Supercritical Fluid Process for the Synthesis of MaleatedPoly(vinylidene fluoride)

Kelly Clark, Sunggyu LeeDepartment of Chemical Engineering, University of Missouri–Columbia, Columbia, Missouri 65211

Poly(vinylidene fluoride) was grafted with maleic anhy-dride monomer via a free-radical mechanism in super-critical carbon dioxide medium. The free-radical initiatorchosen for this study was benzoyl peroxide. The struc-ture of the resultant copolymer pendant groups wasdetermined by 1H NMR spectroscopy to consist of indi-vidual succinyl anhydride functional groups. The degreeof functionalization (graft level) was obtained by FTIRspectroscopy through the correlation of absorbancebands using standard samples. The FTIR analysis indi-cated increased graft level with monomer loading, reac-tion temperature, and treatment time; however, initiatorloading and reaction temperature showed more-com-plex behavior. Graft levels increased at moderate ben-zoyl peroxide initiator loadings (5.0 wt%) and decreasedat the highest initiator loadings (10.0 wt%). POLYM. ENG.SCI., 45:631–639, 2005. © 2005 Society of Plastics Engineers

INTRODUCTION

Poly(vinylidene fluoride) (PVDF) fluoropolymer has ap-plications as a coating material, such as in cable insulationand paints, and as a component of polymer blends becauseit has many desirable properties. These properties includeexcellent electrical, flame, and ultraviolet resistance as wellas low chemical reactivity and high dimensional stability[1]. However, PVDF is inherently incompatible with metalsurfaces (and hydrophilic polymers) and tends to delaminate(or phase separate) because of low interfacial adhesion. It ispossible to improve interfacial adhesion in both laminatesand blends by polymer modification via the addition ofhydrophilic functional groups to the polymer chains, result-ing in reduced interfacial energy. Similar functionalizationtechniques provide the strong interactions necessary to sta-bilize the phase domains of polymer blends and ultimatelyresult in controlled blend microstructures [2, 3].

Maleic anhydride (MA) is one candidate functionalgroup for modification of PVDF, and there is currently onlyone method of synthesizing poly(vinylidene fluoride)-graft-

maleic anhydride (PVDF-g-MA) listed in the open litera-ture. The method uses a microwave plasma grafting processthat treats PVDF film with a low-pressure MA plasma vapor[4]. The microwave plasma process establishes the potentialfor PVDF maleation, although the researchers do not quan-tify the PVDF-g-MA graft level. The omission of a graftlevel makes a comparison with other alternative graftingprocesses impossible. The present study seeks to use super-critical fluid (SCF) graft copolymerization as an alternativemeans of grafting MA onto PVDF homopolymer.

Carbon dioxide is commonly used as the processingmedium in SCF graft copolymerization reactions [5, 6]. Inprior SCF graft copolymerization studies, scCO2 is utilizedas both a solvent for the monomer and initiator and as thereaction medium for graft copolymerization [5, 6]. Thetreated polymer does not dissolve in the scCO2; however,carbon dioxide can function as a plasticizer and increase thefree volume of the polymer during treatment [5, 7, 8]. Thedecreased surface tension and increased polymer surfacearea improve monomer and initiator penetration, resultingin high graft levels [5]. Polymers known to swell in scCO2

include polystyrene, polycarbonate, poly(vinyl chloride),and poly(vinylidene fluoride) [7–11].

There are many additional benefits of using scCO2 as thereaction medium. Initiator efficiency is improved in scCO2

because solvent-solute cage effects are negligible relative toliquid solvents [12–15]. Furthermore, scCO2 does not func-tion as a chain transfer agent in these radical reactions[14–21], unlike other solvents, and it also is easily separatedfrom the products by depressurization. In addition, the dif-fusivity of solute molecules can be an order of magnitudehigher in supercritical fluids than in liquids. Finally, radicaladdition reactions are often accelerated by the thermody-namic pressure effect [15, 22, 23]. For this reason, manyaddition polymerization reactions were first performed atsupercritical fluid conditions, including the polymerizationof low-density polyethylene in supercritical ethylene. Therealization of the benefits of using scCO2 medium in poly-mer synthesis and modification has prompted academic andindustrial researchers to implement these processes.

