Synthesis of Maleic Anhydride Grafted Polyethylene and Polypropylene,with Controlled Molecular StructuresBING LU,T.C.CHUNGDepartment of Materials Science and Engineering,The Pennsylvania State University,University Park,Pennsylvania16802Received4October1999;accepted27January2000ABSTRACT:This article discusses a new chemical route to prepare maleic anhydride(MA)grafted polyethylene and polypropylene polymers with controlled molecular struc-ture,that is,MA grafted content and polymer molecular weight and compositiondistributions.The chemistry involves a free radical graft reaction of maleic anhydridewith poly(ethylene-co-p-methylstyrene)and poly(propylene-co-p-methylstyrene)copoly-mers.Under a suspension reaction condition,the grafting reaction takes place selec-tively on the p-methylstyrene units in the copolymer,due to high reactivity of p-methylgroup and favorable mixing between p-methylstyrene units and chemical reagents inthe swollen amorphous phases.The resulting polymer shows no detectable molecularweight change during the reaction,and the MA grafted content increases with theincrease of initiator and p-methylstyrene concentrations.©2000John Wiley&Sons,Inc.J Polym Sci A:Polym Chem38:1337–1343,2000Keywords:maleated PE;maleated PP;functional polyolefin;polyethylene copoly-mer;polypropylene copolymerINTRODUCTIONMaleic anhydride(MA)modified polyolefins,in-cluding PE,PP,and EPDM,have been one of the most important class of functional polyolefins in industry,because of low cost of maleic anhydride and high activity of the anhydride group.1MA modified polyolefin considerably improve adhe-sion and hydrophilicity of the polymer,and com-patibility with polar polymers.As a results,MA modified polyolefins are used in many commercial applications,such as glassfiber reinforced poly-olefins,2anticorrosive coatings for metal pipes and containers,3multilayer sheets of paper for chemical and food packaging,4and polymer blends with polyamides and polyesters.5–7 MA modified polyolefin is normally prepared by free radical grafting reaction of maleic anhy-dride with the corresponding polyolefin in the presence of organic peroxide initiator either in solution,8–12or in melt.12–23Equation1illus-trates the reaction mechanism of MA graftingpolypropylene.It is generally believed that the grafting reaction starts with hydrogen abstraction by alkoxyl rad-ical,upon the thermal decomposition of peroxideCorrespondence to:T. C.Chung(E-mail:chung@ems.)Journal of Polymer Science:Part A:Polymer Chemistry,Vol.38,1337–1343(2000)©2000John Wiley&Sons,Inc.1337initiator.The formed macroradical1,20–22in poly-olefin subsequently reacts with maleic anhydride monomer.Many experimental results show that the incorporated MA unit is predominately a sin-gle succinic anhydride group,22,23due to poor ho-mopolymerization capability of MA monomer.As expected,many side reactions also take place dur-ing this free radical grafting reaction.In PP case, most of tertiary macroradicals formed involve fast intramolecular-scission reaction and degrade the PP chains,as illustrated in eq1.On the other hand,some of the secondary macroradicals18,19 formed in polyethylene chain couple each other to produce the crosslinked product.Many works have been done in optimizing the reaction condi-tions and extruder parameters to promote the desired reactions while suppressing the unde-sired ones.8–20However,because of the inherent complexity of free radical reactions,it is very dif-ficult to incorporate the desired MA content with-out extensive side reactions.Thefinal product