Cyclopolymerization of Si-Containing α,ω-Diolefins by a

Sep 16, 2014 - School of Material Science and Engineering, and Tianjin Key Laboratory of ... catalytic system with high cyclization selectivity and st...
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Cyclopolymerization of Si-Containing α,ω-Diolefins by a Pyridylamidohafnium Catalyst with High Cyclization Selectivity and Stereoselectivity Bin Wang,†,‡ Yong-Xia Wang,† Jing Cui,∥ Ying-Yun Long,† Yan-Guo Li,† Xiao-Yan Yuan,*,∥ and Yue-Sheng Li*,†,∥ †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ University of the Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China ∥ School of Material Science and Engineering, and Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, China S Supporting Information *

ABSTRACT: Cyclopolymerization of unsymmetric and symmetric Sicontaining α,ω-diolefins by dimethylpyridylamidohafnium/organoboron catalytic system with high cyclization selectivity and stereoselectivity was first reported here. Compared with the constrained geometry catalyst [Me2Si(η5-Me4C5)(NtBu)]TiCl2 and the typical metallocene catalyst racEt(Ind)2ZrCl2, the Hf complex has proved to be one more promising catalyst for the cyclopolymerization of unsymmetric 3,3-dimethyl-3-sila1,5-hexadiene (DMSHD). The poly(DMSHD) with high molecular weight (up to 953 kg/mol), high cyclization selectivity (100%), high cis selectivity (>90%), and high isotactic selectivity (>90%) could be easily obtained by the Hf catalyst under mild conditions. Moreover, the poly(DMSHD) also possessed a high glass transition temperature (up to 99.8 °C) and high melting temperature (Tm >200 °C). The insertion reaction of the vinyl group into Hf−CAr bond was further investigated, and a possible cyclization mechanism of DMSHD was proposed according to density functional theory calculation. Meanwhile, the Hf catalyst also showed notable catalytic activity for the cyclopolymerization of the symmetric Si-containing α,ω-diolefins including 4,4-dimethyl-4-sila1,6-heptadiene, 4-methyl-4-phenyl-4-sila-1,6-heptadiene and 4,4-diphenyl-4-sila-1,6-heptadiene. Although the cyclization selectivity is somewhat low, the copolymerization by Hf catalyst still shows excellent cis/trans selectivity and isotactic selectivity.

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INTRODUCTION The cyclopolymerization of a nonconjugated diene is of much interest because it converts acyclic monomers into a novel family of the polymer materials with cyclic repeating units, namely, the cycloolefin polymers (COPs).1,2 The microstructure of the COPs is significantly more complicated than the linear polymers derived from simple α-olefins.3 First, since the monomer has two vinyl groups, either cyclization or crosslinking can occur. Second, cyclopolymers include the cis and trans configuration of the rings and the relative stereochemistry between the rings. Diastereoselectivity is an important factor in the cyclopolymerization that is not encountered in typical αolefin polymerization. Generally, the stereospecific polymerization of α-olefins will give rise to only two highly ordered microstructures (isotactic or syndiotactic), while the cyclopolymerization of nonconjugated dienes can produce four microstructures of maximum order. Considering that the polymers whose cyclic repeating units are arranged in a controlled manner will exhibit unique properties, the cyclopolymerization with high cyclization and high stereoselectivity is of very importance, and it also has been a great challenge in controlling over stereochemistry of COPs microstructure.4 © 2014 American Chemical Society

Gratifyingly, the occurrence of single-site catalysts including metallocene and nonmetallocene has offered an unprecedented chance to control over the stereochemistry and polymer architecture of COPs.5−7 Recently, Osakada and his co-workers reported that some Pd diimine complexes can efficiently promote the regio- and stereospecific cyclopolymerization of substituted 1,6-heptadienes (1,6-HPD).8a The similar results were also obtained by Fe and Co complexes with bis(imino)pyridine ligands.8b However, both of the catalysts resulted in the formation of the thermodynamically favorable trans-fused cyclopentane rings. Indeed, as far as we concerned, the cyclopolymerization with high degree of cis selectivity was seldom reported. The first example was the cyclopolymerization of 1,5-hexadiene (1,5-HD) with phenoxyamine hafnium complex which was reported by Coates et al.7a A highly stereoregular 1,3-cis-isotactic poly(methylene-1,3-cyclopentane) (PMCP) could be obtained under mild conditions. Lately, Sita and his colleagues reported the use of a C1-symmetric Zr Received: July 16, 2014 Revised: August 26, 2014 Published: September 16, 2014 6627

