Synthesis and Characterization of Maleic Anhydride GraftedPolypropylene with a Well-Defined Molecular StructureMin Zhang,† Ralph H. Colby,† Scott T. Milner,‡ and T. C. Mike Chung*,†

†Department of Materials Science and Engineering and ‡Department of Chemical Engineering, The Pennsylvania State University,University Park, Pennsylvania 16802, United States

Tianzi Huang and Willem deGroot

The Dow Chemical Company, 2301 Brazosport Blvd., Freeport, Texas 77541, United States

*S Supporting Information

ABSTRACT: Despite the commercial importance of maleicanhydride grafted polypropylene (PP-g-MAH), it has long beena scientific challenge to prepare this polymer with a well-controlled molecular structure. This paper discusses a newchemical route that can form PP-g-MAH with desirable MAHcontent, a single MAH incorporated unit, white color, highmolecular weight, and narrow molecular weight and composi-tion distributions. The chemistry involves a unique PP-co-p-BTcopolymer as the “reactive intermediate” that can be effectivelyprepared by metallocene-mediated copolymerization of propy-lene and p-(3-butenyl)toluene (p-BT), with narrow molecularweight and composition distributions, high molecular weight,and a broad range of p-BT contents. The incorporated p-BTcomonomer units provide the reactive sites for the subsequentfree radical MAH graft reaction under a suspension condition at a low reaction temperature. The resulting PP-g-MAH polymerswere carefully examined by a combination of NMR and GPC measurements, which shows almost no change in polymermolecular weight and distribution and a single MAH incorporation (no oligomerization). The incorporated MAH units increasewith the increase of initiator concentration, p-BT content, and reaction time. Evidently, the combination of high reactivity of φ-CH3 moiety, a favorable mixing condition between the reactive sites and chemical reagents in the swollen amorphous phases, andlow reaction temperature results in MAH grafting reaction selectively taking place at the φ-CH3 moieties without side reactions(i.e., chain degradation and MAH oligomerization). In addition, this suspension reaction process presents an economic methodto prepare PP-g-MAH with high polymer content and easy product purification.

■ INTRODUCTION

Isotactic polypropylene (PP) represents a quarter ofcommercial polymers produced in the world and is one ofthe fastest growing thermoplastics due to its uniquecombination of properties, including high melting point, hightensile strength, stiffness, chemical resistance, excellentprocessability and recyclability, and low cost.1 Despite itscommercial success, the functionalization of PP has long been ascientifically challenging and industrially important area.2−5

The constant interest is driven by the strong desire to improvePP’s poor interactive properties and broaden its applications tohigher value products, especially in polymer blends andcomposites those in which adhesion and compatibility withother materials are paramount. Despite significant researchefforts in the past decades by both direct and postpolymeriza-tion approaches, limited success has been yielded.By far, maleic anhydride modified polypropylene (PP-g-

MAH)6 is the most important commercial functionalized PP

polymer due to its unique combination of the low cost ofmaleic anhydride (MAH) reagent, high activity of the resultingsuccinic anhydride moiety, and good processability of the PP-g-MAH polymer. Despite its low molecular weight (most of them<20 kg/mol), deep color (with impurities), and ill-definedmolecular structures, PP-g-MAH is the most popular choice ofmaterial for improving the compatibility, adhesion, andprintability of polypropylene. They can be found in manyimportant commercial products, such as glass fiber reinforcedPP,7 anticorrosive coating for metal pipes and containers,8

metal−plastic laminates,9 multilayer sheets of paper forchemical and food packaging,10 and polymer blends such asPP/polyamide and PP/polyester11−14 as well as polymer/claynanocomposites.15−19

