Synthesis and Inhibition Performance of a Polymer-Supported Inhibitor

PENG YANG,1,2 YUFENG SUN,1,2 JIANYUAN DENG,1,2 WEINA LIU,1,2,* LI ZHANG,1,2,† WANTAI YANG1,2

1Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education, Beijing, China

2Department of Polymer Science, Beijing University of Chemical Technology, Beijing, China, 100029

Received 9 December 2003; accepted 3 April 2004DOI: 10.1002/pola.20238Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The synthesis of a polymer-supported inhibitor (PSI) and its inhibitionperformance for free-radical polymerization are reported for the first time. A specialmethod has been devised to synthesize PSI with pure and abundant hydroquinone (HQ)groups anchored onto the polymer surface. A thin HQ/acetone (AC) solution is sand-wiched between two polymer films. Under ultraviolet irradiation, AC as an photoini-tiator quickly and effectively grafts HQ onto the polymer surface. PSI has been char-acterized with ultraviolet–visible and attenuated total reflectance/Fourier transforminfrared spectroscopy. For potential applications, PSI has been used to inhibit thethermal polymerization of styrene and methyl methacrylate. The corresponding inhi-bition performance has been investigated through the measurement of the inductionperiod with the dilatometer method. With the same absolute amount, the maximuminhibition ability of PSI approaches half that of a free inhibitor. Increasing the disper-sion degree of PSI is favorable for the enhancement of the inhibition ability. © 2004 WileyPeriodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4074–4083, 2004Keywords: functionalization of polymers; inhibitors; photografting; supports; sur-faces

INTRODUCTION

During the transportation and shelf life of purecommercial monomers, the autoinitiated poly-merization of the monomers takes place easilywithout the addition of an initiator added; it isoften induced by heat, ultraviolet (UV) irradia-tion, or some impurities such as peroxy or hy-droperoxy molecules. The aftermath of this auto-initiated polymerization is so serious that certain

procedures must be used to suppress it. Besidesstorage at low temperatures and the avoidance ofdirect irradiation by sunlight, it is necessary toadd inhibitors to the pure monomers. These in-hibitors include stable free radicals (e.g., 2,2,6,6-tetramethylpiperidinyl-1-oxy and 1,1-diphenyl-2-picryhydraz), quinones (e.g., benzoquinone), phe-nolics [e.g., hydroquinone (HQ)], aromatic dinitroand trinitro compounds, and a large number ofother substances such as oxygen, sulfur, and fer-ric chloride.1 Therefore, it is necessary to cleanout these inhibitor molecules from the monomersbefore polymerization. In laboratories and plants,two purifying methods, distillation and columnchromatography, have been used. Some draw-backs are inevitable, such as tedious procedures,a large loss of the monomers, certain environmen-tal pollution, and resultant cost increases.

*Present address: Department of Chemical Engineering,Columbia University, New York, NY 10025

†Present address: State Intellectual Property Office, Peo-ple’s Republic of China

Correspondence to: W. Yang (E-mail: yangwt@mail.buct.edu.cn)Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 4074–4083 (2004)© 2004 Wiley Periodicals, Inc.

4074

Since the solid-phase method for oligopeptidesynthesis was introduced by Merrifield in 1963,2

much attention has been paid to polymer-supportedreagents and catalysts, which are applied in manyareas, including organic synthesis, ion exchange,and pharmaceutical and biological applications.3–5

For example, Tanyeli and Gumus6 prepared a recy-cled polymer-supported 2,2,6,6-tetramethylpiper-idinyl-1-oxy as a catalyst in the Anelli oxidation ofvarious prime alcohols; Chun and Beard7 modifiedthe surface biocompatibility of a polypropylene (PP)film with the UV-induced liquid-phase graft reac-tion of HQ onto a PP film surface with benzophe-none (BP) as the photoinitiator, and Kurth et al.8

prepared water-soluble antioxidants through thecoupling of some antioxidants with Merrifield’sresin and further derivation. The most importantadvantage of a polymer-supported reagent is thesimplification of the reaction workup (e.g., productseparation and isolation) because of its heteroge-neous characteristics.