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complex as the catalyst for living stereospecific cyclopolymerization of 1,6-HPD to provide poly(methylene-1,3-cyclohexane) (PMCH) with an extremely high degree of 1,3-cisisotactic stereoregularity.7b Recently, our group have revealed that pyridylamidohafnium catalyst exhibits not only extremely high activity (up to 106 g of polymer/molHf·h) but also excellent cyclization selectivity as well as high stereoselectivity in the cyclopolymerization of 1,5-HD.9 Organosilicon polymers have attracted considerable attention due to their enhanced properties, some of which can not be achieved by hydrocarbon polymers, such as low surface tension and high hydrophobicity.10 Several synthetic methods have been applied to synthesize such Si-containing polymers with acyclic repeating units.10b,c However, the synthesis of Sicontaining cycloolefin polymers, especially with exclusive cyclic repeating units, via coordination chemistry have been reported rarely. As a pioneer, Naga investigated the copolymerization of disubstituted diallylsilane with ethylene and propylene involving intramolecular cyclization using Zr catalysts.11 Although the molecular weight of the copolymers is relative low (about 8 kg/mol) and the pendent vinyl groups could be observed when the monomer incorporations are higher than 20%, the introduction of Si to polyolefins should be one of the most attractive methods for developing new polyolefins with novel properties. This concept is quite enlightening to prepare Si-containing COPs whose properties will be different from those of carbon-based COPs. The first example of well-defined Si-containing COPs with only cyclic repeating units is the highly stereoregular 3,5-cis,isotactic poly(3,5-methylene-1,1dimethyl-1-silacyclohexane) from cyclopolymerization of bis(2-propenyl)dimethylsilane using C1-symmetric Zr complex documented by Sita.12 In present work, the cyclopolymerizations of two Sicontaining unsymmetric α,ω-diolefins including 3,3-dimethyl3-sila-1,5-hexadiene (DMSHD, 1) and 3,3-diphenyl-3-sila-1,5hexadiene (DPSHD, 2), and three symmetric α,ω-diolefins including 4,4-dimethyl-4-sila-1,6-heptadiene (DMSHP, 3), 4methyl-4-phenyl-4-sila-1,6-heptadiene (MPSHP, 4) and 4,4diphenyl-4-sila-1,6-heptadiene (DPSHP, 5) were conducted by using the typical metallocene catalyst rac-Et(Ind)2ZrCl2 (I), the “constrained geometry” catalyst (II) and dimethylpyridylamidohafnium catalyst (III) (Scheme 1). Compared with the catalysts I and II under the similar conditions, hafnium catalyst (III) has significant preponderance in not only high monomer conversion but also extremely high cyclization selectivity and stereoselectivity in the process of the cyclopolymerization of DMSHD (1). With respect to symmetric α,ω-diolefins (3−5), the cyclization selectivity was somewhat low and some pendent vinyls were present in polymer chains. Additionally, we document the unique thermal properties of these cyclopolymers which are associated with a high Tg and high Tm.

Scheme 1. Structure of Si-Containing Diolefins, Catalysts, and the Corresponding Cyclopolymers

CGC catalysts (Table 1, runs 1−3). Under the mild conditions, the catalytic activity was almost 2 orders of magnitude higher than those of the CGC and Zr catalysts, up to 105 g polymer/ (molHf·h). Monomer conversion can easily reach to 40% in 10 min, while it took more than 12 h to reach the similar conversion using the CGC and Zr catalysts. The resultant poly(DMSHD)s with Zr and Hf catalysts possessed unimodal and relative narrow molecular weight distributions (MWDs). Surprisingly, the weight-average molecular weight (Mw) of the polymer obtained by Hf catalyst was extremely high, up to 953 kg/mol. As shown in Figure 1, 1H NMR spectra confirmed the absence of vinyl end group that might arise from β-H elimination and incomplete cyclization. This observation is significant for the goal of obtaining Sicontaining cyclopolymers with controlled microstructure whose properties will be different from the corresponding carbonbased cyclopolymer such as PMCP. In contrast, the polymers obtained with Ti and Zr catalysts not only displayed low molecular weight but also possessed some pendent vinyl groups (parts b and c, Figure 1). Noticeably, the cyclopolymerization behaviors between CGC and Zr catalyst were also obviously different from each other. As shown in Figure 1, both allyl and vinyl groups were observed in the polymer obtained by Zr catalyst, while only vinyl group was detected in the resultant polymer produced by CGC catalyst. Taking these results into account, it is clear that dimethylpyridylamidohafnium/[Ph3C][B(C6F5)4] is one more promising catalyst for the cyclopolymerization of Si-containing α,ω-diolefins. We further thoroughly characterized the structure of the cyclopolymers obtained by the Hf catalyst using 13C, DEPT-13C, H−H COSY, and C−H COSY NMR analyses (see Figures S1 and S2 in Supporting Information). The typical 13 C NMR spectra are presented in Figure 2, and the resonance peaks were assigned clearly. Two signals derived from methyl groups connected to silicon atom were observed at −1.59 and −4.04 ppm. The split of the signal probably assigned to the

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RESULTS AND DISCUSSION Cyclopolymerization of 3,3-Dimethyl-3-sila-1,5-hexadiene with Zr, Ti, and Hf Catalysts. The cyclopolymerizations of unsymmetric diolefin, DMSHD (1), catalyzed by the typical metallocene catalyst rac-Et(Ind)2ZrCl2 (I), the constrained geometry catalyst [Me2Si(η5-Me4C5)(NtBu)]TiCl2 (II), and hafnium catalyst (III) were investigated. The typical experimental results are summarized in Table 1. The dimethylpyridylamidohafnium, in the presence of [Ph3C][B(C6F5)4], showed excellent performance in the cyclopolymerization using AliBu3 as a scavenger, compared with the Zr and 6628

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Table 1. Cyclopolymerization Data of Unsymmetric α,ω-Diolefinsa run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

diolefin (mol/L) 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2

(0.30) (0.30) (0.30) (0.30) (0.30) (0.30) (0.30) (0.30) (0.30) (0.30) (0.30) (0.30) (0.30) (0.30) (0.30) (0.30) (0.15) (0.15) (0.15) (0.15) (0.30) (0.30)

catal

Al (mmol)

temp (°C)

time (min)

convn (%)