Received: March 31, 2013Revised: May 5, 2013Published: May 20, 2013

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As illustrated in Scheme 1, PP-g-MAH is normally preparedvia a free radical grafting reaction of maleic anhydride with apreformed isotactic PP homopolymer in the presence of anorganic peroxide initiator in homogeneous (solution20−24 ormelt25−35) reaction conditions at elevated temperatures. It isgenerally believed that the grafting reaction starts withhydrogen abstraction by the alkoxyl radical upon the thermaldecomposition of the peroxide initiator. Most of the formedtertiary macroradicals (I) in the PP chain involve a facileintramolecular β-scission reaction and degrade the polymerchain into two shorter PP chains:25−35 one with an unsaturatedchain end (II) and the other having a terminal macroradical(III) that subsequently reacts with maleic anhydride mono-mers. As expected, the incorporated MAH content is reverselyproportional to PP molecular weight. Many experimentalresults also suggest that the incorporated MAH monomer unitsare a mixture of single and multiple succinic anhydride moietiesdue to the oligomerization of maleic anhydride. In addition,other side reactions, such as direct oligomerization of MAHmonomers by the alkoxyl radical to form the ungraftedimpurities (IV), also take place during this free radical graftingreaction, especially under high reaction temperatures. Muchwork has been done in optimizing the reaction conditions andextruder parameters to promote the desired reactions whilesuppressing the undesired ones.20−36

The final product is usually a complicated (deep color)mixture. Unfortunately, the performance of PP-g-MAH as aninterfacial agent is highly dependent on its molecular weightand MAH content as well as the ability to remove impurities. Itis very important to develop a new method that can prepare ahigh PP-g-MAH polymer with a well-controlled molecularstructure.In the past decade, we have been investigating a new

approach by using PP copolymers containing some “reactive”sites that can offer selective MAH modification reactions. Theobjective was to minimize side reactions (PP chain degradation,MAH oligomerization, etc.) and to obtain a white PP-g-MAHpolymer with high molecular weight, desirable MAH content,and a single MAH incorporated structure. The first methodinvolved the borane-containing PP copolymer.37 The incorpo-rated borane groups in the side chains or chain end arespontaneously oxidized by oxygen to form the polymericradicals38−42 that are associated with in situ formed “stable”borinate radicals.43 With the presence of MAH monomers, thepolymeric radical in situ reacts with MAH at ambienttemperature to produce PP-g-MAH without altering the PPmain chain structure. However, the major concerns around thischemistry are the lack of commercial availability, cost, and

handling of borane reagents (highly air sensitive).44 Anotherreactive PP copolymer studied included poly(propylene-co-p-methylstyrene) (PP-co-p-MS) copolymers45 prepared by theheterogeneous Ziegler−Natta catalyst, which has low p-MScontent (<0.8 mol %) and broad molecular weight andcomposition distributions. The subsequent free radical graftreaction46 of PP-co-p-MS copolymerscarried out under asuspension reaction condition using dicumyl peroxide initiatorat 125 °Cshows good selectivity on the p-MS units withminimal reduction of PP molecular weight. Unfortunately, thepreparation of the PP-co-p-MS copolymer was failed by iso-specific metallocene catalysts due to the chain transfer reactionin the presence of styrenic monomers.45,46 Therefore, this PP-co-p-MS method only affords the PP-g-MAH copolymers withlow MAH content and broad molecular weight andcomposition distributions. In fact, due to the broadcomposition distribution in the PP copolymers, the incorpo-rated MAH moieties at p-MS units are mostly located inrelatively low molecular weight polymer chains, making for anonideal molecular structure for interfacial agents. It is stillelusive to prepare the well-defined PP-g-MAH polymers witheconomic process.In this paper, we will discuss three closely related issues in

the formation of the well-defined PP-g-MAH polymer. First isthe redesign and synthesis of the “reactive” PP copolymer thatcontains active benzylic proton (φ-CH3) moieties in the flexibleside chains. This copolymer can be conveniently prepared by ametallocene catalyst to form copolymers with variouscomonomer content, high molecular weight, and narrowmolecular weight and composition distributions. Second is asystematic free radical MAH grafting study to identify the mostsuitable reaction condition for selective free radical grafting anMAH single unit onto the φ-CH3 moieties without sidereactions. In other words, the resulting PP-g-MAH polymersshould have nearly the same molecular weight and compositiondistributions as the starting PP copolymer and MAH contentthat is proportional to the incorporated φ-CH3 units in the PPcopolymer, while exhibiting a white color (without purifica-tion). Third is a comprehensive polymer structure character-ization to confirm the resulting well-defined PP-g-MAHpolymers.