Enlightened by these approaches, we proposefor the first time that if some inhibitors can beanchored onto polymer support, the resulting het-erogeneous phases (a monomer liquid and a solidpolymer material with the inhibitor immobilized)will make it possible for the inhibitors to be re-moved from the monomers by simple procedures,such as filtration, without other boring proce-dures. We have named the immobilized inhibitora polymer-supported inhibitor (PSI). As a firsteffort, referring to Beard’s work, we have synthe-sized PSI with phenolic groups by a surface pho-tografting procedure, and we have evaluated itsperformance as an inhibitor of the free-radicalpolymerizations of methyl methacrylate (MMA)and styrene (St).

EXPERIMENTAL

Materials and Reagents

Casting polypropylene (CPP) films (30 �m thickwith a density of ca. 0.9 g/cm2) were purchased

from Guangdong Foshan Plastic Co., Ltd. (China),and then were subjected to Soxhlet extractionwith acetone (AC) for 24 h to exclude impuritiesand additives before use. AC and HQ were usedas received. Benzoyl peroxide (BPO) and azobi-sisobutyronitrile (AIBN) were purified before useby recrystallization. St and MMA were purchasedfrom Tianjin No. 1 Chemical Reagent Factory andBeijing Chemical Reagents Co., respectively, andthe inhibitors added to the two monomers arelisted in Table 1. To investigate the inhibitionperformance of PSI, we purified the monomers bydistillation under reduced pressure.

Preparation of PSI

A schematic picture of the photografting setup isshown in Figure 1. A predetermined amount of anAC solution of HQ (HQ/AC) was deposited ontothe bottom film (5 � 5 cm2) with a microsyringe.The top film (5 cm � 5 cm) covered this solution,and a drop of the solution was spread into an evenand very thin liquid layer under suitable pressurefrom a quartz plate. Then, the assembly was laidon the holder and was irradiated with a high-pressure mercury lamp (1000 W; the UV intensityat � � 254 nm was ca. 8000 �w/cm2) from thetopside. The reaction temperature was controlledwith a water bath. After the irradiation, the topfilm was rinsed with copious AC for the absoluteremoval of unreacted HQ and dried under theatmosphere to a constant weight. The graftingyield (GY) of the HQ groups was determined ac-cording to the following formula:

GY (%) � �M1 � M0�/M0 � 100% (1)

where M0 (g) is the mass of the original CPP filmand M1 (g) is the mass of the HQ-g-PP film.

Characterization of PSI

The ultraviolet–visible (UV–vis) absorption spec-tra were recorded with a GBC Cintra 20 spectro-

Table 1. Formulas of the Commercial Monomer and Corresponding Inhibitor Added

Monomer Amount I/Ma Density

MMA 500 mL 0.94 g/cm3

p-Methoxyphenol 0.12–0.16 g ca. 0.026–0.035% —St 500 mL 0.91 g/cm3

HQ 0.05–0.22 g ca. 0.011–0.048% —

a Mass ratio of the inhibitor to the monomer.

POLYMER-SUPPORTED INHIBITOR 4075

photometer (Australia). Fourier transform infra-red (FTIR) spectra were recorded on a NicoletNexus 670 spectrometer with a 4-cm�1 resolution,and a variable-angle attenuated total reflectance(ATR) accessory (Pike ATRMax II) was used withZnSe (n � 2.43) as an internal reflection elementwafer. Gravimetric analysis was performed witha BP211D electrobalance (Sartorius AG, Ger-many) with an accuracy of 0.00001 g.

Evaluation of PSI

The inhibition performance of PSI was investi-gated with the common dilatometer method. Pu-rified MMA (6.5 mL) was polymerized at 50 °Cwith BPO as an initiator (1 wt %) and a certainamount of PSI (GY � 0.8 wt %) added. The poly-merization temperature was controlled to within0.1° with a platinum temperature sensor. Similardilatometer procedures were performed at 60 °Cfor 6.5 mL of St with AIBN as an initiator (2 wt %)and a certain amount of PSI (GY � 0.8 wt %)added. An inert gas was not used to saturate thesolution because HQ did not operate as an inhib-itor without oxygen.9,10

RESULTS AND DISCUSSION

Synthesis and Characterization of PSI

The pioneering work on the immobilization of HQon polymer supports was performed by Chun andBeard.7 They performed the solution photograft-ing of HQ onto a PP film surface with BP as thephotoinitiator to modify the surface biocompat-ibility of the PP film, and the maximum graft

yield was about 7.5 wt % after 200 min of irradi-ation in a liquid photoreactor.