Mwb (kDa)

Mw/Mnb

[C]c (%)

cisd (%)

Tge (°C)

Tme (°C)

I II III III III III III III III III III III III III III III III III III III III III

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1.0 2.0 4.0 4.0 4.0 4.0 4.0 4.0 0.5 0.5 4.0 4.0 0.5 0.5

10 10 10 10 25 25 50 50 10 10 10 10 25 25 50 50 25 25 25 25 25 25

720 720 10 60 10 60 10 60 10 10 10 60 10 60 10 60 10 60 10 60 10 60

40 56 41 52 66 78 76 86 48 57 65 83 72 91 80 96 82 99 88 99 2.4 5.0

5.00 87.0 835 953 766 847 651 792 769 643 597 802 470 546 258 392 479 513 220 250 10 12

1.68 2.55 1.73 1.87 2.22 2.23 2.67 3.18 1.75 1.91 1.84 2.13 1.96 2.40 2.06 2.66 2.00 2.50 2.12 2.25 1.28 1.25

59.0 90.0 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 96.0 95.0

76.9 70.9 93.2 93.0 92.4 92.6 92.4 92.2 92.8 92.7 92.3 92.9 93.0 93.4 93.1 93.2 92.6 92.7 93.2 92.8 n.d.f n.d.

37.8 83.4 99.5 99.8 98.8 99.3 96.5 99.0 97.9 96.7 95.6 98.0 94.8 95.2 93.6 94.2 94.9 95.0 92.5 92.8 n.d. n.d.

− − 223.1 223.7 222.5 222.8 221.6 222.4 221.8 219.6 217.5 222.0 216.2 218.7 210.3 214.2 216.5 218.6 210.0 210.2 n.d. n.d.

a Reaction conditions: catalyst 5 μmol, [Ph3C][B(C6F5)4] 10 μmol, AliBu3 as a scavenger, Vtotal = 20 mL. bGPC data in trichlorobenzene at 150 °C vs narrow PS standards. cCyclization ratio was determined by 1H NMR spectra. dThe cis ring content was determined by 13C NMR spectra. eTg (second heating) and Tm (first heating) was determined by DSC at a heating rate of 10 °C/min. fNot determined.

Figure 1. 1H NMR spectra of the poly(DMSHD)s using different catalysts. (a) dimethylpyridylamidohafnium (III) (Table 1, run 3), (b) [Me2Si(η5-Me4C5)(NtBu)]TiCl2 (II) (Table 1, run 2), and (c) racEt(Ind)2ZrCl2 (I) (Table 1, run 1).

Figure 2. 13C NMR and DEPT (135°) spectra (b) of the cyclic repeating units region for the poly(DMSHD)s prepared with (a) dimethylpyridylamidohafnium (III) (run 3, Table 1), (c) [Me2Si(η5Me4C5)(NtBu)]TiCl2 (II) (run 2, Table 1), and (d) rac-Et(Ind)2ZrCl2 (I) (run 1, Table 1).

carbon of equatorial position and axial position in silacyclopentane units, respectively. It was also confirmed that the complete cyclization of DMSHD occurred during the cyclopolymerization. As mentioned above, the microstructure of the cyclopolymer includes the cis/trans configuration of the rings and the relative stereochemistry between rings. The latter, the tacticity of the polymer, is determined by the enantiofacial selectivity of cyclopolymerization catalysis in the first insertion reaction; the cis/trans stereochemistry among the rings, on the other hand, arises from the diastereoselectivity of the cyclization step.3 Two parameters, α and σ, were defined by Waymouth and Coates to

describe the tacticity of the cyclopolymers. The parameter σ, equaling to the cis-rings content in the cyclopolymer, can be determined by 13C NMR. The parameter α, indicating the enantioface selectivity, can be calculated according to the equation developed by Waymouth and Coates.1b Previous investigation and modeling have suggested that the ligand structure of the catalyst is the main factor in determining the diastereoselectivity of the cyclization step.1c In order to inspect the stereoselectivity of the cyclopolymerization catalysis, the resultant polymers with different catalysts were further characterized by 13C NMR at high temperature, respectively 6629

