Preparation of a Copolymer of Methyl Methacrylate witha New Monomer, 2,2,6,6-Tetramethyl-4-benzyloxyl-piperidinyl Methacrylate, and Its Initiation of theGraft Copolymerization of Styrene with a ControlledRadical Mechanism

HAITAO ZHANG, ZHENRONG GUO, JUNLIAN HUANG

Key Laboratory of the Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University,State Education Ministry of China, Shanghai 200433, People’s Republic of China

Received 16 April 2002; accepted 23 September 2002Published online 00 Month 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pola.10525

ABSTRACT: A copolymer [P(MMA-co-TBPM)] was prepared by the radical polymeriza-tion of methyl methacrylate (MMA) and 2,2,6,6-tetramethyl-4-benzyloxyl-piperidinylmethacrylate (TBPM) with azobisisobutyronitrile as an initiator. TBPM was a newmonomer containing an activated ester. Both the copolymer and TBPM were charac-terized with NMR, IR, and gel permeation chromatography in detail. It was confirmedthat P(MMA-co-TBPM) could initiate the graft polymerization of styrene by the cleav-age of the activated ester of the TBPM segment. This process was controllable, and themolecular weight of the graft chain of polystyrene increased with the increment ofconversion. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4398–4403, 2002Keywords: copolymerization; graft copolymers; nitroxide free radical; radical poly-merization; controllability

INTRODUCTION

Recent progress in the field of grafted (or brush)copolymers has led to a need to develop efficientmethods for synthesizing a wider variety of ma-terials with the same basic architectural design.In the past few years, a variety of brushlike mac-romolecules have been prepared with the mac-romonomer method.1–6

Georges et al.7 first reported the controlled rad-ical polymerization of styrene (St) by stable free-radical polymerization. The mechanism involvesthe reversible capping of the growing chain by acounter stable free radical such as 2,2,6,6-tetra-

methyl-1-piperidinyloxy (TEMPO). Recently, var-ious methods have been used to introduceTEMPO onto the chains of (co)polymers, andTEMPO adducts have been widely used to pro-mote the controlled radical graft polymerizationof St.8–10 According to the traditional method, thepolymer or copolymer is first prepared, and thenTEMPO is introduced to it.11 With this method, itis obvious that the amount and site of the acti-vated species are very difficult to control.

The approach described here involves graft-ing from a copolymer synthesized by the copo-lymerization of methyl methacrylate (MMA)with a newly developed activated ester monomer,2,2,6,6-tetramethyl-4-benzyloxyl-piperidinylmethacrylate (TBPM), with the copolymer usedas a macroinitiator. In comparison with tradi-tional methods, the amount of TEMPO intro-duced into the backbone of the macroinitiator was

Correspondence to: J. Huang (E-mail: jlhuang@fudan.edu.cn)Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 40, 4398–4403 (2002)© 2002 Wiley Periodicals, Inc.

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easier to control, and the polymerization wasmore efficient.

EXPERIMENTAL

Materials

4-Hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy(HTEMPO) was prepared by the oxidation of4-hydroxy-2,2,6,6-tetramethylpiperidine (BeijingChaoyang Huashan Auxiliary Factory) with hy-drogen peroxide, with sodium tungstate as a cat-alyst9 (mp � 71–72 °C). Azobisisobutyronitrile(AIBN) was purified by recrystallization frommethanol. All other reagents were purified bystandard methods.

Preparation of 4-Benzyloxy-2,2,6,6-tetramethylpiperidinyl-1-oxy (BTEMPO)

The synthesis of BTEMPO is outlined in Scheme1. HTEMPO (19.5 g, 0.113 mol), 25.4 g (0.149 mol)of benzyl bromide, 16.7 g (0.298 mol) of powderedKOH, and 5.5 g of poly(ethylene glycol) (molecu-lar weight � 600; used as a phase-transfer cata-lyst in the procedure12,13) were added to 70 mL ofbenzene. The mixture was stirred violently for 5 hat 40 °C, and then the mineral was filtered. Theoil layer was washed with water several timesuntil the water was colorless, then it was driedwith CaCl2 overnight. Rude BTEMPO was ob-tained after benzene and excessive benzyl bro-mide were distilled under reduced pressure. Itwas purified by recrystallization in hexane for ayield of 74.8%.