Referring to the aforementioned work and forPSI applications, we have established a new syn-thesis strategy, as shown in Scheme 1. In thisstrategy, there are two unique points: (1) replac-ing BP with AC as a photoinitiator and (2) usinga sandwich assembly instead of a liquid reactor.In addition, a PSI inhibition principle is expected.The photoinitiation performance of AC in surfacephotografting polymerization has been report-ed.11,12 Yang and Rånby11 pointed out that someketones (including AC) could be decomposed un-der UV irradiation to yield fragment radicals,which were mainly capable of initiating homopo-lymerization; on the other hand, these ketoneswere activated by UV to abstract hydrogen fromthe polymer surface, and this was followed bysurface-initiated radical grafting. However, wehave no way of proving that fragment radicalscannot abstract hydrogen from the substrate, inthat we have proposed that the possible synthesismechanism of PSI is based on abstracting hydro-gen cocontributed by the activated AC (route A)and the fragment radicals (route B), as shown inScheme 1.

In another work,13 the special reaction charac-ter of this sandwich setup was introduced in de-tail: the confined interface reaction and strongUV radiation made the reactions between SO4

�

and the polymer surface, including the abstract-ing surface hydrogen atom and coupling with thesurface free radical, become the main part in thewhole oxidation course. Herein with the sandwichreaction setup, we expected that, under strongUV irradiation, a great amount of AC could be

Figure 1. Setup used in the photografting of HQ groups onto PP films.

4076 YANG ET AL.

activated in a short time, which could result inthe formation of both phenolic radicals and sur-face radicals, and at last, desired photograftingHQ reactions could be achieved. Another merit ofthe strategy is that PSI, having a pure array ofHQ groups without other interfering groups,could be obtained. If BP were used, it could attachitself onto the polymer surface as a semibenzopi-nacol group by a similar mechanism, as reportedin some references (including ours).14

Figures 2 and 3 show the UV–vis absorptionspectra of HQ/AC and HQ-g-PP films, respec-tively. In Figure 2, the absorption bands of HQ inthe AC solvent appear at about � � 220 nm and �� 290 nm. Similar absorption bands also appear

in the UV–vis absorption spectra of HQ-g-PP, andwith increasing irradiation time, the intensity ofthe absorption peak increases (cf. Fig. 3). Thespectra of the HQ-g-PP film (230 and 290 nm) area little different from the spectra of pure HQdissolved in AC (220 and 290 nm). It is wellknown that UV–vis spectra are notably influ-enced by the measuring environment (e.g., a sol-vent) and the molecular structure of the chro-mophore. Some changes in a HQ-g-PP film can beattributed to this difference, such as an immobi-lized chromophore (HQ) on a solid surface in theatmosphere; one of two phenolic hydroxyl groupsare chemically bonded to the substrate surface

Figure 2. UV–vis absorption spectra of HQ dissolvedin AC (10 wt %).

Figure 3. UV–vis absorption spectra of HQ-g-PPfilms with different irradiation times: (a) 10 and (b) 20min (10 wt % HQ and 40 °C).

Scheme 1. Synthesis strategy and inhibition mechanism of PSI.

POLYMER-SUPPORTED INHIBITOR 4077

carbon atom. Contrasting Figure 3 with Figure 2,we can believe that HQ groups have been addedto the surface of the CPP film successfully. Theabsorption band of carbonyl in AC is at about �� 270 nm, and this band does not appear in theabsorption spectra of the HQ-g-PP film. This find-ing shows that after irradiation, carbonyl groups(AC fragments) are not implanted onto the mod-ified surface.

Further evidence proving this conclusion isprovided by ATR–FTIR. Figure 4 presents ATR–FTIR spectra of PP and HQ-g-PP samples withdifferent GY values. The absorption in the regionof 1600 cm�1 might be related to the aromaticCAC band of HQ.15 The aromatic COO band16

