mg for 10b, 14 mg for 10c) in DCM (15 mL) under nitrogen was stirred for 1.5 h. Part of the solvent was removed under reduced pressure to a final volume about 3

arylene)s in a controlled manner leading to well-defined degree of polymerization and narrow polydispersity.

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CONCLUSION In summary, we have demonstrated the use of sequential polymerizations for the synthesis of three different kinds of polymers with well-defined degree of polymerization and narrow polydispersity. Both conjugated and nonconjugated functional polymers are obtained selectively. Monomers containing both norbornene moiety and the second polymerizable groups that are connected by ester linkages have been employed. ROMPs of a norbornene moiety have offered a unique entry for the synthesis of the parent template scaffold where the molecular weights are well-controlled. The presence of N-arylpyrrolidene fused at 5,6-endo positions of the norbornene appears to be essential to direct the stereoselectivity of the polymerization. The polymerizable groups attached to such aryl pendants via an ester linkage then proceeded second polymerization leading to unsymmetrical double-stranded ladderphanes. Hydrolytic cleavage of the ester linkages in these ladderphanes yields the corresponding neutral daughter polymers, which can easily be separated from the polynorbornene template containing carboxylic acid functions. Cross metathesis of divinylarene pendants provides a convenient route for the synthesis of m-arylene-vinylene copolymers 12 with narrow polydispersity and controlled chain lengths. The daughter polymers in this study contain hydroxyl group in each of the monomeric units. Accordingly, this functional group could be used for further transformations. This strategy has indeed been used to produce highly fluorescent conjugated benzofuranylene-ethynylene copolymers 22 by intramolecular annulation processes. The STM image of 20a offers a direct evidence for the formation of unsymmetrical double-stranded ladderphanes. The present study suggests that the span for each of monomeric units in daughter polymers could range from 5.5 to 8.5 Å. Claisen condensation reaction of arylene-bisacetate has been employed to construct nonconjugated alt-poly(acetonylene-m-arylene)s 36 selectively. It seems likely that a range of condensation polymerizations could be conveniently used in these sequential polymerizations. The potentials emanating from this approach for the synthesis of other functional polymers with well-defined chain lengths and polydispersity abound. Further elaboration of this strategy for the synthesis of well-controlled functional polymers is in progress.

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EXPERIMENTAL SECTION

Single Stranded Polymer 34a−c. A mixture of 3329 (600 mg, 0.86 mmol) and 7a (15 mol % for 34a, 10 mol % for 34b, 5 mol % for 34c) in DCM (60 mL) was stirred for 2 h under nitrogen. Ethyl vinyl ether (2 mL) was then added, and stirring was continued for 20 min; then, the mixture was concentrated in vacuo. The precipitate was collected, redissolved in DCM, and reprecipitated by adding the DCM solution to methanol. The solid was washed with Et2O to afford 34 as a grayish powder (541−550 mg, 87−92%). 34a: (541 mg, 90%), IR (KBr) ν 3055, 2982, 2914, 2851, 1732, 1606, 1524, 1471, 1434, 1370, 1292, 1179, 1147, 1105, 1034, 973, 909, 861, 828, 768, 704, 584 cm−1; 1H NMR (400 MHz, CDCl3) δ 0.80− 1.00 (br, 3 H), 1.10−1.50 (br, 25 H), 1.70−1.90 (br, 3 H), 2.70−3.40 (br, 8 H), 3.50−3.70 (br, 4 H), 3.70−3.90 (br, 2 H), 4.10−4.30 (br, 4 H), 5.10−5.50 (br, 4 H), 6.50−6.60 (br, 2 H), 7.40−7.50 (br, 2 H), 7.90−8.10 (br, 2 H); 13C NMR (100 MHz, CDCl3) δ 14.6, 14.8, 23.0, 26.2, 29.6, 29.8, 29.90, 29.94, 29.95, 30.5, 32.2, 35.9, 36.4, 37.2, 44.9, H