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Mws and the broadening MWDs of poly(DMSHD)s obtained at higher temperature. Another remarkable difference is the cis ring contents in poly(DMSHD)s and PMCPs. The cis ring contents of poly(DMSHD)s range from 92.2 to 93.4%, whereas from 67.8 to 69.5% in PMCP, indicating that the Hf catalyst exhibits higher cis selectivity in the cyclopolymerization of DMSHD. Generally, the cis/trans diastereoselectivity can be strongly influenced by the structure of organometallic catalyst. Cyclopolymerization with more hindered catalyst leads to polymers containing predominantly cis rings.3 Therefore, we proposed that DMSHD-modified pyridylamidohafnium catalyst shows more steric encumber than 1,5-HD-modified counterpart, which facilitate forming more cis ring structures in poly(DMSHD)s than PMCP. As for DPSHD (2), the cyclopolymerization behavior is very different from that of DMSHD under the same condition (runs 21−22, Table 1). Both low catalytic activity and decreased conversion were observed for DPSHD cyclopolymerization. Further GPC analysis showed that the poly(DPSHD) only exhibits low molecular weight (10 kg/mol) and relatively narrow MWDs. The degree of polymerization (DP) of the poly(DPSHD) is about 40. Moreover, the pendent vinyl group also was observed in the cyclopolymers according to the 1H NMR spectra, indicative of incomplete cyclization (see Figure S4 in Supporting Information). We proposed that the steric repulsion of the phenyl group between the adjacent rings has a negative influence on chain propagation, which resulted in the polymers with low molecular weight ultimately and incomplete cyclization. Having a sample of highly stereoregular 1,3-cis-isotactic poly(DMSHD)s in hand provided an opportunity to further investigate the thermal properties of this polymer material. In brief, DSC analysis of the sample (run 4, Table 1) displayed unique thermal property, namely, a reproducible Tg of 97.9 °C and a transient Tm of 221.8 °C that was observed only in the first heating cycle. We envisioned that the thermal motion of the cyclopolymer chains is constrained because of the methyl group in the silicon atom, which results in the intrinsically slow crystallization rates. Moreover, the high Tg of 97.9 °C indicated that rapid cooling from the melt coupled with intrinsically slow crystallization rates may provide the cyclopolymer in an amorphous glassy state. The hypothesis was verified by repeating the DSC analysis of the sample using the same temperature cycle at first heating but with an isothermal crystallization at 160 °C for 5 h before the second heating, which provided a reproducible Tm value of 211.8 °C (Figure 3). The same results were observed in analyzing the samples with lower molecular weights, which suggested the unique thermal property arises from the structure complexity of the polymer chains rather than the increase of molecular weight. It was noteworthy that the thermal properties of poly(DMSHD)s with controlled microstructure were remarkable different from those of the carbon-based cyclopolymer PMCPs obtained by the same Hf catalyst. The Tg and Tm of poly(DMSHD) were much higher than those of PMCP (Tg: 14 °C, and Tm: 121 °C, respectively). Especially, the crystallizability and crystallization rates of poly(DMSHD) decreased and its thermal property was highly dependent upon the exact thermal history. The distinct thermal properties of poly(DMSHD) from PMCP may be ascribed to the more complicated structure of poly(DMSHD) in which the thermal motion and the arrangement of the polymer chains was

(parts a, c, and d, Figure 2). The ratio of the cis and trans stereochemistry in the obtained polymer chains was estimated by integration ratio of the C3 resonance at 39.8 and 38.1 ppm. The cis ring content (σ) of poly(DMSHD) obtained with the Hf catalyst is up to 93.2% (run 3, Table 1), which is higher than those of the polymers obtained with the Ti and Zr catalysts (the cis ring content is 70.9% and 76.9%, respectively). The high cis selectivity of Hf catalyst may be attributed to the more sterically encumbered DMSHD-modified pyridylamidohafnium catalyst, which favors forming cis-rich ring structure by facilitating a homofacial insertion/cyclization process in the polymerization of DMSHD. Moreover, the polymers obtained with the Hf catalyst showed extremely high degree of stereoregularity. The parameter α for the cyclopolymer obtained by Hf catalyst (run 4, Table 1) is up to 92.8%, indicating the high enantioface selectivity of this catalyst. It has been shown that C1-symmetric dialkyl pyridylamidohafnium complexes exhibit unexpected isoselectivity for cyclopolymerization of 1,5-HD and α-olefin (co)polymerization.9,13 The isoselectivity of polymerization catalyzed by the Hf catalyst was proposed to result from the conformation imposed on the active species by the bulky substituent on the bridge joining pyridine and amido moieties.14 Therefore, the cyclopolymerization of DMSHD is expected to proceed with isotactic enchainment. In contrast, the Ti and Zr catalysts showed apparently poor capability in stereocontrol, as an evidenced by both the relative low 1,3-cis/ 1,3-trans selectivity and the low stereoregularity (38.2% for Ti catalyst and 40.5% for Zr catalyst respectively). These results further confirmed that dimethylpyridylamidohafnium complex is one more promising catalyst for the cyclopolymerization of Si-containing α,ω-diolefins. Cyclopolymerization of Unsymmetric Diolefins with the Hf Catalyst. On the basis of the results aforementioned, as the cyclopolymerizations by dimethylpyridylamidohafnium catalyst exhibit high cyclization selectivity and stereoselectivity, the effects of reaction conditions (AliBu3 loading, polymerization temperature etc.) on cyclopolymerization behaviors of unsymmetric diolefins were further investigated in detail. The conversion increased slightly with the increase of AliBu3 dosage (runs 9−11, Table 1), suggesting AliBu3 could act as a scavenger in the polymerization of DMSHD. Besides, the Mw of the resultant polymer gradually decrease, and the MWDs increase slightly, suggesting that AliBu3 could also act as a chain transfer agent, and the chain transfer to AliBu3 is irreversible. In previous work, we utilized the Hf catalyst to cyclopolymerize 1,5-HD, and the highest Mw of the PMCP was 154 kg/mol. Surprisingly, the Mw of the poly(DMSHD) is up to 835 kg/ mol, which is much higher than that of PMCP under the similar condition (see Figure S3 in Supporting Information). These results strongly supported that the steric effect arised from methyl groups in silicon atom prevents the chain transfer to AliBu3 at a large extent. It is noteworthy that the Mws of poly(DMSHD)s decreased not as sharply as that of PMCP with the increase of AliBu3 content. Therefore, AliBu3 can mainly act as scavenger in polymerization of DMSHD, whereas mainly as chain transfer agent in polymerization of 1,5-HD. Additionally, temperature is an important parameter for cyclopolymerization. The conversion increased obviously with the increase of temperature form 10 to 50 °C (runs 3, 5, and 7), suggesting that the diffusions of monomer to the active site might be improved significantly. Meanwhile, increasing temperature can promote chain transfer to AliBu3, as evidenced by the decreased 6630

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increase of space length of the two vinyl compared with the unsymmetric diolefins.