around 1250 cm�1 and the OH band around 3300

cm�1 are also shown in the spectra. The intensityof these bands increases with increasing GY. TheCAO band around 1715 cm�1 does not appear inthe spectra of the HQ-g-PP samples, and thisfurther supports the idea that no carbonyl groupsare added to the HQ-g-PP surfaces. The distribu-tion of HQ along the profile of a modified CPP filmhas been investigated preliminarily with ATR–FTIR with variant incidence angles, as shown inFigure 5. The absorbance ratio of the aromaticCAC band at 1600 cm�1 to the methyl stretchingband at 2915 cm�1, that is, I(HQ/CH3), increasesas the incidence angle increases gradually. Ac-cording to the nature of the ATR–FTIR technique,the sampling depth decreases with an increasingincidence angle, so the results show that the mod-ification layer is not a single molecular array butinstead is multilayer with a certain depth atwhich the number of HQ groups decreases grad-ually from the outer surface to the subsurface. Apossible interpretation is based on the good affin-ity between the AC and CPP substrate. In gen-eral, the better the affinity is, the more easily the

Figure 4. ATR–FTIR spectra of HQ-g-PP with differ-ent GY and PP samples: (a) PP, (b) HQ-g-PP (GY� 0.8%), and (c) HQ-g-PP (GY � 1.7%).

Figure 5. Profile analysis for the HQ distribution onan HQ-g-PP sample (GY � 1.7%): (a) incidence angle� 45°, (b) incidence angle � 55°, and (c) incidence angle� 65°.

Figure 6. Effect of the HQ concentration on GY ofHQ-g-PP (irradiation time � 20 min, temperature � 40°C).

Figure 7. Effects of the reaction temperature andirradiation time on GY of HQ-g-PP (10 wt % HQ): (F)20, (Œ) 40, and (■) 60 °C.

4078 YANG ET AL.

substrates are wetted. A high level of affinity isthus favorable for HQ in AC to penetrate thesubstrate. Accordingly, grafting occurs under theoutmost layer and even into the deeper innerlayer.

Figure 6 presents the effect of the HQ concen-tration on GY of a HQ-g-PP film at a certainirradiation time (20 min). GY increases with in-creasing HQ concentration up to the maximum(ca. 10 wt % in the AC solvent).

Figure 7 shows that GY can be convenientlycontrolled by changes in the irradiation time. At acertain temperature, GY increases from 0 to

about 1.7% and reaches a plateau at about 20min. At the initial stage, the reaction extent pro-ceeds as the irradiation time is prolonged; this isreflected in the increase of GY. When the irradi-ation time is up to about 20 min, GY reaches aconstant value, and this means that the numberof HQ groups attached to the CPP film is satu-rated because of the limited CPP film surfacearea. With the irradiation time unchanged, GYincreases from 20 to 40 °C and then decreasesfrom 40 to 60 °C. For a certain irradiation time,the order of GY is as follows: 40 � 60 � 20 °C.This phenomenon could be similar to some re-

Figure 8. Dilatometer experimental plots (scatter) and linear fitting curve (linear) forMMA: the capillary level (height) versus the time with different x values and freeinhibitor added.

POLYMER-SUPPORTED INHIBITOR 4079

ports about the photografting of vinyl acetate ontolow-density polyethylene films by Deng andYang17 and could be explained as follows. As aresult of increasing temperature, the activity ofthe macroradicals and HQ molecules possessingoxy radicals increases greatly. In addition, in-creasing the temperature causes HQ moleculespossessing oxy radicals to distribute rapidly, andthis results in a greater opportunity to couplewith surface macroradicals. As a result, increas-ing the temperature is favorable for implantingHQ groups onto the CPP surface; for example, GYincreases from 20 to 40 °C. However, if the reac-tion temperature is too high (e.g., up to 60 °C), ACvolatizes violently; on the other hand, the extentof the thermodegradation of a PP film and thethermally induced dissociation of an HQ groupfrom a PP chain increase with increasing irradi-ation time. Both result in the reduction of GYfrom 40 to 60 °C.

Evaluation of the Inhibition Performance of PSI

HQ is a phenolic inhibitor, and the accepted opin-ion about the inhibition mechanism is that a poly-mer chain-carrying radical or peroxy radical canbe deactivated through the abstraction of a hydro-gen atom from a hydroxyl group of HQ. The re-sulting oxy radical (semiquinone radical) com-bines with another active chain free radical or isoxidized further into a quinone group, and theresulting quinone can inhibit free-radical poly-merization further.18 Accordingly, a possible inhi-bition mechanism of anchored HQ is summarizedin Scheme 1.