is usually a complicated mixture.Unfortunately, the performance of MA modified polyolefin is very much dependent on its molecular structure,as well as the MA content.It is therefore very important to develop the new method that can prepare MA modified polyolefin with controllable molecular structure.In our previous article,24we have showed a synthetic route for the preparation of maleic an-hydride terminated PP polymers with single or multiple MA units located at the polymer chain end.The chemistry involves the borane-termi-nated PP and a free radical graft from reaction of maleic anhydride.In this article,we will discuss a new chemical route for preparing maleic anhy-dride grafted PE and PP polymers,having the MA units pending along the polymer chain.The reac-tion allows good control of MA grafted copolymers with desirable molecular structure,that is,MA content and molecular weight and composition distributions.RESULTS AND DISCUSSIONThe chemistry is based on the“reactive”polyole-fin copolymers containing p-methylstyrene(p-MS)units that provide the selective reaction sites for free radical modification reaction.Equation2 illustrates the main differences between this ap-proach and the current commercialroutes.Both poly(ethylene-co-p-methylstyrene)(PE-p-MS)25,26and poly(propylene-co-p-methylstyrene)(PP-p-MS)copolymers,27with a broad range ofcopolymer compositions and well-controlled mo-lecular structures,are available in our laboratory.Ideally,only the highly reactive benzylic protons(-CH3)in p-MS unit involve the hydrogen ab-straction by alkoxyl radical.The formed relativelystable benzylic radical then reacts with facile ma-leic anhydride monomer.The overall processavoids any side reaction in the polymer backbone,therefore,backbone degradation and crosslinkingcan be largely prevented.In the early experiments,we carried out theMA graft reactions of PE-p-MS and PP-p-MS co-polymers in the homogeneous solution conditionsat elevated temperatures.Both PE-p-MS and PP-p-MS copolymers showed much higher MA incor-poration than the corresponding PE and PP ho-mopolymers,under the same reaction conditions.However,a certain degree of side reactions didtake place,that result in the change of polymermolecular weight.A small portion of hydrogenabstractions may take place at the secondary CH2or tertiary CH units in the polymer backbone,other than the p-MS side groups.Despite thereactivity difference,p-CH3Ͼbenzylic CH(back-bone)ӷCH(backbone)ϾCH2(backbone),theproton concentrations of CH2(backbone)and CH(backbone)are much higher than that of p-CH3inthe PE-p-MS and PP-p-MS copolymers,contain-ing only few mol%of p-MS units.To enhance the selectivity on p-MS units,theMA grafting reactions were usually carried out inthe suspension(heterogeneous)reaction condi-tions at lower reaction temperature.The startingsemicrystalline PE-p-MS and PP-p-MS copoly-1338LU AND CHUNGmers,in afine powder form,were suspended in solution,containing solvent,initiator and MA. Since the p-MS side groups only located in the amorphous phase,the swollen amorphous do-mains provide the physical contacts between p-MS groups,initiator,and MA reagent.On the other hand,the secondary CH2(backbone)or ter-tiary CH(backbone)units in the crystalline do-mains are largely intact.Therefore,the suspen-sion reaction condition physically enhances the grafting reaction on p-MS units.Table I summarizes the results of MA grafting reactions of three PE-p-MS copolymers,containing 2,5,10mol%p-MS units,in a suspension condition at75°C.Benzene and benzoyl peroxide(BPO)were