Figure 3. DSC curves of the resultant poly(DMSHD) (run 4, Table 1) by the catalyst dimethyl pyridylamidohafnium (III). Figure 4. 1H NMR spectra of the resultant (a) poly(DMSHP)s (in oC6D4Cl2, run 1, Table 2), (b) poly(MPSHP)s (in o-C6D4Cl2, run 9, Table 2), and (c) poly(DPSHP)s (in o-C6D4Cl2, run 12, Table 2).

constrained significantly, resulting in higher Tg and Tm and its intrinsically slow crystallization rates. Cyclopolymerization of Symmetric Diolefins with Dimethylpyridylamidohafnium Catalyst. Owing to the relatively high catalytic activity, high cyclization, and high cisenriched isotactic content of poly(DMSHD)s produced using III/[Ph3C][B(C6F5)4] system, we proceeded to investigate the cyclopolymerizations of symmetric α,ω-diolefins, DMSHP (3), MPSHP (4), and DPSHP (5). As shown in Table 2, the Hf catalyst also exhibited high performance in cyclopolymerization of symmetric Si-containing diolefins. Especially, there was a significant increase in the conversion of DPSHP (5) (runs 12− 14, Table 2) compared with DPSHD (2) (runs 21−22, Table 1), as well as the Mw of the resultant polymer. It was reasoned that the steric repulsion of the phenyl group between the adjacent six-numbered rings has been decreased significantly, which resulted in the higher conversion and molecular weight ultimately. However, the 1H NMR spectra showed the existence of the pendant double bond as shown in Figure 4, which suggested that the cyclization selectivity decreased with the

To understand the polymerization behavior of the symmetric diolefins more profoundly, the kinetic studies of the polymerization of DMSHP (3) were further conducted (see Figure S5 in Supporting Information). Different form DMSHD (1), the polymerization of DMSHP (3) did not obey the first-order kinetics of the monomer concentration, suggesting that the rate-determining step of the chain propagation is the intramolecular cyclization rather than the intermolecular insertion of the allyl group into the Hf−polymer bond.8c This result means that intermolecular insertion of the allyl group in next coming monomer is more preferred than intramolecular cyclization at high concentration. And the cyclization selectivity would be increased under more dilute conditions, since the rate of intramolecular cyclization/ intermolecular insertion increased. Experimentally, the polymers with higher cyclo-structure content can be easily prepared under the more dilute conditions (run 4, Table 2).

Table 2. Cyclopolymerization Data of Symmetric α,ω-Diolefinsa run 1 2 3 4 5 6f 7 8 9 10 11 12 13 14

diolefin (mol/L) 3 3 3 3 3 3 3 3 4 4 4 5 5 5

(0.25) (0.25) (0.25) (0.13) (0.50) (0.25) (0.25) (0.25) (0.25) (0.25) (0.25) (0.25) (0.25) (0.25)

Al (mmol)

time (min)

convn (%)

Mwb (kDa)

Mw/Mnb

[C]c (%)

cisd (%)

Tge (°C)

Tme (°C)

0.5 1.0 2.0 0.5 0.5 0.5 0.5 0.5 0.5 1.0 2.0 0.5 1.0 2.0

10 10 10 10 10 10 30 60 10 10 10 10 10 10

40.0 37.2 34.3 34.1 41.3 38.6 61.1 71.4 51.5 49.5 45.7 57.6 52.2 48.3

546 316 255 296 1042 920 589 668 668 490 382 450 435 282

1.87 1.89 1.94 1.85 2.01 2.05 2.01 2.23 9.86 7.60 5.16 6.04 4.43 3.35

90.1 90.3 90.5 93.2 88.9 89.1 90.2 90.1 85.2 85.5 85.8 80.0 80.4 80.7

100 100 100 100 100 100 100 100 100 100 100 100 100 100

127.5 126.4 125.5 126.0 127.0 126.6 126.6 127.3 139.5 138.9 138.4 163.8 163.3 162.3

258.7 258.5 258.1 262.8 262.9 261.5 258.8 261.6 − − − − − −

General conditions: catalyst III 5 μmol, [Ph3C][B(C6F5)4] 10 μmol, AliBu3 as the scavenger, Vtotal = 20 mL, and polymerization at 10 °C. bGPC data in trichlorobenzene at 150 °C vs narrow PS standards. cCyclization ratio was determined by 1H NMR spectra. dThe cis ring content was determined by 13C NMR spectra. eTg (second heating) and Tm (first heating) was determined by DSC at a heating rate of 10 °C/min. fThe temperature is −10 °C. a