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mL. cis-3-Hexene (10 equiv) was added, and stirring was continued for another 12 h at rt. The mixture was concentrated, and the residual solution was added to methanol. Repeated precipitation in DCM/ methanol twice afforded 10 as a grayish powder. 10a (276 mg, 92%): IR (KBr) ν 2927, 2926, 2854, 1703, 1606, 1523, 1479, 1447, 1378, 1270, 1179, 1137, 1096, 995, 969, 865, 839, 768, 731, 695, 632, 506 cm−1; 1H NMR (400 MHz, CDCl3) δ 0.17 (s, 18 H), 0.89 (m, 3 H), 1.28−1.80 (m, 24 H), 2.75−2.89 (m, 4 H), 3.27 (br, 4 H), 3.75 (br, 2 H), 5.26−5.37 (m, 4 H), 6.50 (d, J = 19.2 Hz, 2 H), 7.23 (d, J = 19.2 Hz, 2 H), 7.55 (s, 2 H), 7.92 (br, 2 H); 13C NMR (100 MHz, CDCl3) δ-0.7, 14.6, 23.1, 26.6, 26.9, 29.7, 30.0, 30.7, 32.3, 36.3, 37.0, 45.0, 46.7, 46.9, 49.8, 66.3, 75.4, 111.6, 117.2, 125.9, 126.1, 128.7, 131.3, 131.7, 132.0, 132.5, 138.0, 151.0, 154.3, 166.8; GPC (THF, polystyrene standard) Mn = 8300, PDI = 1.17. 10b (271 mg, 90%): GPC (THF, polystyrene standard) Mn = 11 000, PDI = 1.16. 10c (270 mg, 90%): GPC (THF, polystyrene standard) Mn = 18 700, PDI = 1.12. Double Stranded Ladderphane (11). Under nitrogen, to a solution of 10 (100 mg, 0.14 mmol [calculated based on the molecular weight of corresponding monomeric unit 9]) and DCM (10 mL) was added TFA (0.3 mL, 4 mmol). The resulting mixture was stirred for 30 min at rt. The mixture was concentrated, and the residue was added to methanol. Repeated precipitation in methanol afforded desilylated polymer as a gray powder (60−66 mg, 75−83%), which was used without further purification. Under nitrogen atmosphere, a mixture of the desilylated polymer (30 mg, 0.05 mmol) and 7b (3 mg, 0.003 mmol) in DCM (10 mL) was stirred at 40 °C for 1 h. The solution was concentrated under reduced pressure, and fresh DCM (10 mL) was then added. This cycle was repeated every 1 h in the first 5 h, and every 2 h in the remaining time. After 24 h, ethyl vinyl ether (2 mL) was added, and the mixture was stirred at rt for 1 h and concentrated to give the residue, which was added to methanol. The solid was collected, and this workup procedure was repeated 3 times to afford 11 as a white powder. 11a (38 mg, 65%): IR(KBr) ν 2924, 2852, 1706, 1605, 1522, 1479, 1453, 1376, 1269, 1178, 1139, 1096, 967, 828, 767, 721, 694 cm−1; 1H NMR (400 MHz, CDCl3) δ 0.87(m, 3 H), 1.26−1.80 (m, 22 H), 2.82−3.32 (m, 8 H), 3.75−3.78 (m, 2 H), 5.30 (m, 4 H), 6.53 (m, 2 H), 7.37−7.94 (m, 6 H). 13C NMR (100 MHz, CDCl3) δ 14.1, 22.7, 26.3, 29.3, 29.6, 30.2, 31.9, 45.9, 49.6, 65.8, 74.7, 111.6, 117.0, 124.2, 126.1, 126.6, 127.6, 128.7, 131.4, 137.6, 151.1, 155.0, 166.7. GPC Mn = 6600, PDI = 1.33. 11b (35 mg, 60%): GPC Mn = 7800, PDI = 1.35. 11c (34 mg, 58%): GPC Mn = 14 200, PDI = 1.30. alt-Poly{[3,5-(4-dodecyloxy-1-hydroxymethyl])phenylene]vinylene} (12). To a solution containing 11 [30 mg, 0.05 mmol (based on the molecular weight of the monomeric unit)] and [nBu4N]Br (1.6 mg, 0.005 mmol) in THF (10 mL) and DCM (5 mL) under nitrogen was added 10% NaOH solution (6 mL) and methanol (5 mL). The resulting mixture was refluxed for 3 days and then cooled to rt. Water was added, and the mixture was extracted with DCM, dried (MgSO4). After removing the solvent in vacuo, the resulting solid was precipitated from DCM/methanol several times to afford 12 as a white solid. 12a (8 mg, 47%): IR (KBr) ν 3391, 2923, 2852, 1458, 1378, 1260, 1209, 1133, 1021, 972, 865, 800, 721 cm−1; 1H NMR (400 MHz, CDCl3) δ 0.84 (m, 3 H), 1.20−1.79 (m, 20 H), 3.75 (m, 2 H), 4.67− 4.74 (m, 2 H), 7.35−7.54 (m, 4 H); 13C NMR (100 MHz, CDCl3) δ 14.1, 22.7, 26.4, 29.3, 29.7, 30.2, 32.0, 65.4, 75.6, 124.3, 126.6, 127.7, 128.8, 131.4, 137.0, 154.0. GPC Mn = 3400, PDI = 1.22. 12b (8 mg, 47%): GPC Mn = 4600, PDI = 1.17. 12c (10 mg, 58%): GPC Mn = 7300, PDI = 1.22. alt-Poly{[3,5-(4-dodecyloxy-1-acetoxymethyl])phenylene]vinylene} (14). Under nitrogen, a mixture of 13 (30 mg, 0.07 mmol) and 7b (3 mg, 0.003 mmol) in DCM (0.3 mL) was stirred at 40 °C for 30 min, the solvent was removed under reduced pressure, and DCM (0.3 mL) was added. This cycle was repeated at 1 h, 2 h, 12 h, and 24 h intervals. Ethyl vinyl ether (0.5 mL) was then added, and the mixture stirred at rt for 1 h. The mixture was added to methanol. Repeated