IR (cm�1): 1597, 1497, 1463, 746, 696 (benzenering). UV (nm): 230 (benzene ring), 450 (NO � , log� � 1.37).14 mp: 63–64 °C.

Preparation of TBPM

BTEMPO (5.0 g, 0.02 mol) and 2.0 g of ascorbicacid were added to 40 mL of distilled water. Themixture was stirred for 8 h until the orange-redcolor vanished at 25 °C. Ether was used to extractthe solution three times; it was combined with theorganic layers, dried with MgSO4, and filtered forthe removal of the solid. Methacryloyl chloride(2.5 mL, 0.025 mol) was added dropwise to theether solution mixed with 2.4 mL (0.02 mol) oftriethylamine in an ice bath. The mixture wasstirred for 4 h, washed with distilled water, anddried with CaCl2. The solid was filtered, and the

solvent was removed by distillation in vacuo. Theresidue crystallized from methanol to yield 3.7 g(55% yield).

NMR (�): 7.22 (m, 5H, phenyl), 6.05 (s, 1H,CH2AC), 5.50 (s, 1H, CH2AC), 4.47 (s, 2H,OCH2OOO), 3.67 (m, 1H, piperidinyl), 1.92 (s,3H, CH3OCA), 1.88 (d, 2H, piperidinyl), 1.69 (t,2H, piperidinyl), 1.18 (s, 3H, piperidinyl), 1.03 (s,3H, piperidinyl). IR (cm�1): 1746 (CAO), 1637(CAC), 1603, 1503, 740, 698 (benzene ring). mp:73–75 °C.

Preparation of the Macroinitiator [P(MMA-co-TBPM)]

To a 100-mL ampule, accurately weighed MMA,TBPM, and AIBN in a definite molar ratio werecharged; the ampule was degassed by three freez-e–pump–thaw cycles at 77 K and then sealedunder very pure N2 (99.99%). The ampule wasplaced in an oil bath at 60 � 0.1 °C for a period oftime. The copolymerized product was diluted withCHCl3, precipitated with petroleum ether (bp� 30–60 °C), purified twice by the procedures ofdissolution and precipitation with CHCl3/petro-leum ether, and then dried to a constant weight at60 °C.

Graft Copolymerization of St by the Initiation ofP(MMA-co-TBPM)

The preparation of the grafter copolymer can bedescribed as follows. A charged molar ratio of Stand P(MMA-co-TBPM) was placed in an ampule.The ampule was degassed by three freeze–pump–thaw cycles at 77 K and then sealed under verypure N2 (99.99%). The ampule was placed in anoil bath at 125 � 0.1 °C for a period of time. Thegraft copolymers were diluted with CHCl3 andprecipitated with petroleum ether (bp � 30–60°C). The copolymer was purified twice by the pro-cedures of dissolution and precipitation withCHCl3/petroleum ether, extracted with ethanol(95%) for 48 h for the removal of the small mole-cules, and then dried to a constant weight at60 °C.

Cleavage of the Grafted Polystyrene (PS) Chain15,16

In a 10-mL flask, LiAlH4 (0.38 g, 10.0 mmol) in2.4 mL of tetrahydrofuran (THF) and the graftcopolymer (100 mg) in 3.6 mL of THF were intro-duced and then refluxed for 3 h with stirring. Tothe solution, 2.0 mL of ethyl acetate and distilled

COPOLYMER OF METHYL METHACRYLATE 4399

water were slowly added, respectively. The bro-ken PS chain was extracted by cyclohexane sev-eral times (10 mL � 6), and the combined ex-tracted solutions were concentrated to about 1/3of the original volume and then precipitated withpetroleum ether (30–60 °C). The precipitant waspurified by dissociation and precipitation withchloroform/petroleum ether.

Measurements

IR spectra were obtained on a Magna 550 Fouriertransform infrared spectrometer. NMR spectrawere obtained on a Bruker MSL 300 spectrometerwith tetramethylsilane as an internal standardand CDCl3 as a solvent. Gel permeation chroma-tography (GPC) was performed on an HP 1100with PS standards with THF as an eluant, a UVdetector, and a column (6.2 mm inner diameter� 25 mm) packed with small (5-�m) porous si-

lanized silica microspheres (injection volume� 20 �L, flow rate � 1.0 mL/min).