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respectively. The phenomenon supports the proposal that the melting temperature of the cyclopolymer is more dependent on the overall cis ring content than the tacticity of the polymer.7a It is noteworthy that the structure of the polymer chain exerts profound effects on the Tg. The introduction of sterically encumbered side chains will restrict the chain conformational mobility significantly, resulting in the increased Tg. Recently, Sita and his co-workers reported that the C1-symmetric (η5C5Me5)Zr(Me)2[N(Et)C(Me)N(tBu) could catalyze the cyclopolymerization of DMSHP in a living manner to provide highly stereoregular 3,5-cis,isotactic poly(DMSHP)s with complete cyclization.12 However, the Mw of the resultant polymer was relative low (about 14.8 kDa). By contrast, the Mws of the poly(DMSHP)s obtained by Hf catalyst were extremely high (up to 1042 kDa). Although the cyclization selectivity is somewhat low, the Hf catalyst still showed excellent cis/trans selectivity and isotactic selectivity in cyclopolymerization of DMSHP. Furthermore, the poly(DMSHP)s obtained by Hf catalyst also possessed high Tg (126.0 °C) and high Tm (260.0 °C), which was in good agreement with the results documented by Sita et al. Insertion into Hf−Aryl Bond and Cyclization Mechanism of Unsymmetric Diolefins. It has shown that the cyclopolymerization behavior of DMSHD (1) is very different from that of DMSHP (3) using Hf catalyst. Since there are two types of double bond including allyl and vinyl in unsymmetric diolefin, their reactivities are definitely different from each other. Thus, we are more interested in which double bond will insert into active species in chain propagation first, which is benefit for the further investigation on the cyclization mechanism. It is difficult to determine the first insertion of the double bonds in chain propagation or predict it from the structure of the poly(DMSHD). Fortunately, we can set out our investigation in the first stage, the activation of the catalyst, according to the special mechanism of polymerization catalysis by the Hf complex. The presence of Hf−CAryl is one distinct structural feature of the Hf catalyst from the other catalyst for olefin polymerization. It has been confirmed that the active species was generated by insertion of a single monomer into the Hf−CAryl bond based on the computational and experimental data.9,17 According to these documents, at least two distinct active species corresponding to vinyl-insertion and allyl-insertion complex would be generated (see Figure S7 in Supporting Information), respectively, in the cyclopolymerization of DMSHD, and provide bimodal polymers. However, the poly(DMSHD)s with high molecular weight and unimodal MWDs were obtained via the cyclopolymerization of DMSHD using the Hf catalyst. The result strongly suggested that the cyclopolymerization took place with single catalytically active species and one of the two active species arised from the insertion of double bond into Hf−CAr is formed exclusively. With this in mind, we analyzed the filtrates of a relative large scale polymerization experiment by ESI-MS. The results showed a combined compounds including original ligand and DMSHD inserted products (640.5 m/z) (see Figure S6 in Supporting Information). The mixture was further purified and each fraction was analyzed by NMR spectra. Besides the original ligand (yield: 56%), vinyl group inserted ligand with pendent allyl group (yield: 35%) was also obtained and the corresponding resonance peaks were assigned clearly (Figure 6a). The MS and NMR data clearly demonstrated that a measurable fraction of catalyst has been modified by DMSHD. Although the large unmodified fraction is present during

The data listed in Table 2 also indicate that the cyclopolymerization behaviors largely depended on the substituent groups on the silicon atom. The content of pendent vinyl groups also increased with the increase in the steric bulk of the substituent groups. The poly(DMSHP)s possessed relative narrow MWDs, and the cyclization selectivity was up to 90%. By contrast, the MWDs of the poly(MPSHP)s and poly(DPSHP)s were rather broad and the cyclization selectivity was somewhat low (runs 9−14, Table 2). It was assumed that the increased ring strain with the steric bulk decreased the intramolecular cyclization rates. Furthermore, the incorporation of the pendent double bond into other propagation centers resulted in microcross-linking structure, inducing the relative broad MWDs of the poly(MPSHP)s and poly(DPSHP)s. These observations strongly supported that the intramolecular cyclization, depending on the substituent groups on the silicon atom, is the rate-determining step in the polymerization of symmetric diolefins. Also, the detection of both cyclic units and pendent vinyl groups suggests that the rates of intramolecular cyclization and intermolecular insertion are comparable.15 Accordingly, the cyclization selectivity of the resultant polymers could be modulated at some range by controlling the initial concentration and changing the substituent groups on the silicon atom in the polymerization of symmetric diolefins. However, the poly(DMSHP)s, poly(MPSHP)s, and poly(DPSHP)s with complete cyclization would not be obtained, because the intermolecular insertion is competitive with intramolecular cyclization in the polymerization. The cyclization selectivity decreased with lowering reaction temperature as well (run 6, Table 2). A similar observation was reported by Waymouth et al. in the cyclopolymerization of 1,5-hexadiene.1b The 13C NMR spectra of the cyclopolymers obtained from monomers 3−5 exhibit exclusively cis ring in repeating units because of the preferential homofacial insertion/cyclization process with the increase of the diene length (Figure 5).16 The isotacticity of the cyclic units is higher than 99%. Inspection of the thermal properties of the poly(DMSHP)s revealed reproducible Tg about 126.0 °C and transient Tm about 260 °C (runs 1−8, Table 2). However, the poly(MPSHP)s and poly(DPSHP)s exhibited only reproducible Tg about 139.0 °C (runs 9−11, Table 2) and 163.0 °C (runs 12−14, Table 2),

Figure 5. 13C NMR spectra of (a) poly(DMSHP)s (run 1, Table 2), (b) poly(MPSHP)s (run 9, Table 2), and (c) poly(DPSHP)s (run 12, Table 2). 6632

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Figure 7. Potential energy surface for the cyclization reaction of 3,3dimethyl-3-sila-1,5-hexadiene. Figure 6. 1H NMR spectra (CDCl3) for (a) vinyl-inserted ligand and (b) original ligand.