For common inhibition, both inhibitor mole-cules and small or short-chain active free radicalscan move freely in three-dimensional (3D) spacein a monomer solution, and the inhibition reac-tion is achieved through a collision between them.Conventional inhibition is a model in which in-hibitors in 3D space react with free radicals in 3Dspace to terminate further free-radical polymer-ization (i.e., a 3D-to-3D model).

Different from common small-molecule free in-hibitors, PSI is a kind of inhibitor anchored onto atwo-dimensional (2D) surface of solid support ma-terials. The inhibition reaction is achievedthrough the collision of active free radicals in 3Dspace and inhibitor molecules on a 2D surface(3D-to-2D model). The transformation addressesthe following important problems, which we callimmobilization effects: (1) the moving ability ofsolid PSI is depressed largely because of the in-troduction of an enormous mass of polymer sup-port and will increase with a reduction in the PSIsize and (2) a volume-exclusion or steric hin-drance effect arises when free radicals are closedto inhibitors. All these factors may greatly reducethe probability of collision to achieve the inhibi-tion reaction successfully. Therefore, the inhibi-tion performance of PSI must have a significantrelationship with the dispersion situation of PSIin a monomer liquid.

On the basis of these considerations, we pro-pose the concept of inhibition factor F(x), which isdefined as the ratio of induction period T(x) of PSIwith dispersion degree x to induction period Tf ofa small-molecule free inhibitor at the same con-centration:

Figure 9. Experimental (scatter) and exponential fit-ting curve (dashed) plots of F(x) versus x for MMA.

Table 2. Dataset of x, Sp, T(x), and F(x) for MMA

x(number) 0 16 32 64 128 �

Sp (cm2) Sw � 25 3.125 1.563 0.7813 0.3906 0T(x) (min) 11.62 13.90 16.23 21.35 24.81 29.06F(x) 0.1784 0.2134 0.2492 0.3278 0.3809 0.3544

4080 YANG ET AL.

F�x� � T�x�/Tf (2)

Apparently, F(x) is a measurement of the immo-bilization effect. Because of this effect, F(x) is lessthan 1; that is, the inhibition performance of PSIis lower than that of the free inhibitor [T(x) � Tf].Changing x, we could obtain experimental plots ofF(x) and x, and after curve fitting, a formula couldbe obtained to express the correlation between

the inhibition ability and dispersion degree ofPSI.

Experimentally, we used the induction periodobtained by the dilatometer polymerizationmethod to measure the immobilization effectquantitatively. According to the inhibitor concen-tration in the commercial monomer, we took twoHQ-g-PP films in every polymerization sample(the total grafted HQ amount was about equal to

Figure 10. Dilatometer experimental plots (scatter) and linear fitting curve (linear)for St: the capillary level (height) versus the time with different x values and freeinhibitor added.

POLYMER-SUPPORTED INHIBITOR 4081

that of the free inhibitor dissolved in the commer-cial monomer) and then divided the HQ-g-PP film(the area is defined as Sw) to obtain x number ofsmaller pieces with certain area Sp. When thepolymerization was performed, these dividedfilms could be suspended in a monomer (MMA orSt) solution without aggregating or floating (be-cause of similar density) and played their inhib-iting function:

x � 2Sw/Sp (3)

Inhibition Performance of PSI with MMA as aMonomer and Mathematical Analysis

According to the procedures reported in the Ex-perimental section, a bulk MMA solution (afterthe removal of the inhibitor by distillation) withBPO as an initiator (1 wt %) at 50 °C was poly-merized to investigate the induction period withdifferent values of x. Figure 8 summarizes theexperimental results with different values of xand with free inhibitor being added (i.e., themonomers were used directly without distilla-tion).

Through the linear fitting of the scatter plots,we obtained the intercept at the X axis of thelinear fitting equation, that is, induction periodT(x). With eq 2, F(x) has been calculated and islisted in Table 2. As dispersion degree x increases,the induction period of PSI increases and F(x)increases. For example, when x increases from 16to 64, the induction period changes from 13.90 to21.35 min, and F(x) increases from 0.2134 to0.3278.