■ EXPERIMENTAL DETAILSMaterials and Instrumentation. All O2- and moisture-sensitive

manipulations were carried out inside an argon-filled VacuumAtmospheres drybox. The metallocene catalyst rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2 was prepared using the published procedures.47

Polymerization-grade propylene (from Matheson Gas) was used asreceived. Toluene (Wiley Organics) was distilled over sodium

Scheme 1. Reaction Mechanism of a Free Radical Mediated MAH Grafting Reaction on PP

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benzophenone under argon. Allylmagnesium bromide (1.0 M solutionin diethyl ether), TiCl3·AA, AlEt2Cl (10 wt % in toluene), calciumhydride, diethyl ether (anhydrous), 4-methylbenzyl chloride, chlor-obenzene (anhydrous), (trimethylsilyl)methylamine, benzyl peroxide(BPO), AlEt2Cl (10 wt % in toluene), and methylaluminoxane (MAO,10 wt % in toluene) were purchased from Sigma-Aldrich and wereused as received. Maleic anhydride (MAH) was purchased fromSigma-Aldrich and was purified by sublimation before use. 3-Butenylbenzene (BB) was purchased from Sigma-Aldrich and wasdistilled with calcium hydride before use.All high-temperature 1H NMR spectra were recorded on a Bruker

AM-300 instrument in 1,1,2,2-tetrachloroethane-d2 at 110 °C. Themelting temperatures of the polymers were measured by differentialscanning calorimetry (DSC) using a PerkinElmer DSC-7 instrumentcontroller with a heating rate of 10 °C/min. The MAH content wascalculated from FTIR by the following equation: MAH wt % = K ×(A1780/d), where A1780 is the absorbance of carbonyl group at 1780cm−1 and d is the thickness (mm) of the film; K constant (0.25) isdetermined by a calibration curve of the known MAH content of PP-g-MAH polymers.48 The polymer molecular weights were determined byintrinsic viscosity of polymer measured in decahydronaphthalene(Decalin) dilute solution at 135 °C with a Cannon-Ubbelohdeviscometer. The viscosity molecular weight was calculated by theMark−Houwink equation: [η] = KMv

α where K = 1.05 × 10−4 dL/gand α = 0.80.49 The polymer molecular weights were also analyzed ona PL-220 series high-temperature gel permeation chromatography(GPC) unit equipped with triple detectors, including a PrecisionDetectors 2-angle laser light scattering detector Model 2040, aViscotek model 210R viscometer, a differential refractive indexdetector, and four PLgel Mixed-A (20 μm) columns (PolymerLaboratory Inc.). The oven temperature was at 150 °C, and thetemperatures of autosampler’s hot and warm zones were at 135 and130 °C, respectively. The solvent 1,2,4-trichlorobenzene (TCB)containing ∼200 ppm tris(2,4-di-tert-butylphenyl)phosphite (Irgafos168) was nitrogen purged. The flow rate was 1.0 mL/min, and theinjection volume was 200 μL. A 2 mg/mL sample concentration wasprepared by dissolving the sample in N2 purged and preheated TCB(containing 200 ppm Irgafos 168) for 2.5 h at 160 °C with gentleagitation.Synthesis of p-(3-Butenyl)toluene. To a dry 500 mL three-

necked round-bottom flask equipped with an addition funnel,condenser, and magnetic stir bar, 200 mL (0.2 mols) ofallylmagnesium bromide solution was transferred. A 20 mL (0.14mol) aliquot of 4-methylbenzyl chloride diluted with 50 mL of diethylether was added dropwise to allylmagnesium bromide at ice bath

temperature. After complete addition of 4-methylbenzyl chloride, themixture was warmed to room temperature and refluxed for 12 h. Then,200 mL of distilled water was slowly added to the mixture. Theaqueous layer was separated and washed three times with diethyl ether.The organic phases were combined and dried with anhydrous sodiumsulfate. After evaporation of the solvent the residue was purified on asilica column with hexane as eluent to give the crude product a paleyellow oil. The crude product was further dried with calcium hydrideand distilled under vacuum before use. Yield: 20.1 g, 94%. 1H NMRspectrum: δ 7.11 (m, Ph−H), 5.87 (m, 1 H, −CHCH2), 5.04 (m, 2H, −CHCH2), 2.70 (t, 2 H, Ph−CH2), 2.39 (m, 2 H, −CH2−CHCH2), 2.34 (s, 3 H, Ph−CH3).