precipitation in DCM/methanol afforded 14 as a white powder (21 mg, 77%): IR (KBr) ν 2923, 2853, 1742, 1457, 1378, 1363, 1224, 1140, 1025, 974, 915, 865, 720, 570, 528 cm−1; 1H NMR (400 MHz, CDCl3) δ 0.84−0.88 (m, 3 H), 1.22−1.89 (m, 20 H), 2.10−2.17 (m, 3 H), 3.86−3.88 (m, 2 H), 5.08−5.15 (m, 2 H), 7.44−7.59 (m, 4 H); 13 C NMR (100 MHz, CDCl3) δ 14.5, 21.5, 23.0, 23.1, 26.5, 26.6, 26.8, 26.9, 27.5, 29.56, 29.63, 29.74, 30.1, 30.4, 30.7, 32.3, 66.5, 75.6, 124.7, 125.7, 125.9, 126.0, 126.8, 128.1, 128.9, 131.8, 131.99, 132.05, 154.9, 155.1, 155.2, 170.8; GPC Mn = 6600, PDI = 2.69. Polynorbornenes 19a−b. A mixture of 1829 (212 mg, 0.3 mmol) and 7a (27 mg, 0.03 mmol for 19a, 0.019 mmol for 19b) in DCM (10 mL) under nitrogen was stirred for 2 h. Ethyl vinyl ether (1.0 mL) was then added and stirring was continued for 20 min. The mixture was concentrated, and the residual solution was added to methanol. Precipitate was collected and redissolved in DCM. Reprecipitation by adding the DCM solution to methanol afforded the 19 as a grayish powder. 19a (191 mg, 90%): IR (KBr) ν 2156, 1727, 966 cm−1; 1H NMR (400 MHz, CDCl3) δ 0.04 (s, 9 H, Si(CH3)3), 0.24 (s, 9 H, Si(CH3)3), 0.89 (m, 3 H, CH3), 1.28−1.52 (m, 18 H, CH2), 1.63−1.84 (m, 4 H), 2.77−2.91 (m, 4 H), 3.30 (m, 4 H), 3.98 (m, 2 H, CH2), 5.06−5.37 (br m, 2 H), 6.53 (m, 2 H), 6.91 (s, 1 H), 7.25 (s, 1 H), 8.02 (m, 2 H). End group analysis by measuring the relative intensities at d 5.37/5.07 = 10.4 corresponded to about 10 repetitive units; 13C NMR (100 MHz, CDCl3) δ 0.1, 1.2, 14.2, 22.8, 26.1, 26.2, 26.4, 27.0, 27.1, 29.3, 29.4, 29.5, 29.7, 32.0, 35.2, 35.8, 44.6, 46.6, 49.5, 69.2, 99.9, 100.2, 100.7, 100.9, 111.3, 114.0, 115.4, 116.0, 118.0, 125.9, 127.0, 128.4, 132.0, 145.7, 151.0, 157.0, 164.6; GPC Mn = 7400, PDI = 1.18. 19b (195 mg, 92%): End group analysis by measuring the relative intensities at δ 5.38/5.06 = 15.7 corresponded to about 16 repetitive units; GPC Mn = 10 700, PDI = 1.29. Double Stranded Ladderphanes 20. A mixture of Cu(OAc)2 (55 mg, 0.3 mmol) and 19 (70 mg, 0.1 mmol [calculated based on the molecular weight of the monomeric unit 19]) in pyridine (120 mL) and DCM (40 mL) was stirred at rt. Then, [n-Bu4N]F (1 M in THF, 0.3 mL, 0.3 mmol) was added. The mixture was kept stirring for 24 h and washed with water, an aqueous 7% HCl, and brine. The organic phase was evaporated in vacuo and precipitated in DCM/methanol twice. After filtration, the solid was collected and dried in vacuo to afford the desired product 20 as a yellow powder. 20a (48 mg, 85%): IR (KBr) ν 2206, 2108, 1731, 953 cm−1; 1H NMR (400 MHz, CDCl3) δ 0.89 (m, 3 H, CH3), 1.27−1.47 (m, 20 H, CH2), 1.82 (m, 2 H, CH2), 2.83−3.35 (m, 8 H), 4.00 (m, 2 H, CH2), 5.08−5.46 (m, 2 H), 6.58 (br, 2 H), 6.98 (br, 1 H), 7.21 (br, 1 H), 8.04 (m, 2 H); 13C NMR (100 MHz, CD2Cl2) δ 14.6, 23.4, 26.5, 29.6, 30.0, 30.3, 32.6, 37.4, 45.5, 46.2, 50.2, 70.0, 79.4, 83.6, 84.2, 112.1, 114.3, 115.8, 116.5, 118.1, 126.4, 128.2, 128.9, 132.3, 146.1, 151.9, 157.7, 165.0; GPC Mn = 5000, PDI = 1.28. 20b (46 mg, 82%): GPC Mn = 9100, PDI = 1.33. alt-Poly(benzofuranylene-ethynylene)s (22). To a solution of 20 (45 mg, 0.08 mmol [calculated based on the molecular weight of the monomeric unit]) in THF (10 mL) was added 10% aq. NaOH (1 mL) and [n-Bu4N]F (1 M in THF, 0.1 mL, 0.1 mmol). The mixture was refluxed for 24 h and cooled to rt. After removing the solvent in vacuo, the mixture was dissolved in DCM and washed with water, 7% HCl, and brine. The organic phase was evaporated in vacuo and precipitated in DCM/methanol twice. After filtration, the solid was collected and dried in vacuo to afford the desired product 22 as a yellow powder. 22a (19.2 mg, 74%): IR (KBr) ν 2201, 2104 cm−1; 1H NMR (400 MHz, CDCl3) δ0.86−0.91 (m, 3 H, CH3), 1.25−1.60 (m, 18 H, CH2), 1.87−1.92 (m, 2 H, CH2), 3.34−3.35 (m, 0.13 H, terminal alkyne-H), 4.04−4.12 (m, 2 H, CH2), 6.70−6.72 (m, 0.1 H, terminal-H), 6.91− 7.09 (m, 2 H), 7.54−7.65 (m, 1 H). End group analysis by measuring the relative intensities at δ 4.08/(3.34 + 6.71) = 8.7 corresponded to about 9 repetitive units. 13C NMR (200 MHz, CD2Cl2)δ 14.1, 22.7, 25.9, 26.0, 26.06, 26.11, 29.2, 29.3, 29.4, 29.5, 29.7, 31.9, 69.6, 69.7, 78.9, 79.7, 80.5, 81.4, 83.9, 93.8, 103.0, 103.4, 104.4, 104.5, 104.6, 109.7, 110.3, 110.5, 111.4, 115.1, 116.0, 116.1, 129.4, 129.8, 130.0, I