The goal of this study was to utilize an scCO2 process forthe synthesis of PVDF-g-MA copolymer. Both the synthesis

Correspondence to: S. Lee; e-mail: leesu@missouri.eduDOI 10.1002/pen.20318Published online in Wiley InterScience (www.interscience.wiley.com).© 2005 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—2005

and characterization of the copolymer are described, as wellas an evaluation of the synthesis process based on treatmentconditions. The two characterization techniques used are 1HNMR for structural determination and FTIR spectroscopyfor determination of the graft level. The independent vari-ables investigated in this study include monomer and initi-ator loadings, experimental temperature, and treatmenttime. The effects of monomer and initiator loadings areexamined independent of the effects of temperature andtime.

EXPERIMENTAL

Materials

The PVDF base polymer used for this study is Kynar�761, obtained in powder form (with a 40-�m average par-ticle size) from Atofina Chemicals, Inc. Maleic anhydridebriquettes, benzoyl peroxide (BPO) granules, PRA-gradechloroform (99.9%), and poly(maleic anhydride-alt-1-octa-

decene) (PMAO) powder were obtained from AldrichChemical Co. The PVDF is functionalized with MA byutilizing BPO as a free-radical initiator. Extractions of re-sidual reactants from the copolymer product are performedwith chloroform, and PMAO standards are used in graftlevel calibration. Praxair Inc. supplied the instrument-gradecarbon dioxide (CO2) used in the experiments.

Copolymer Synthesis

For each experiment, 20 g of PVDF powder is mixedwith predetermined amounts of MA and BPO. Maleic an-hydride briquettes and BPO granules are crushed to powderbefore mixing. The quantities of MA and BPO are varied foreach experiment as shown in Table 1. Figure 1 shows the300-cm3 reactor setup used for the experiments. The SS-316bolt-closure vessel, from Autoclave Engineers, Inc., is out-fitted with a MagneDrive impeller system and a thermowell.Pressure measurement is by a Bourdon tube pressure gauge(�7 kPa), and temperature measurement is by a thermocou-

TABLE 1. Copolymer graft levels obtained.

Treatment number MA loading (wt%) BPO loading (wt%) Time (min) Temperature (°C) Pressure (MPa) Avg. graft level (wt%)

D2-1 5 2.5 90 80 12.41 0.118D2-5 5 2.5 180 80 12.41 0.241D2-2 5 2.5 90 90 13.25 0.269D2-3 5 2.5 90 100 14.00 0.311D1-1 5 2.5 180 100 14.00 0.454D1-3 5 10.0 180 100 14.00 0.457D2-6 5 2.5 180 90 13.25 0.559D2-4 5 2.5 90 110 14.63 0.565D1-2 5 5.0 180 100 14.00 0.902D2-8 5 2.5 180 110 14.63 0.957D2-7 5 2.5 180 100 14.00 0.977D1-6 15 10.0 180 100 14.00 1.390D1-4 15 2.5 180 100 14.00 2.600D1-5 15 5.0 180 100 14.00 3.257

FIG. 1. High Pressure Reactor System: 1,CO2 cylinder; 2, nitrogen cylinder; 3, gasbooster; 4, intermediate storage vessel; 5, stor-age vessel pressure gauge; 6, relief valve; 7,high pressure check valve; 8, reactor isolationvalve; 9, MagneDrive assembly; 10, thermo-couple; 11, reactor pressure gauge; 12, impel-ler; 13, furnace; 14, filter; 15, exit valve.

632 POLYMER ENGINEERING AND SCIENCE—2005

ple inserted in the thermowell and attached to an OmegaCN76000 temperature controller. Heating tapes are coupledto the temperature controller. The PVDF, MA, and BPOpowders are vigorously agitated for 5 min in a cellulose bag,and then the resulting mixture is charged to the vessel. Afterthe reactor is sealed, air is purged from the system withCO2.

The reactor is then heated to the predetermined experi-mental temperature while the vessel is pressurized with CO2

from the upstream intermediate storage vessel via the inlettube. BPO decomposition is negligible below 80°C, wherethe half-life (t1/2) is over 120 min, based on published BPOdecomposition data [24]. Heating time from 80°C to theexperimental temperature is consistently less than 10 minfor experiments carried out at reactor temperatures of 80, 90or 100°C. The quantity of BPO that decomposes before thereactor reaches the experimental temperature does not ex-ceed 10%. Also, experiments are carried out at 110°C andrequire an additional 15 min of heating time between 100and 110°C. Although the additional heating time does in-troduce a systematic error, the error is small because theadditional heating time is short relative to the measuredtreatment time. Furthermore, the effects of monomer andinitiator loadings were determined with experiments utiliz-ing a constant 100°C treatment temperature and therefore aconstant heating time.