used as solvent and initiator,respectively.Three commercial PE(HDPE,LDPE,and LLDPE)samples and a poly(ethylene-co-styrene) (PE-S)copolymer were also modified under the same reaction conditions and used as control re-actions to study the role of p-MS in the modifica-tion reaction.It is clearly shown that the MA modified PE-p-MS copolymers(PE-p-MS-g-MA) had higher MA graft contents than the MA mod-ified PE(HDPE,LDPE,and LLDPE)and PE-S polymers.In addition,each PE-p-MS-g-MA copoly-mer shows similar intrinsic viscosity as the cor-responding PE-p-MS copolymer,strongly indicat-ing that there is no detectable crosslinking in the MA modified PE-p-MS copolymer.With the in-crease of the initiator concentration or p-MS units in the copolymer,the MA graft content increases (compared PE-p-MS-1,2and3series).The MA graft content was determined by IR spectrum. Figure1compares the IR spectra of two PE-p-MS-g-MA copolymers and the starting PE-p-MS.The new absorbances at1860and1780cmϪ1,corre-sponding to the symmetric and asymmetric stretchings of the two carbonyl groups in succinic anhydride,were used in determining the incorpo-rated MA content in the copolymer(see the Ex-perimental section).Table I.A Summary of MA Modification a of PE Homo-and CopolymersSample[Comonomer](mol%)BPO(wt%)MA Modified PE PolymersMA(wt%)Mv(ϫ104g/mol)Tm(°C)HDPE0——14.2135 HDPE-g-MA-100.2014.3135 HDPE-g-MA-200.5014.2135 LDPE——— 4.38115 LDPE-g-MA-1—0.20.4 4.52115 LDPE-g-MA-2—0.5 1.1Crosslinking b—LLDPE———11.4125 LLDPE-g-MA-1—0.20.312.4125 LLDPE-g-MA-2—0.50.8Crosslinking b—PE-S-1[S]ϭ5—— 3.54122 PE-S-1-g-MA-1[S]ϭ50.2 1.0 3.61122 PE-S-1-g-MA-2[S]ϭ50.5 1.6Crosslinking b—PE-S-1-g-MA-3[S]ϭ5 1.0 2.3Crosslinking b—PE-p-MS-1[p-MS]ϭ2—— 6.10130 PE-p-MS-g-MA-1[p-MS]ϭ20.2 1.5 6.23129 PE-p-MS-g-MA-2[p-MS]ϭ20.5 2.4 6.40128 PE-p-MS-g-MA-3[p-MS]ϭ2 1.0 3.2 6.41128 PE-p-MS-2[p-MS]ϭ5—— 3.83117 PE-p-MS-2-g-MA-1[p-MS]ϭ50.2 2.9 3.94117 PE-p-MS-2-g-MA-2[p-MS]ϭ50.5 3.7 4.06116 PE-p-MS-2-g-MA-3[p-MS]ϭ5 1.0 6.4 4.07117 PE-p-MS-3[p-MS]ϭ10—— 3.1897 PE-p-MS-3-g-MA-1[p-MS]ϭ100.2 5.6 3.2396 PE-p-MS-3-g-MA-2[p-MS]ϭ100.57.9 3.3597 PE-p-MS-3-g-MA-3[p-MS]ϭ10 1.08.8 3.6397a Reaction condition:5-g polymer,2-g MA,50-mL benzene,75°C,3hr.b Crosslinking is indicated by the poor solubility of polymer in decalin at135°C.MALEIC ANHYDRIDE GRAFTED POLYETHYLENE1339The HDPE sample showed almost no graft re-action under the same reaction condition,possibly due to its high crystallinity and thus less swelling at 75°C.The LDPE and LLDPE samples showed some graft reactions at a low initiator concentra-tion,but had extensive crosslinking side reactions at a high initiator concentration.It is very inter-esting to compare the modification of PE-p -MS copolymers with PE-S copolymers.In Table I,un-der the identical reaction conditions and comono-mer contents,the MA modified PE-p-MS-2co-polymers consistently have higher MA grafted contents than the modified PE-S-1copolymers.In addition,the MA modified PE-p-MS-2copolymers show no crosslinking in all the reactions,but the modified PE-S-1has crosslinking at high initiator concentrations.This strongly indicates the ad-vantage of p-methylstyrene units.Because of the steric effect,the backbone tertiary benzylic pro-ton has less reactivity than the pendant p-methyl group.This reactivity difference of two types of protons was also observed in free radical bro-mination of poly(p-methylstyrene),28where the dominated bromination occurs on the p-methyl group.In general,the melting point of PE-p-MS-g -MA copolymer is similar with that of the starting PE-p -MS copolymer,indicating