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130.1, 130.4, 130.7, 141.5, 148.9, 149.0, 149.3, 149.4, 156.8, 157.0, 157.2, 157.8, 158.0; GPC Mn = 3000, PDI = 1.22. 22b (17.4 mg, 67%): End group analysis by measuring the relative intensities at δ4.08/(3.33 + 6.71) = 15.4 corresponded to about 15 repetitive units. GPC Mn = 5000, PDI = 1.37. STM Images. An aliquot of 10 mL of 20a in phenyloctane was placed on HOPG (highly orientated pyrolytic graphite, Advanced Ceramics, ZYH grade) with a micropipet. A piece of tissue paper was employed to remove excess solvent and to generate a shear flow that would align the molecules on the HOPG suface.41 STM imaging was carried out with a NanoScopeIIIa controller (Veeco Metrology Group/Digital Instruments) at rt. The STM probes were commercially available Pt/Ir tips (PT, Nanotips, Veeco Metrology Group/Digital Instruments). The images were filtered with a first-order flattening to minimize noise and without further processing.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details for monomeric precursors and 1H and 13C NMR spectra of all new compounds and GPC curves for all polymers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions §

These authors (Y.Z.K., R.J.J., and T.C.W.) contribute equally to this paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Council (Taipei) and the National Taiwan University for support. REFERENCES

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