The vessel is then thermostated at the experimental tem-perature, and the reaction medium is agitated. The interme-diate storage vessel is used to compensate for the pulsatingeffect of the pressurized CO2 delivered by the gas booster.The steady-state treatment time is recorded, and reactorconditions are maintained for the duration of the experi-ment. After each experiment, the reactor is cooled to 25°Cand then the CO2 is discharged, in a controlled manner, overa 1-h interval. The exit line is equipped with a 5 �m filterto collect any entrained polymer or copolymer. The reactorand filter contents are removed and Soxhlet extracted at60°C for 24 h by using chloroform solvent to remove thespent initiator, unreacted monomer, and residual initiatorfrom the copolymer sample. Extracted samples are vacuum-dried to constant weight before analysis. Reactant loadingsand reactor conditions for the various experiments are listedin Table 1. Temperature, time, and reactant loadings arevaried across the experiments, whereas the CO2 density ismaintained at 290 kg/m3 for each experiment. MA and BPOwere shown in prior scCO2 grafting studies to dissolvehomogeneously into CO2 at the loadings and conditionslisted in Table 1 [25].

1H NMR Spectroscopy

Spectra of both neat PVDF and grafted PVDF samplesare obtained by using 1H NMR spectroscopy with a BrukerARX250 NMR spectrometer. Each 10 mg sample is placedin 1 g of acetone-d6 and gently heated for 10 min to dissolvethe polymer. After solvation, each solution is pipetted into aseparate 5 mm NMR tube, and then the NMR tube is placed

in the spectrometer. A 3.146-s acquisition time is used onthe 250-MHz spectrometer, and 1024 transients are super-imposed before Fourier transform for the copolymer sam-ples, whereas 64 transients are used to obtain the neat PVDFspectrum. The low concentrations of grafted species in thecopolymer samples produces small resonance signals andrequires the accumulation of additional transients to im-prove the signal-to-noise ratio of the resultant 1H NMRspectra.

FTIR Spectroscopy

Standard samples of poly(vinylidene fluoride)/poly(ma-leic anhydride-alt-1-octadecene) (PVDF/PMAO) blends areused for the graft level determination. Films of PVDF,PVDF-g-MA, and PVDF/PMAO are prepared for FTIRanalysis. The 40–60 �m films are formed by compressing5 g samples at 44.5 kN and 170°C for 5 min followed byquenching to 80°C before removal.

Each film is scanned with a Thermo Nicolet spectrometerover the frequency range of 650–4000 cm–1 by using a dataresolution of 0.482 cm–1. Gaussian curves are used to fit thepeak areas of the IR absorption bands in both the 1650–1900 and 2800–3300 cm–1 regions of the spectra. The areasof the Gaussians are considered the actual absorbances. Agraft level calibration curve is then generated by using theabsorbance data of both the neat PVDF and standard blendspectra. Graft levels for the experimental copolymer sam-ples are then obtained by using the peak areas of thecopolymer IR spectra and the graft level calibration curvegenerated with the standards.

Graft Level Determination—FTIR

The graft level of each copolymer sample was deter-mined by using FTIR spectroscopy. Graft level is a quan-tification of the functionalization of a copolymer, and it isdefined for PVDF-g-MA as the ratio of the mass of succinylanhydride units to the mass of copolymer, written as aweight-percent as indicated in Eq. 1.

mass of succinic anhydride unit �g�

mass of copolymer �g�� 100% (1)

The copolymer graft levels were determined from the 1783and 2983 cm–1 IR absorbance bands. Figure 2 shows the1650–1900 cm–1 absorbance spectra for both a neat PVDFsample and a PVDF-g-MA copolymer sample. The 1783cm–1 absorbance band in the copolymer spectrum of Fig. 2results from symmetric carbonyl stretching in 5-membercyclic anhydride rings (succinyl anhydride groups) [26] andfrom absorbance by PVDF, observed as the 1783 cm–1

shoulder in the PVDF spectrum. The absorbance at 1783cm–1 is expressed by using the Beer–Lambert law as:

POLYMER ENGINEERING AND SCIENCE—2005 633

A1783 � log�I0

I � � K1�CCAO � K2�CPVDF (2)

where A1783, I, Io, Ki, �, CC�O, and CPVDF are the absor-bance, transmitted intensity, incident intensity, absorptioncoefficients, path length, functional group concentration,and PVDF monomer unit concentration, respectively. Sim-ilarly, the absorbance at 2983 cm–1 can be expressed byusing the Beer–Lambert law as:

A2983 � K3�CPVDF (3)

where A2983 is the absorbance at this frequency. The 2983cm–1 absorbance band is due to the symmetric C-H stretch-ing band [26] of PVDF methylene units, and Fig. 3 shows

the 2800–3300 cm–1 spectrum for a copolymer sample. TheC-H stretching band is shifted to a higher wavenumberrelative to C-H stretching in alkanes because of the masseffect [26]. The ratio of the 1783 cm–1 absorbance over the2983 cm–1 absorbance for each sample spectrum is calcu-lated to remove the path length dependence from the dataanalysis. The resulting equation is:

A1783

A2983�

K1

K3

CCAO

CPVDF�

K2

K3(4)

where A1783/A2983 is the absorbance ratio and CC�O/CPVDF

is the ratio of functional group concentration to PVDFmonomeric unit concentration. The latter ratio is convertedto a graft level as defined in Eq. 1. Absorbance ratios were

FIG. 2. FTIR spectra of PVDF and PVDF-g-MA inthe 1650–1900 cm–1 region.

FIG. 3. FTIR spectrum of PVDF-g-MA copolymerin the 2800–3300 cm–1 region.

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obtained for standard samples, and the ratios of absorptioncoefficients were determined by least-squares regression. Graftlevels were then calculated for the copolymer samples by usingEq. 4 and the absorbance ratios for each sample spectrum.

RESULTS AND DISCUSSION

Copolymer Structure — 1H NMR

A 1H NMR spectrum for copolymer sample D1-4 isshown in Fig. 4, and the corresponding structures are listedin Table 2. The 2.93 ppm quintet and 2.33 ppm triplet peaksare the resonance bands of methylene hydrogen in head-to-tail and head-to-head polymerized VDF monomer units ofthe PVDF base polymer, respectively [27]. The multiplet at2.04 ppm is the resonance peak for acetone-d5 hydrogenresulting from the presence of partially deuterated acetonesolvent. The acetone-d5 peak is the reference peak used forchemical shift determination.

The presence of the peaks at 4.09 and 7.29 ppm is ofinterest, as is the absence of a peak centered at 7.04 ppm.The equivalent hydrogen of monomeric MA resonates at7.04 ppm [28], and the lack of this peak indicates theabsence of unbound MA in the copolymer sample, thusverifying successful extraction of residual monomer. Thebroad peak centered at 7.29 ppm is due to hydrogen attachedto carbon-carbon double bonds along the polymer back-bone. Dehydrofluorination during PVDF heating is a deg-radation mechanism resulting in carbon-carbon double bondformation [29]. The existence of this peak indicates thatsome polymer degradation occurs during grafting. Thebroad peak at 4.09 ppm is the resonance peak of methinehydrogen geminal to the pendant succinyl anhydride groupsof the copolymer samples. Chemical shift prediction soft-ware [30] has confirmed the 4.09 ppm resonance of themethine hydrogen in this structure. The shift of the methinehydrogen resonance to low field is attributed to the presenceof electron withdrawing fluorines at the �-position, relativeto the methine hydrogen. The chemical shift predictionsoftware has also indicated that single succinyl anhydride

TABLE 2. Described structures with resonating hydrogen indicated.

Peak DescriptionLocation

(ppm) Resonating hydrogen

AHead-to-tail PVDF

segment2.93

BHead-to-head

PVDF segment2.33

C Acetone-d5 2.05

DDehydrofluorinated

PVDF segment7.29

ESuccinyl anhydride

bound to PVDF4.09

Maleic anhydride(unbound)

7.04

FIG. 4. 1H NMR spectrum of PVDF-g-MA copolymer sample D1-4 in acetone-d6.

