Scandium-Catalyzed Syndiospecific Chain-Transfer Polymerization of

Article pubs.acs.org/Macromolecules

Scandium-Catalyzed Syndiospecific Chain-Transfer Polymerization of Styrene Using Anisoles as a Chain Transfer Agent Atsushi Yamamoto,†,§ Masayoshi Nishiura,†,‡ Juzo Oyamada,† Hiroyuki Koshino,‡ and Zhaomin Hou*,†,§,‡ †

Organometallic Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan § Department of Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan ‡

S Supporting Information *

ABSTRACT: The polymerization of styrene in the presence of various anisole derivatives has been examined by using the h a l f - s a n d wi c h s c a n di u m di a m i n ob e n zy l c om plex (C5Me4SiMe3)Sc(CH2C6H4NMe2-o)2 with borate [Ph3C][B(C6F5)4]. The syndiospecific chain transfer polymerization of styrene proceeded efficiently via the ortho-C−H bond activation of anisoles, affording the corresponding end-ortho-anisyl-functionalized syndiotactic polystyrenes. The molecular weight of the resulting polymers could be controlled in a wide range by changing the styrene/anisole feeding ratio. Propenyl and halogen (F, Cl, Br, and I) substituents on the anisole compounds are compatible with the present catalyst system, thus enabling easy introduction of unsaturated CC double bond or halogen moieties together with the anisole functionality to the chain end of syndiotactic polystyrene.

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INTRODUCTION Syndiotactic polystyrene (sPS)1 is an attractive engineering plastic because it shows unique properties such as high melting point, high crystallization rate, high modulus of elasticity, low dielectric constant, and excellent chemical/solvent resistance.2,3 However, further application of sPS has been limited because of its poor adhesion and incompatibility with other materials such as pigments, glass fibers, clays, and metals, due to the lack of chemical functionality or polar groups. The introduction of polar functional groups into sPS is expected to overcome these drawbacks. There are several methods for the functionalization of sPS, such as polymerization of prefunctionalized monomers, post chemical modification of sPS, block copolymerization of styrene with polar monomers, and the use of chain transfer agents (CTAs) in the polymerization process.4−8 Among these approaches, the chain transfer reaction has significant advantages such as the direct regioselective introduction of a functional group into the polymer chain end during the polymerization reaction, catalytic synthesis of end-functionalized polymers, and the ability to control molecular weight. Previously, cationic halfsandwich titanium complexes were reported to catalyze the syndiospecific chain transfer polymerization of styrene using organosilanes and organoboranes as CTAs, which afforded the corresponding end-functionalized silyl-sPS5 and boryl-sPS,6 respectively. However, a large quantity ([CTA]/[catalyst] = 30−100) of the corresponding CTA was needed for the polymerization reaction to produce more than one polymer chain per catalyst ([polymer chains]/[catalyst] = 2−3),9 possibly because the cationic half-sandwich titanium catalysts are not © XXXX American Chemical Society

good enough in livingness and relatively poor in reactivity toward the CTAs. Attempts to control the molecular weight of the resulting polymers by changing the CTA/monomer ratio seemed unsuccessful in this regard. The highly efficient syndiospecific chain-transfer polymerization of styrene has not been reported to date, as far as we are aware.10 To achieve efficient synthesis of end-functionalized sPS, the catalyst system should not only show high syndiotactic selectivity for styrene polymerization, but it should also show excellent livingness and efficient reactivity with the chain-transfer agent. Moreover, the active catalyst species generated after the chaintransfer reaction should also show high activity and selectivity. To date, the catalyst systems reported for the syndiospecific polymerization of styrene have been mainly based on titanium complexes, which usually do not show sufficient living character.4−6 In addition, the chain transfer agents used for these catalyst systems have generally been limited to organosilane and organoborane compounds.10 Therefore, the search for new catalysts for highly efficient syndiospecific chain-transfer polymerization of styrene is of much interest and importance. We recently reported that cationic half-sandwich rare-earth metal alkyls can serve as excellent catalysts for the polymerization and copolymerization of a wide range of olefin and diene monomers.11,12 In particular, the half-sandwich scandium catalysts showed high activity, high syndiotactic selectivity, and Received: January 13, 2016 Revised: March 10, 2016