that the crystal-line phase did not change significantly during the modification.This may be attributed to the sus-pension process,which keeps the crystalline phase untouched during the reaction.Table II summarizes the MA modification of PP-p -MS copolymers,containing 0.8and 1.5mol %p -MS units,in suspension condition at 125°C.Biphenyl and dicumyl peroxide (DCP)were used as solvent and initiator,respectively.A commercial PP polymer was also modified under the same reaction conditions and used as the control reactions.The MA modified PP-p -MS copolymers (PP-p-MS-g -MA)also show much higher MA graft contents than the corresponding MA modified PP homopolymers.In addition,the PP-p-MS-g -MA copolymer exhibits similar intrin-sic viscosity with the corresponding starting PP-p -MS copolymer,indicating no detectable side re-action,whereas,the MA modified PPhomopoly-Figure 1.IR spectrum of (a)the starting PE-p -MS copolymer with 10mol %of p -MS and two resulting PE-p-MS-g -MA containing (b)5.6,and (c)8.8wt %MA.Table II.A Summary of MA Modification a of PP Homo-and CopolymersSample [p -MS](mol%)Initiator (wt %)MA Modified PP PolymersM v (ϫ10Ϫ5g/mol)MA (wt %)T m (°C)PP0— 2.14—162PP-g -MA-10 1.0 1.320.7161PP-g -MA-20 2.00.61 1.1158PP-p -MS-10.8— 1.60—157PP-p -MS-1-g -MA-10.8 1.0 1.57 1.6156PP-p -MS-1-g -MA-20.8 2.0 1.53 2.2155PP-p -MS-21.5— 1.31—153PP-p -MS-2-g -MA-1 1.5 1.0 1.302.1152PP-p -MS-2-g -MA-21.52.01.262.8152aReaction conditions:125°C,polymer 5g,MA 2g,biphenyl 50g,DCP initiator.1340LU AND CHUNGmer shows severe degradation,especially at a high initiator paring PP-p-MS-1and PP-p-MS-2sets,higher p -MS and ini-tiator concentrations also result in higher MA graft content.To identify the grafting point,the MA modified copolymers were analyzed by 1H NMR spectra.The Figures 2and 3compare the 1H NMR spectra of PE-p-MS-g -MA and PP-p-MS-g -MA copolymers with the corresponding PE-p -MS and PP-p -MS copolymers,respectively.There are three new chemical shifts at 2.7,3.5,and 3.7ppm in both MA modified samples.The peak at 2.7ppm corresponds to methylene pro-tons of -CH 2-MA,and two new peaks at 3.5and 3.7ppm are assigned to the methylene and me-thine protons of succinic anhydride,respectively.Overall,NMR results clearly indicate that the MA grafting reaction takes place on p-methyl groups of the copolymers.It is known that it is difficult to observe the resonance of protons of MA on the commercial MA grafted polymers,possibly due to the dipolar broadening of resonance near the graft points that have restricted mobil-ity.22,29,30In fact,we did not observe the same proton peaks at 3.5and 3.7ppm in our MA mod-ified PE,PP,and PE-S polymers.Thus,the obser-vation of MA protons in both PE-p-MS-g -MA and PP-p-MS-g -MA copolymers may be attributed to the fact that they are on the flexible pendant p-methyl groups and have high mobility.It is very interesting to note that some short chain MA oligomers may also form in both PE-p-MS-g -MA and PP-p-MS-g -MA par-ing the integrated peak intensities between 2.3(-CH 3)and 2.7ppm (-CH 2-MA)in Figure 3(b)with the corresponding protons,about 60%(0.5mol %)of p -MS units were involved in the free radical grafting reaction.However, 2.2wt %(ϳ1.0mol %)of MA units were observed in PP-p-MS-g -MA,in average about 2MA units per p -MS activated site.The low grafting tempera-tures (75and 125°C for PE-p -MS and PP-p-MS,respectively)may offer the favorable reaction con-dition for the oligomerization 22,31of MA monomers.EXPERIMENTALMaleic anhydride,benzene,biphenyl,benzoyl peroxide,and dicumyl