Slurry Copolymerization of Propylene and p-(3-Butenyl)-toluene with Metallocene Catalyst. In a typical copolymerizationreaction, 50 mL of toluene and 4.2 mL of MAO (Al/Zr = 3000) werecharged into a Parr 450 mL stainless autoclave equipped with amechanical stirrer in a drybox. After removal from the box, the reactorwas injected with a certain amount of p-(3-butenyl)toluene and thencharged with 2.72 atm of propylene to saturate the toluene solution atambient temperature. About 2 × 10−6 mol of rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2 in 2 mL of toluene solution was then injected into thereactor with 130 psi of propylene under rapid stirring to initiate thecopolymerization. Additional propylene was fed continuously into thereactor to maintain a constant pressure (8.84 atm) during the entirecourse of the polymerization. After a 15 min reaction time elapse at 45°C, the reaction solution was quenched by methanol (10 mL), and theproduct was filtered, washed with a large amount of acidified methanolsolution, and then dried under vacuum at 60 °C for 24 h. Similarsynthetic procedures were used to prepare propylene/3-butenylben-zene copolymers, except changing the comonomer to 3-butenylben-zene.

Bulk Copolymerization of Liquid Propylene with p-(3-Butenyl)toluene with Metallocene Catalyst. In a typicalcopolymerization reaction, 50 mL of toluene and 14 mL of MAO(Al/Zr = 10 000) were charged into a Parr 450 mL stainless autoclaveequipped with a mechanical stirrer in a drybox. After removal from thebox, the reactor was injected with 3 mL of p-(3-butenyl)toluene andthen charged with 60 mL of liquid propylene at ambient temperature.The reactor was heated to 45 °C within 3 min. Then 2 × 10−6 mol ofrac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2 in 2 mL of toluene solution wasthen injected into the reactor by the use of 19.04 atm of argon gasunder rapid stirring to initiate the copolymerization. The reaction wasstopped by the addition of methanol (10 mL) and cooled; the pressurewas released, and the product was taken out, washed with a largeamount of acidified methanol solution, and dried in vacuum at 60 °C

Scheme 2. New Reaction Mechanism To Prepare PP-g-MAH Polymer and Derivative

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for 24 h. Similar procedures were used to prepare propylene/3-butenylbenzene copolymers.Ziegler−Natta Mediated Propylene Copolymerizations.

Similar copolymerization procedures were used to prepare bothpropylene/p-(3-butenyl)toluene and propylene/3-butenylbenzene co-polymers, except the catalyst system of metallocene/MAO, which waschanged to TiCl3·AA (0.10g) and AlEt2Cl (5 mL, 10 wt % toluene).The reaction temperature was kept at 60 °C for 15 min with 75 mL oftoluene as the total volume. The pressure of propylene gas wasmaintained at 4.08 or 8.84 atm.Maleic Anhydride Grafting Reaction of Poly(propylene-co-p-

(3-butenyl)toluene). In a typical reaction, 2 g of the propylene/p-(3-butenyl)toluene copolymer (powder form) was suspended in 40 mL ofchlorobenzene at 25 °C under nitrogen in a 100 mL flask equippedwith a magnetic stirrer. Then, 2.5 g of maleic anhydride and 0.02 or0.04 g of BPO were added. The reactor was heated to a hightemperature (75 or 85 °C), and the reaction was stirred for anotherperiod of time (3 or 9 h) at that temperature before precipitating thereaction mixture into 200 mL of acetone. The resulting PP-g-MAHwas isolated by filtration, washed with acetone four times, and driedunder vacuum at 60 °C for 24 h.Reaction between PP-g-MAH Polymer and (Trimethylsilyl)-

methylamine. Maleic anhydride modified polymer (1.0 g) wassuspended in 20 mL of chlorobenzene at 25 °C under nitrogen in a 50

mL two-necked flask equipped with a magnetic stirrer and a condenseralong with a water knockout trap. Subsequently, 1.0 mL of(trimethylsilyl)methylamine was added to the flask. The mixture washeated to 110 °C under nitrogen for 8 h before precipitating thereaction mixture into 200 mL of acetone. The final product wasisolated by filtration, washed with acetone three times, and finally driedunder vacuum at 60 °C for 24 h.