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DOI: 10.1021/acs.macromol.6b00083 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules excellent livingness for the polymerization of styrene.7,12a,e,i More recently, we found that related half-sandwich rare-earth catalysts could also efficiently catalyze the C−H addition of functionalized aromatic compounds such as anisoles to styrene and other olefins through ortho-C−H bond activation and subsequent insertion of a CC double bond.11c,13 From these results, we envisioned that anisole might serve as an efficient chain transfer agent for the rare-earth-catalyzed syndiospecific polymerization of styrene (cf. Scheme 1). In this paper, we report

Table 2. Polymerization of Styrene with Anisole as a Chain Transfer Agent by Using Complex 2 under Low Conversion Condition (Conversion ≪ 10%)

Scheme 1. A Possible Scenario for Catalytic Synthesis of Anisyl-Functionalized SPS by Using Anisole as a Chain Transfer Agent in Sc-Catalyzed Polymerization of Styrene

run

anisole (mmol)

1 2 3 4 5

0.1 0.2 0.3 0.4 0.5

[St]:[Ani]:[Sc]

reaction time (min)

yield (mg)

Mnb [GPC] (×103)

Mw/Mnb

500:5:1 500:10:1 500:15:1 500:20:1 500:25:1

0.5 1 2 3 3

38 20 71 98 55

12.3 7.1 5.2 4.1 3.6

2.0 1.8 1.7 1.7 1.6

Conditions: Sc, 20 μmol; [Ph3C][B(C6F5)4], 20 μmol; styrene, 10 mmol (1.04 g); toluene/styrene = 5/1 (v/v). bDetermined by GPC in o-dichlorobenzene at 145 °C against a polystyrene standard. a

the half-sandwich scandium-catalyzed chain-transfer polymerization of styrene with various anisole derivatives as a chain transfer agent, which afforded a new family of end-ortho-anisylfunctionalized sPS polymer materials with high efficiency and controllable molecular weight in a catalytic fashion.

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Figure 1. Plot of the reciprocal number of the number-average degree of polymerization of styrene (1/Pn) vs the ratio of anisole to styrene ([anisole]/[styrene]).

RESULTS AND DISCUSSION At first, we examined the polymerization of styrene in the presence of anisole by using a combination of the half-sandwich scandium dialkyl complexes (C5Me4SiMe3)Sc(CH2SiMe3)2(thf) (1)12a and (C5Me4SiMe3)Sc(CH2C6H4NMe2-o)2 (2)12c with

[Ph3C][B(C6F5)4] as a cocatalyst. Some representative results are shown in Table 1. In the absence of anisole, both catalysts 1

Table 1. Polymerization of Styrene with Anisole as a Chain Transfer Agenta

run f

1 2 3f 4 5h 6i 7j 8 9l 10m

[Sc] 1 1 2 2 2 2 2 2 2 2

[St]:[3a]:[Sc] 500:0:1 500:5:1 500:0:1 500:5:1 1000:5:1 2000:5:1 4000:5:1 500:10:1 500:25:1 150:25:1

yield (g) 0.99 0.02 1.02 1.03 2.08 4.24 8.42 1.02 0.97 0.17n

Mnb [GPC] (×103)

Mnc [NMR] (×103)

115 11.5 228 11.7 24.1 50.5 87.6 6.5 2.8 1.6

n.d. n.d. n.d. n.d. n.d. n.d. n.d. 7.1 3.3 2.0

g

Mw/Mnb

Tgd (°C)

Tmd (°C)

chain/Sce

1.2 1.7 1.7 2.0 1.8 1.8 1.9 1.9 1.7 1.2

99 n.d. 95 87 95 94 96 −k − 41, 103

273 n.d. 273 271 273 274 275 266 251 206

0.4