Furthermore, the energies of the Pcis‑twist and Ptrans‑chair are 5.82 and 18.82 kJ/mol, indicating that the product derived from the cis form cyclization Pcis‑twist is more stable. Additionally, the cyclization process of cis-form is endothermic, as evidenced by the stability of Pcis‑twist is 6.51 kJ/mol lower than the π-complex Ccis‑twist. In contrast, the reaction of trans configuration needs more energy, because the π-complex Ctrans‑chair is 27.51 kJ/mol more stable than Ptrans‑chair. Therefore, the DFT calculation gives an account of the high cis selectivity exhibited by the Hf catalyst in the cyclopolymerization of DMSHD. According to the experimental data and DFT calculations, a possible cyclization mechanism involving sequential insertion step was proposed (see Scheme 2).

polymerization, the resultant polymers still possess unimodal molecular weight distributions, indicating that the cyclopolymerization took place with single catalytically active species. These observations were consistent with the results documented by Dow scientists who found that the Hf complex could not be modified by monomer completely and the unmodified Hf complex was inactive for polymerization.17 It was noteworthy that the allyl group inserted ligand with pendent vinyl group was not observed in the product fraction. The results undoubtedly revealed that the reactivity of vinyl group is much higher than allyl group, and the vinyl-inserted complex generated exclusively in chain initiation, although the steric effects was unfavorable for the insertion of vinyl group into Hf−CAryl bond. Additionally, density functional theory (DFT) calculation was conducted to verify the experimental results (see Figure S7 in Supporting Information).18−20 According to DFT data, the DMSHD-modified complex is more stable than the complex without DMSHD inserted into the ligands. Noticeably, the stability of the vinyl-inserted complex is higher than that of allyl-inserted complex, and there is a low energy difference between the DMSHD-modified complex and the DPSHDmodified complex. These results further support the conclusion that the vinyl group inserted complex with pendent allyl group is the true catalyst active species in the cyclopolymerization of DMSHD using the Hf catalyst. Further investigation of the cyclization reaction was also conducted based on DFT calculation through nonbonded interactions.1c,9 Figure 7 shows the potential energy surface for the cyclization process of DMSHD. A twist-boat conformation or a chair conformation will be formed during the second double bond coordinates with the metal center (see Figure S8 in Supporting Information). The π-complex Ccis‑twist and Ctrans‑chair corresponding to the twist boat and chair conformation respectively are formed in an exothermic reaction (ΔH = −0.69 and −8.69 kJ/mol). In light of the whole cyclization reaction, the twist boat conformation is more favorable than the chair one. As shown in Figure 7, the activation barrier of the insertion transition state TScis‑twist for the twist boat model is much lower than that of TStrans‑chair for the chair model (43.94 and 53.47 kJ/mol, respectively).

Scheme 2. Possible Mechanism for the Cyclopolymerization of 3,3-Dimethyl-3-sila-1,5-hexadiene

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CONCLUSIONS The cyclopolymerizations of unsymmetric and symmetric Sicontaining α,ω-diolefins by dimethylpyridylamidohafnium/ organoboron catalytic system are documented here. The dimethylpyridylamidohafnium complex, activated with [Ph3C][B(C6F5)4], showed not only high catalytic activity but also high cyclization selectivity and high stereoselectivity for the 6633