The relationship between F(x) and x is plottedin Figure 9, and by exponentially fitting thesescatter plots, we obtained a special equation:

F�x� � 0.3544 � 0.1828 e�0.0246x (4)

The fitting curve (dashed) is close to the experi-mental plot (except for the data at x � 128); thisshows that the distribution law of these experi-mental data agrees with eq 4. With eq 4, we havederived two limiting values about F(x): (1) when xapproaches 0, F(x) approaches F(0), which is0.1784, and (2) when x approaches infinity, F(x)approaches F(�), which is 0.3544. F(0) representsthe inhibition efficiency of oxygen dissolved in amonomer solution. F(�) may represent the maxi-mum inhibition efficiency of PSI corresponding to

the commercial free inhibitor for the MMA mono-mer.

Table 2 shows an interesting phenomenon:F(128) is larger than F(�). After investigating theinhibition performance of PSI with St as themonomer, we found a similar phenomenon. Thismay due to some inevitable inaccuracies in theexperiments and the faultiness of the selectivefitting equation and needs to be further explored.

Inhibition Performance of PSI with St as aMonomer and Mathematical Analysis

Similarly to the analysis with MMA, a bulk Stsolution (after the removal of the inhibitor bydistillation) with AIBN as an initiator (2 wt %) at60 °C was used to investigate the induction periodwith different values of x. Figure 10 summarizesthe results with different x values and with freeinhibitor being added (i.e., the monomers wereused directly without distillation). The relation-ship between F(x) and x is plotted in Figure 11,and by exponentially fitting the scatter plots inFigure 11, we obtained a special equation of F(x)for St:

F�x� � 0.3991 � 0.1421 e�0.0347x (5)

Although some disagreements exist, the fittingcurve (dashed) is close to the experimental plot.With eq 5, we have derived two limiting valuesabout F(x): (1) when x approaches 0, F(x) ap-proaches F(0), which is 0.2217, and (2) when xapproaches infinity, F(x) approaches F(�), whichis equal to 0.3991.

Figure 11. Experimental (scatter) and exponentialfitting curve (dashed) plots of F(x) versus x for St.

4082 YANG ET AL.

Table 3 lists data for F(x) and its related pa-rameters. We have found that with increasing x,the induction period of PSI is prolonged; this issimilar to the situation for MMA. The limitingvalue is approaching 0.5, which is similar to thatfor MMA. A possible interpretation is that an-chored HQ has only one hydroxyl group in thebenzene ring, whereas free HQ has two suchgroups. Under limiting conditions, the dispersiondegree has little influence on the inhibition per-formance because the dispersion degree of PSIhas reached microscopic distribution; this is sim-ilar to the distribution of the free inhibitor in 3Dspace. Therefore, in contrast with the free inhib-itor, the fact that the functionality number of PSIdecreases 50% may result in a certain reductionof the inhibition performance.

CONCLUSIONS

PSI has been synthesized through the photograft-ing of HQ groups onto a CPP film surface (HQ-g-PP), and this has been confirmed with UV–visand ATR–FTIR spectroscopy. Increasing the irra-diation time, HQ concentration, and reaction tem-perature obviously increases GY. With an in-creasing dispersion degree, the inhibition abilityof PSI is enhanced. The maximum inhibition abil-ity of PSI obtained by us approaches half that of afree inhibitor.

As an inhibitor for the autoinitiated free-radi-cal polymerization of a commercial monomer, PSIcan be separated from the monomer by simplefiltration, instead of conventional and boring pu-rification methods such as column chromatogra-phy and distillation. This function comes from itsoriginal heterogeneous character. Based on a spe-cial heterogeneous inhibition model, a newmethod has been devised to evaluate the inhibi-tion performance of PSI quantitatively. There-fore, it is quite believable that if much attention ispaid to this research by a polymer chemist, apolymer physicist, a material scientist, an engi-

neer, and others, a new and unexplored researcharea concerning immobilized inhibitors will beopened up.

The authors gratefully acknowledge the financial sup-port of the Chinese State Outstanding Youth Founda-tion (20025415).

REFERENCES AND NOTES

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Table 3. Dataset of x, Sp, T(x), and F(x) for St

x (number) 0 16 32 64 128 �

Sp (cm2) Sw � 25 3.125 1.563 0.7813 0.3906 0T(x) (min) 2.88 4.26 4.61 4.86 5.52 5.33F(x) 0.2217 0.3279 0.3548 0.3739 0.4242 0.3991

POLYMER-SUPPORTED INHIBITOR 4083