RESULTS AND DISCUSSION

Copolymerization of MMA with TBPM andCharacterization of the Macroinitiator P(MMA-co-TBPM)

The copolymerization of MMA and TBPM wasperformed with a common radical mechanism. InNMR spectra (Fig. 1) of the monomer (TBPM) andcopolymer, the double bonds of TBPM at 6.05 and5.50 ppm disappeared, and that at 4.5 ppm forprotons of methylene of the benzyloxy group re-mained. Furthermore, protons for OCOOCH3 ofMMA at 3.6 ppm were observed. In IR spectra ofthe copolymer, we also found that the absorptionof the double bonds around 1637 cm�1 for TBPMand around 1640 cm�1 for MMA disappeared. All

Figure 1. 1H NMR spectra of TBPM and poly(TBPM-co-MMA).

4400 ZHANG, GUO, AND HUANG

this information confirmed that the copolymeriza-tion was conducted successfully.

The composition ratio of MMA and TBPM inthe copolymer was calculated with followingequation by means of NMR:

r �[TBPM]

[MMA] � [TBPM] �A/2

B/3 � A/2

where r is the fraction of TBPM units in thecopolymer, A is the peak area of the protons ofmethylene of the benzyloxy group at 4.5 ppm, andB is the peak area of methyl protons (OCOOCH3)of the MMA segment at 3.6 ppm. The contents ofTBPM in the macroinitiator P(MMA-co-TBPM),shown in Table 1, increased with the increment ofTBPM in the feed ratio of the monomers.

Graft Copolymerization of St by the Initiation ofP(MMA-co-TBPM)

Tables 2 and 3 show the data for the graft copo-lymerization initiated by P(MMA-co-TBPM). Themolecular weight of P(MMA-co-TBPM)-g-PS in-

creased with the polymerization time and withthe conversion of St.

The purified graft copolymers were character-ized in detail with GPC and 1H NMR. In the GPCdiagram, only a peak attributed to the graft co-polymer was observed, and no trace of the macro-initiator P(MMA-co-TBPM) was left. In 1H NMR,the protons of the benzene ring at 6.5–7.3 ppm,attributed to the PS graft chain, appeared next tothe chemical shifts of protons of P(MMA-co-TBPM), as illustrated in Figure 1.

So that the controllability of the graft copoly-merization initiated by TBPM segments inP(MMA-co-TBPM) could be investigated, PS graftchains were broken from the grafter copolymerwith LiAlH4 as a reductant. It was confirmed byGPC that the molecular weight of the PS graftchain was proportional to the conversion and thatthe molecular weight distribution was rather nar-rower; in all cases, its values were within 1.21–1.5, even for a molecular weight as high as230,000. Therefore, it can surely be said that thegraft copolymerization of St initiated by P(MMA-co-TBPM) was controllable.

Table 1. Copolymerization of MMA and TBPM with AIBN as an Initiator at 60 °Ca

TBPM in the Feed(mol %)

Copolymer CompositionTBPM (mol %)b Mn �10�4 Mw/Mn

P(MMA-co-TBPM)1 10.0 9.1 4.2 1.79P(MMA-co-TBPM)2 6.0 4.3 4.7 1.76P(MMA-co-TBPM)3 4.8 3.7 5.1 1.82P(MMA-co-TBPM)4 2.4 1.8 4.9 1.85

a [AIBN] � 20 mg (0.12 mmol); [TBPM] � [MMA] � 50 mmol.b The copolymer compositions were calculated from 1H NMR

Table 2. Effect of the Concentration of P(MMA-co-TBMP)2 on the Grafted Chain of PS at 125 °C

Concentration(� 10�4 mol/L)

Time(h)

Molecular Weight(� 10�4)