■ RESULTS AND DISCUSSIONPoly(propylene-co-p-(3-butenyl)toluene) and Poly-

(propylene-co-3-butenylbenzene) Copolymers. Scheme2 illustrates the new reaction scheme. Compared to theprevious reaction using p-methylstyrene (p-MS) comonomer,this chemistry is centered on a new p-(3-butenyl)toluene (p-BT) comonomer (I′) that contains α-olefin moiety (instead ofstyrene). The similar α-olefin moieties in both propylene and p-BT monomers offer a favorable metallocene-mediated copoly-merization to form PP-co-p-BT copolymers50 (II′) with a broadrange of p-BT contents and narrow molecular weight andcomposition distributions. It is logical to predict that theincorporated bulky p-BT comonomer units (located in theflexible side chains) shall be excluded from the PP crystalline

Figure 1. 1H NMR spectra of (a) PP-co-p-BT copolymer containing 0.8 mol % of p-BT units (run B-3) and (b) a PP-co-BB copolymer containing 2mol % BB units (run C-1 in Table 1).

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phases and will position themselves into the amorphousdomains.In the subsequent free radical MAH grafting reaction to

prepare PP-g-MAH polymer (III′), this polymer morphology,in conjunction with a suspension reaction condition, directs thegrafting reaction selectively at the “reactive” φ-CH3 moieties inthe swollen amorphous domains under low reaction temper-atures to avoid the side reactions. For comparative studies tounderstand the free radical grafting reaction, the correspondingcontrol copolymerization reactions using 3-butenylbenzene(BB) comonomer were also carried out to obtain the PP-co-BB copolymers that contain no reactive φ-CH3 moieties.Figure 1 compares the 1H NMR spectra between a PP-co-p-

BT copolymer (run B-3 in Table 1) and a PP-co-BB copolymer(run C-2 in Table 1), both containing 0.8 and 2 mol %comonomer units (with and without φ-CH3 group, respec-tively). They are prepared by the same rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/MAO catalyst under similar copolymerizationconditions. Both spectra show three major chemical shifts at0.95, 1.35, and 1.65 ppm, corresponding to the methine,methylene, and methyl groups in the PP chain. In Figure 1a,there are three additional chemical shifts around 2.35, 2.65, and7.1 ppm, corresponding to φ-CH3, −CH2-φ-CH3, and aromaticprotons, respectively, in the PP-co-p-BT copolymer. In Figure1b, there are only two additional chemical shifts around 2.65and 7.1 ppm, corresponding to the −CH2-φ and aromaticprotons, respectively, in the PP-co-BB copolymer. Theintegrated intensity ratio of the chemical shifts between 0.9and 1.7 ppm and the chemical shift at 7.1 ppm, and the numberof protons both chemical shifts represent, determines theconcentration of p-BT and BB units in the copolymers.

Tables 1 summarizes two comparative propylene/p-BTcopolymerization sets (A and B) using a heterogeneousTiCl3·AA/Et2AlCl Ziegler−Natta catalyst and a homogeneousrac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/MAO metallocene catalystto prepare the “reactive” PP-co-p-BT copolymers, respectively.Four corresponding control reactions in set C using the 3-butenylbenzene (BB) comonomer were also carried out toobtain the PP-co-BB copolymers, without reactive φ-CH3

moieties. The resulting polymers were analyzed by 1H NMR,differential scanning calorimetry (DSC), and intrinsic viscosity(η) to determine polymer composition, melting temperature,and average molecular weight (Mv). For several comparativesets, the high-temperature gel permeation chromatography witha series of detectors, including deferential reflection index, lightscattering, and intrinsic viscosity (GPC-triple detectors), wasapplied to obtain detailed information about the change ofpolymer molecular weight and molecular weight distributionbefore and after MAH grafting reactions (discussed later).In general, two p-BT and BB comonomers with the same α-

olefin moiety exhibit similar copolymerization reactivity (run A-2 vs C-1, run B-1 vs C-2, run B-4 vs C-3, and run B-6 vs C-4),showing similar polymer yield, comonomer incorporation, andcopolymer molecular weight and crystallinity, in both theZiegler−Natta and metallocene catalyst systems. In otherwords, the resulting PP-co-p-BT and PP-co-BB copolymers arevery similar, except having the “reactive” benzylic protons (φ-CH3) in PP-co-p-BT copolymers. Comparing runs in set A, theheterogeneous Ziegler−Natta catalyst offers good catalystactivities and high molecular weight copolymers. Thecomonomer incorporation is proportional to the comonomerconcentration, while the molecular weight and polymer yield