peroxide were purchased from Aldrich Chemical Company and used with-out further purification.Poly(ethylene -co-p-meth-ylstyrene)and poly(ethylene-co -styrene)copoly-mers were prepared by [Me 2Si(C 5Me 4)N(t -Bu)]-TiCl 2/Methylaluminoxane (MAO)catalysts.25,26Poly(propylene -co-p-methylstyrene)copolymers 27were prepared by TiCl 3-AA/AlEt 2Cl mercial HDPE (T m ϭ135°C),LDPE (T m ϭ115°C),LLDPE (5wt %1-hexene,T m ϭ125°C),and PP (T m ϭ162°C)from Aldrich Chemical Company were purified by dissolving polymer in xylene at the elevated temperature and then pre-cipitated out from solution at roomtemperature.Figure 3.1H NMR spectra of (a)the starting PP-p -MS copolymer with 0.8mol %of p -MS,and (b)the resulting PP-p-MS-g -MA containing 2.2wt %MA.Figure 2.1H NMR spectra of (a)the starting PE-p -MS copolymer with 5mol %of p -MS,and (b)the resulting PE-p-MS-g -MA containing 3.7wt %MA.MALEIC ANHYDRIDE GRAFTED POLYETHYLENE 1341In each MA modification reaction,about5-gpolymer powders were suspended in50-mL sol-vent at a certain temperature under nitrogen,then2g MA and a certain amount of initiatorwere added.For PE polymers,benzene was usedas the solvent,and BPO was the initiator,and thereaction temperature was at75°C.For PP poly-mers,biphenyl was used as the solvent,DCP wasthe initiator,and the reaction temperature was at125°C.The reaction was stirred for3h beforeprecipitating the reaction mixture into200mLacetone.The polymer was isolated byfiltration,washed with acetone four times,and dried undervacuum at50°C for24h.1H NMR samples were prepared in d2-tetra-chlorethane solution(20wt%)at120°C andmeasured by a Bruker AM-300MHz NMR spec-trometer.IR was detected by a Bio-Rad FTIR-60spectrometer using a polymer thinfilm(about2to8m),which was prepared by compression-mold-ing polymer powders between PTFE coated alu-minum sheets at190°C and25000psi.The MAcontent was calculated from FTIR by the follow-ing equation:MA wt%ϭK(A1780/d),where A1780is the absorbance of carbonyl group at1780cmϪ1and d is the thickness(mm)of thefilm,K isconstant(ϭ0.25)and detected by calibration ofthe known MA content of MA grafted PP.Al-though,the correlation32between absorbance andMA content orfilm thickness may not be perfectlinear,especially for the samples with high MAcontents,the general trends of this free radicalMA grafting reaction are valid.The intrinsic vis-cosity of polymer was measured in decalin dilutesolution at135°C with a Cannon–Ubbelohde vis-cometer.The viscosity molecular weight was cal-culated by the Mark–Houwink equation:[]ϭKM␣,where for PP Kϭ1.05ϫ10Ϫ4dL/g and␣ϭ0.80;for PE Kϭ6.2ϫ10Ϫ4dL/g and␣ϭ0.70.33The melting point of the polymer was measured under nitrogen by a differential scan-ning calorimetry(Perkin–Elmer DSC-7)with arate of20°C/min.CONCLUSIONThe reactive p-MS comonomer units in both poly-(ethylene-co-p-methylstyrene)and poly(propylene-co-p-methylstyrene)copolymers facilitate the MAgrafting reaction.Under suspension condition,the MA grafting reaction selectively takes placeat the-CH3group,which avoids the backbone side reactions,such as crosslinking and degrada-tion.The resulting MA modified copolymers basi-cally keep the same molecular weight as the start-ing PE-p-MS and PP-p-MS copolymers,which can be easily controlled in the copolymerization reac-tion,especially using metallocene catalyst.With the increase of the concentration of p-MS in the copolymers,the MA graft content increases.The authors would like to thank the Petroleum Re-search Foundation for thefinancial support. 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