POLYMER ENGINEERING AND SCIENCE—2005 635

units are grafted during the reaction rather than oligomericor polymeric MA chains. A chain structure was ruled outbecause the hydrogen geminal to a grafted oligomeric MAchain is predicted to resonate at 4.32 ppm instead of at theobserved 4.09 ppm value.

The 1H NMR findings suggest a grafting mechanism asshown in Fig. 5. In step 1 of the mechanism, free-radicalinitiator decomposes into primary radicals. These phenyland benzoyl radicals subsequently (step 2) abstract hydro-gen from the PVDF polymer, forming a macroalkyl radical.Succinyl anhydride groups are grafted when the macroalkylradical adds to the double bond of an MA monomer in step3. Radical termination of the succinyl anhydride radicalsoccurs by chain transfer to a species containing abstractablehydrogen, as shown in step 4. Two additional competingreactions are included in the grafting mechanism. Primaryradical recombination is shown in step 5, and it results in theformation of stable byproducts, with an attendant loss ofinitiator efficiency. Radical recombination is an unavoid-able problem associated with free-radical polymerizationand grafting mechanisms. Step 6 shows a competing reac-tion whereby the carbon-carbon double bond structures ob-served in the 1H NMR spectrum result from loss of fluorinefrom a carbon neighboring the macroalkyl radical producedin step 2. The dehydrofluorination competes for macroalkylradicals and reduces the efficiency of the grafting reactions.

Effects of Treatment Conditions on Graft Level

The graft levels for each of the copolymer samples arelisted in Table 1. The effects of monomer and initiator

loadings on graft level were determined while maintaining areaction temperature of 100°C and a treatment time of 180min. Figure 6 is a plot of copolymer graft level as a functionof the monomer and initiator loadings. Graft level increaseswith MA loading for each of the different initiator loadings.The explanation for this trend is that increased MA concen-tration drives the macroalkyl radical addition (step 3 of Fig.5) toward the grafted product, according to Le Chatelier’sprinciple. The same trend is observed in scCO2 grafting ofstyrene monomer onto polypropylene base polymer [5].

The effect of initiator loading on graft level is also shownin Fig. 6. At each monomer loading, graft level increaseswith small increases in BPO and then decreases at higherinitiator loadings. The increase in graft level at moderateBPO loadings is due to the increased radical concentrationand a corresponding increase in hydrogen abstraction fromthe PVDF homopolymer. The subsequent decrease in graftlevel at higher BPO loadings results from the prevalence ofradical reactions that compete with initiation and radicaladdition (steps 5 and 6 in Fig. 5). Similar behavior can beseen for several free-radical grafting systems including sol-id-phase grafting of acrylic acid on polystyrene, whereincreasing radical concentrations resulted in a homopoly-merization of the acrylic acid monomer instead of grafting[31]. This behavior suggests that graft level can be opti-mized with respect to BPO loading.

The effects of temperature and time on the graft levelwere determined while the monomer and initiator loadingswere held constant at 5 and 2.5 wt%, respectively. The CO2

density was maintained at a constant 290 kg/m3 for theseexperiments. Figure 7 shows the graft level as a function of

FIG. 5. Suggested PVDF-g-MA grafting mechanism.

636 POLYMER ENGINEERING AND SCIENCE—2005

temperature for both 90- and 180-min treatments. As shownin Fig. 7, graft level increases with increasing treatmenttime at each experimental temperature. However, consider-ing the t1/2 of BPO at 100°C (�20 min) and 110°C (�10min), it is likely that free-radical concentrations diminish inthe 100 and 110°C samples after 180 min and that thepotential for further grafting is reduced. Previous scCO2

grafting work has shown that at long treatment times, graftlevel ceases to increase with increasing time because of theexhaustion of initiator in the system [5]. In contrast to the100 and 110°C treatments, considerable initiator remainsafter 180 min at 80 and 90°C because the t1/2 of BPO isapproximately 150 and 50 min at these respective temper-atures. This leaves the possibility open for graft level in-creases beyond 180 min for both 80 and 90°C treatments.