Table 1. Comparison of the Experimental Results in the Propylene Polymerization with and without p-BT and BB Comonomers

polymerization conditions polymer products

run catalystapropyleneb

(atm/M)comonomerc

(mL)temp(°C)

time(min)

polymer yield(g)

comonomerd

(mol %)Mv

e

(kg/mol)Tm(°C)

ΔH(J/g)

control-A A 4.08/1.64 60 15 10.5 881 162.9 78.9A-1 A 4.08/1.64 p-BT/1 60 15 6.9 0.7 433 155.3 61.7A-2 A 4.08/1.64 p-BT/2 60 15 5.0 1.3 401 150.1 49.9A-3 A 8.84/3.68 p-BT/1 60 15 8.3 0.5 517 156.2 73.2A-4 A 8.84/3.68 p-BT/2 60 15 6.2 0.8 482 153.8 57.1A-5 A 14.96/6.48 p-BT/2 60 5 7.3 0.3 581 156.9 77.8A-6 A 14.96/6.48 p-BT/3 60 5 5.6 0.4 532 156.4 74.7control-B-1 B 4.08/2.29 45 15 9.3 202 156.8 110.2control-B-2 B 8.84/5.26 45 15 13.3 253 156.7 106.3control-B-3 B 14.96/9.58 45 5 12.1 682 156.9 88.3B-1 B 4.08/2.29 p-BT/1 45 15 4.3 2.3 178 140.3 50.3B-2 B 4.08/2.29 p-BT/2 45 15 3.4 4.2 157 132.7 40.8B-3 B 8.84/5.26 p-BT/0.5 45 15 8.4 0.8 217 148.8 91.3B-4 B 8.84/5.26 p-BT/1 45 15 5.8 1.4 182 144.3 70.9B-5 B 8.84/5.26 p-BT/2 45 15 3.9 2.5 164 139.1 47.1B-6 B 14.96/9.58 p-BT/2 45 5 8.1 1.0 608 144.9 77.8B-7 B 14.96/9.58 p-BT/3 45 5 5.3 1.4 540 143.2 54.9C-1 A 4.08/1.64 BB/2 60 15 4.4 1.0 374 152.4 53.8C-2 B 4.08/2.29 BB/1 45 15 3.8 2.0 180 142.3 58.3C-3 B 8.84/5.26 BB/1 45 15 5.1 1.2 191 145.6 73.8C-4 B 14.96/9.58 BB/2 45 5 7.8 0.7 477 149.0 78.1

aCatalyst A: 0.1 g of TiCl3·AA, 5 mL of Et2AlCl, and 75 mL of toluene. Catalyst B: 2 μmol of rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2 catalyst, 50 mL oftoluene; 4.2 mL of MAO (Al/Zr = 3000). bPropylene pressure and concentration51 used in all reactions except runs A-5, A-6, control-B-3, B-6, B-7,and C-4 using liquid propylene. cComonomer p-BT: p-(3-butenyl)toluene; comonomer BB: 3-butenylbenzene. dComonomer content (mol %) inthe copolymers determined by 1H NMR under 110 °C in 1,1,2,2-tetrachloroethane-d2.

eEstimated by intrinsic viscosity of polymer/decalin dilutesolution at 135 °C with K = 1.05 × 10−4 dL/g and α = 0.8.