PS Mw/Mn

Conversion(%)Grafter PS

7.5 1.5 10.7 7.5 1.31 5.47.5 3.0 14.5 11.3 1.35 9.87.5 5.0 18.8 15.7 1.34 18.77.5 7.0 21.9 18.1 1.36 25.97.5 12.0 25.5 21.9 1.40 31.415.0 1.5 9.2 6.4 1.29 8.215.0 5.0 10.1 7.5 1.30 14.815.0 7.0 14.0 11.6 1.30 19.315.0 12.0 18.7 16.3 1.31 27.2

COPOLYMER OF METHYL METHACRYLATE 4401

Effect of the Concentration of the P(MMA-co-TBPM) and TBPM Contents in P(MMA-co-TBPM)on the Graft Copolymerization of St

The concentration of the P(MMA-co-TBPM) andTBPM contents in P(MMA-co-TBPM) exerted agreat influence on the graft copolymerization of St,as shown in Tables 2 and 3. For example, when theconcentration of P(MMA-co-TBPM)1 increased from7.5 � 10�4 to 15 � 10�4 mol/L, after 5 h, the mo-lecular weight of the PS grafted chain and themonomer conversion dropped from 15.7 � 104 and18.7% to 7.5 � 104 and 14.8%, respectively (Table2). We also found that under similar polymerizationconditions and conversions, the molecular weight ofthe graft copolymer and PS grafted chain decreasedwith the increment of TBPM in P(MMA-co-TBPM)(Table 3). In general, in common radical polymer-izations, when the reaction conditions (e.g., themonomer concentration and polymerization tem-perature) are constant, the conversion of the mono-mer decreases with the decreasing initiator concen-tration, whereas the molecular weight of the poly-mer increases. However, in our system, theconversion of St and the molecular weight of thegrafter and PS grafted chain increased simulta-neously with the decreases in the concentration ofP(MMA-co-TBPM) and in the content of TBPM inP(MMA-co-TBPM). For a low P(MMA-co-TBPM)concentration and low TBPM contents, the dormant

amount from coupling of the propagating PS specieswith BTEMPO � formed by decomposition of theTBPM segment in the copolymer was very low, sothe concentration of propagating radical specieswas high, and the polymerization rate, molecularweight, and conversion were also higher,17 but themolecular weight distribution was broader. When ahigh P(MMA-co-TBPM) concentration and highcontents of TBPM in P(MMA-co-TBPM) were used,the concentration of BTEMPO � radicals was alsohigher, and the dormant amount from coupling ofpropagating PS species with BTEMPO � was in-creased; the polymerization of St was strictly con-trolled because of the low concentration of propa-gating radicals, which led to the decrease in themolecular weight and conversion of St.

The authors are thankful for the financial support ofthe Natural Science Foundation of China and the Doc-tor Training Foundation of the Education Ministry ofChina.

REFERENCES AND NOTES

1. Sheiko, S. S.; Gerle, M.; Fischer, K.; Schmidt, M.;Moller, M. Langmuir 1997, 13, 5368.

2. Tsukahara, Y.; Tsutsumi, K.; Yamashita, Y.; Shi-mada, S. Macromolecules 1990, 23, 5201.

Table 3. Bulk Polymerization of St Initiated by P(MMA-co-BTMP)a

Time(h)

Molecular Weight(� 10�4)

PS Mw/Mn

Conversion(%)Grafter PS

P(MMA-co-TBMP)1 1.5 6.3 3.5 1.25 3.93.0 8.7 5.9 1.24 8.25.0 12.8 8.2 1.27 15.67.0 15.9 12.1 1.26 20.7

12.0 17.7 13.9 1.31 27.1P(MMA-co-TBMP)2 1.5 10.7 7.5 1.31 5.4

3.0 14.5 11.3 1.35 9.85.0 18.8 15.7 1.34 18.77.0 21.9 18.1 1.36 25.9

12.0 25.5 21.9 1.40 31.4P(MMA-co-TBMP)4 1.5 11.7 8.1 1.35 7.9

3.0 16.8 12.5 1.34 11.45.0 19.2 15.7 1.38 20.17.0 23.0 19.5 1.41 27.7

12.0 27.3 23.6 1.45 44.1

a Polymerization conditions: [P(MMA-co-TBMP)] � 7.5 � 10�4 mol/L (1.0 mL, 8.9 � 10�3 mol) and temperature � 125 °C.

4402 ZHANG, GUO, AND HUANG

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