Figure 7 shows the effect of temperature and time on thegraft level of the copolymer samples. For both 90 and 180

min treatments, increasing the experimental temperaturealong the series 80, 90, and 100°C results in increased graftlevels. This result is supported by previous grafting work inwhich poly(vinyl chloride)-graft-polystyrene (PVC-g-PS)and polypropylene-graft-polystyrene (PP-g-PS) were syn-thesized separately in scCO2 media [5]. Graft levels ofPVC-g-PS and PP-g-PS copolymer both increased withincreasing reaction temperature in the previous work [5].The increased graft level in the current work, as well as inthe prior work, results from an increase in free-radicalconcentrations in the reactor with increased temperature,which helps to drive the reaction toward the grafted product.The additional experiments at 110°C produced copolymerwith average graft levels of 0.565 (at 90 min) and 0.957wt% (at 180 min). Although this result fits the trend shownin Fig. 7, it is important to note that the initial heating periodfor the 110°C experiments is 15 min longer, and significant

FIG. 6. Effects of BPO and MA loadings on graftlevel. The data points shown include only experimentscarried out at 100°C for 180 min.

FIG. 7. Effects of reaction temperature and treat-ment time on graft level. The data points shown in-clude only experiments carried out with 5 wt% MAand 2.5 wt% BPO.

POLYMER ENGINEERING AND SCIENCE—2005 637

BPO decomposition occurs during the 15 min period. Al-though increasing the temperature promotes grafting for theexperimental conditions used in this study, there is littlebenefit in increasing the temperature above 110°C. As themelting point of PVDF (�160–170°C) is approached, thepolymer flows and does not retain a powder form.

One additional aspect to consider is the efficiency ofBPO utilization. Four factors contribute to the generally lowinitiator efficiency of the scCO2 grafting process as prac-ticed in this study. The first factor is that initiator decom-position reactions (especially for experiments performed at80 and 90°C), as well as grafting reactions, were not al-lowed to go to completion. Instead, the experiments wereterminated after designated treatment times. As an example,D2-5 and D2-1 treatment conditions differ only by thelength of treatment time, and the D2-5 samples were treatedfor 180 min and have a higher average graft level than D2-1samples that were treated for only 90 min. Allowing D2-1samples to react for an additional 90 min would haveresulted in increased graft levels. The balance of the BPO inthese experiments is extracted from the sample prior toanalysis. The second factor is that BPO participates inreactions that do not lead to grafting. An example of thesetypes of reactions is radical recombination, whereby one ofthe BPO decomposition products recombines with anotherdecomposition product to produce a stable species. In-creased BPO loadings increase the likelihood of the occur-rence of radical recombination events. These BPO decom-position and recombination products are removed from thecopolymer samples during chloroform extraction. The thirdfactor is that there is an important distinction between thegraft copolymerization of easily polymerizable species(e.g., styrene or acrylics) and species for which monomeraddition is sterically hindered (i.e., maleic anhydride). Lowinitiator loadings are required when grafting easily poly-merizable species such as styrene because monomers rap-idly attach to the growing pendant radical groups in apolymerization reaction. Increased graft levels of MA arenot achieved by MA polymerization; rather, bound MAunits undergo chain transfer reactions and do not polymer-ize. Repeated addition of single MA units to the backboneis therefore necessary, and this process requires higherconcentrations of initiator relative to the initiator concen-trations required when polymerizing other species. Thefourth factor contributing to the use of excess BPO is thefact that dehydrofluorination of PVDF also contributes to aloss in initiator efficiency. Dehydrofluorination, followingmacroalkyl radical formation, reduces the number of radicalsites on the polymer where grafting can occur.

CONCLUSIONS

Grafting of PVDF with MA via an scCO2 process issuccessful, producing copolymer with a range of graft levelsbetween 0.118 and 3.26 wt%. 1H NMR spectroscopy indi-cates a resonance signal at 4.09 ppm that is consistent withsingle succinyl anhydride functional groups, as opposed to

oligomers or chains. The absence of a 1H NMR peak cen-tered at 7.09 ppm verifies complete extraction of unboundMA from the copolymer samples. FTIR analysis of thecopolymer samples shows that the effect of increasing themonomer loading is to increase graft level, while the effectof increasing the initiator loading is an initial gain followedby a drop in graft level at higher BPO loadings. The FTIRanalysis also confirms that graft level increases with reac-tion time and temperature. Furthermore, increasing the re-action time for 80 and 90°C treatments has a potential forincreasing graft level above the level obtained at 180 min.

ACKNOWLEDGMENT

Partial funding of the 250-MHz NMR facility is providedby National Science Foundation grant number NSF CHE-92-2183526.

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