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(4) (a) Natori, I.; Imaizumi, K.; Yamagishi, H.; Kazunori, M. J. Polym. Sci.Part B, Polym. Phys. 1998, 36, 1657−1668. (b) Janiak, C.; Lassahn, P. G. Macromol. Rapid Commun. 2001, 22, 479−492. (5) (a) Makowski, H. S.; Wilchinsky, Z. W.; Shim, B. K. C. J. Polym. Sci. Part A 1964, 2, 1549−1566. (b) Doi, Y.; Tokuhiro, N.; Soga, K. Makromol. Chem. 1989, 190, 643−651. (c) Resconi, L.; Waymouth, R. M. J. Am. Chem. Soc. 1990, 112, 4953−4954. (d) Coates, G. W.; Grubbs, R. H. J. Am. Chem. Soc. 1996, 118, 229−230. (e) Resconi, L.; Coates, G. W.; Mogstad, A.; Waymouth, R. M. J. Macromol. Sci., Chem. 1991, A28, 1225−1234. (6) (a) Naga, N.; Shiono, T.; Ikeda, T. Macromol. Chem. Phys. 1999, 200, 1466−1472. (b) Jayaratne, K. C.; Keaton, R. J.; Henningsen, D. A.; Sita, L. R. J. Am. Chem. Soc. 2000, 122, 10490−10491. (c) Nomura, K.; Liu, J. Y.; Fujiki, M.; Takemoto, A. J. Am. Chem. Soc. 2007, 129, 14170−14171. (7) (a) Edson, J. B.; Coates, G. W. Macromol. Rapid Commun. 2009, 30, 1900−1906. (b) Crawford, K. E.; Sita, L. R. J. Am. Chem. Soc. 2013, 135, 8778−8781. (8) (a) Park, S.; Takeuchi, D.; Osakada, K. J. Am. Chem. Soc. 2006, 128, 3510−3511. (b) Takeuchi, D.; Matsuura, R.; Park, S.; Osakada, K. J. Am. Chem. Soc. 2007, 129, 7002−7003. (c) Okada, T.; Takeuchi, D.; Osakada, K. Macromolecules 2010, 43, 7998−8006. (9) Shi, X.; Wang, Y.; Liu, J.; Cui, D.; Men, Y.; Li, Y. Macromolecules 2011, 44, 1062−1065. (10) (a) Mark, J. E.; Allcock, H. R.; West, R. Inorganic Polymers; Oxford University Press Inc.: New York, NY, 2005. (b) Matloka, P. P.; Wagener, K. B. J. Mol. Catal. A- Chem. 2006, 257, 89−98. (c) Brook, M. A. Silicon in Organic, Organometallic and Polymer Chemistry; John Wiley: New York, 2000. (11) (a) Naga, N. Macromol. Chem. Phys. 2005, 206, 1959−1966. (b) Naga, N. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 6083−6093. (12) Crawford, K. E.; Sita, L. R. ACS Macro Lett. 2014, 3, 506−509. (13) (a) Domski, G. J.; Lobkovsky, E. B.; Coates, G. W. Macromolecules 2007, 40, 3510−3513. (b) Wang, X.; Wang, Y.; Shi, X.; Liu, J.; Chen, C.; Li, Y. Macromolecules 2014, 47, 552−559. (14) Boussie, T. R.; Diamond, G. M.; Goh, C.; Hall, K. A.; LaPointe, A. M.; Leclerc, M. K.; Murphy, V.; Shoemaker, J. A. W.; Turner, H.; Rosen, R. K.; Stevens, J. C.; Alfano, F.; Busico, V.; Cipullo, R.; Talarico, G. Angew. Chem., Int. Ed. 2006, 45, 3278−3283. (15) Kodaira, T. Prog. Polym. Sci. 2000, 25, 627−676. (16) (a) Naga, N.; Yabe, Y.; Sawaguchi, A.; Sone, M.; Noguchi, K.; Murase, S. Macromolecules 2008, 41, 7448−7452. (b) Naga, N.; Shimura, H.; Sone, M. Macromolecules 2009, 42, 7631−7633. (17) (a) Froese, R. D. J.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. J. Am. Chem. Soc. 2007, 129, 7831−7840. (b) Zuccaccia, C.; Busico, V.; Cipullo, R.; Talarico, G.; Froese, R. D. J.; Vosejpka, P. C.; Hustad, P. D.; Macchioni, A. Organometallics 2009, 28, 5445−5458. (18) (a) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41− 51. (b) Baerends, E. J.; Ros, P. Chem. Phys. 1973, 2, 52−59. (c) te Velde, G.; Baerends, E. J. Comput. Phys. 1992, 99, 84−98. (d) Fonseca, C. G.; Visser, O.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Methods and Techniques in Computational Chemistry, METECC-95; Clementi, E., Corongiu, G., Eds.; STEF: Cagliari, Italy, 1995. (19) Becke, A. D. Phys. Rev. A 1988, 38, 3098−3100. (20) Perdew, J. P. Phys. Rev. B 1986, 33, 8822−8824.

cyclopolymerization of 3,3-dimethyl-3-sila-1,5-hexadiene (DMSHD), compared with the typical metallocene catalyst rac-Et(Ind)2ZrCl2 and the constrained geometry catalyst [Me2Si(η5-Me4C5)(NtBu)]TiCl2 under the same conditions. The cyclopolymers with high molecular weight (up to 953 kg/ mol), high cis selectivity (>90%), and high isotactic selectivity (>90%) could be easily obtained under mild conditions. The poly(DMSHD)s obtained by the Hf catalyst exhibited the unique thermal properties, namely, a reproducible Tg 92.8−99.8 °C and a transient Tm 210−223.7 °C due to intrinsically slow crystallization rate. NMR spectra and DFT calculation data demonstrated that the vinyl is more active than the allyl group, and the vinyl-inserted complex with pendent allyl group is the true active species in the cyclopolymerization of DMSHD. Thus, a possible cyclization mechanism involving sequential insertion step was proposed. Additionally, the Hf catalyst also showed notable activity for the cyclopolymerizations of the symmetric Si-containing diolefins including 4,4-dimethyl-4-sila1,6-heptadiene, 4-methyl-4-phenyl-4-sila-1,6-heptadiene and 4,4-diphenyl-4-sila-1,6-heptadiene. Compared with the unsymmetric diolefin cyclopolymerization, the cyclization selectivity is somewhat low, depending on the substituent groups on the silicon atom. Nonetheless, the Hf catalyst still shows excellent control over the cis/trans selectivity and isotactic selectivity. To the best of our knowledge, this is the first example of efficient stereospecific cyclopolymerization of unsymmetric Si-containing α,ω-diolefins, affording Si-containing cyclopolymers with well-defined structure.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental section, characterization of compounds and polymer samples and detailed information on DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Authors

*(X.-Y.Y.) E-mail: [email protected]. *(Y.-S.L.) E-mail: [email protected]. Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS The authors are grateful for financial support by the National Natural Science Foundation of China (No. 21234006). REFERENCES

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