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systematically decreased. There are no issues around preparinga high molecular weight PP-co-p-BT copolymer with >1 mol %comonomer content and high crystallinity (run A-2). However,as will be discussed later, the major concern in this PP-co-p-BTcopolymer is the broad molecular weight and compositiondistributions. On the other hand, the metallocene catalyst offersa much better copolymerization capability to prepare thecopolymers with narrow molecular weight and compositiondistributions. In the comparative set B, the increase of the p-BTcomonomer feed results in the systematical increase ofcomonomer content (up to 4 mol % in run B-2). Thecopolymerization results are very different from those using thep-methylstyrene (p-MS) comonomer feed, showing almost nocatalyst activity. Evidently, the α-olefin moiety in p-BTcomonomer (vs styrenic moiety in p-MS) provides effectivecomonomer incorporation in the metallocene-mediatedpropylene copolymerization. Even a small amount ofcomonomer incorporation (1 mol %) has a significant effectto the melting temperature (Tm) and heat of fusion (ΔH) ofpolypropylene, indicating the homogeneous distribution ofbulky comonomers in the resulting PP-co-p-BT copolymers.The increase of the p-BT comonomer feeding also results in thesystematical reduction of catalyst activity and polymermolecular weight. The incorporation of bulky p-BT como-nomer slows down the polymerization rate and increases of thechain transfer reaction. Both can be compensated byintroducing high propylene concentration (runs B-6 and B-7using liquid propylene feed).47 Similar results were observed inthe PP-co-BB copolymer (run C-4).Reactivity Ratios. The optimal way to investigate a

copolymerization reaction is to measure the reactivity ratio ofthe comonomers. To obtain meaningful results, a series ofexperiments were carried out by varying monomer feed ratio ( f= [propylene]/[p-BT] = [M1]/[M2]) and comparing theresulting copolymer composition (F = d[M1]/d[M2]) at a lowmonomer conversion (<5%) in order to maintain a constantcomonomer mole ratio ([M1]/[M2]). In the SupportingInformation, Tables S1 and S2 summarize two experimentalsets of propylene/p-BT copolymerization using a heteroge-neous TiCl3·AA/Et2AlCl Ziegler−Natta catalyst and a homoge-neous rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/MAO metallocenecatalyst, respectively. The reactivity ratios between propylene(r1 = k11/k12) and p-BT (r2 = k22/k21) were estimated using theMayo−Lewis equation. As shown in Figure 2, both plots of f(1− F)/F vs f 2/F give straight lines with slope for r1 and interceptfor r2. The linear slope in Figure 2a shows r1 = 1.535 and r2 =0.425, with r1 × r2 = 0.65 (not far from unity), indicating arelatively good random copolymerization reaction with aslightly higher propylene reactivity in the rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/MAO mediated propylene/p-BT homogene-ous copolymerization reactions. In other words, the propylene/p-BT copolymers shown in runs B (Table 1) are mostlyrandom copolymers. As will be discussed later, they also exhibitnarrow molecular weight distribution. On the other hand, thesimilar linear propylene/p-BT copolymerization plot in Figure2b results in r1 = 23.638 and r2 = 0.75, with r1 × r2 = 17.73 (farfrom unity), indicating the strong tendency of propyleneconsecutive insertion while using a heterogeneous Ziegler−Natta (TiCl3·AA/Et2AlCl) catalyst system. The crossoverreactions from the propagating propylene site to p-BTcomonomer are small. Overall, the results suggest thepropylene/p-BT copolymers shown in runs A (Table 1) havinginhomogeneous copolymer structure and a broad composition

distributions. They are also exhibit broad molecular weightdistributions (discussed later).

PP-g-MAH Polymers. The availability of a wide range ofwell-defined PP-co-p-BT copolymers, prepared by a homoge-neous (single-site) metallocene catalyst, offers us with anexcellent opportunity to systematically study the free radicalMAH grafting reaction and examine their resulting PP-g-MAHmolecular structures. The objective is to develop an effectivechemical route to prepare desirable white PP-g-MAH polymerswith a single MAH grafting enchainment, controlled MAHconcentration, high polymer molecular weight, and narrowmolecular weight and composition distributions (which havenever been reported before). Table 2 summarizes theexperimental results of MAH modification reactions involvingthree PP-co-p-BT copolymers with 0.8, 1.0, and 1.3 mol % p-BTcontents (runs B-3, B-6, and A-2 in Table 1) and two controlpolymers, including a PP homopolymer (control-B-2) and aPP-co-BB copolymer (run C-3), using benzyl peroxide (BPO)initiator under a temperature range between 75 and 95 °C. TheMAH contents and locations in the resulting PP-g-MAHpolymers were determined by FTIR and 1H NMR measure-ments. The polymer molecular weights were determined byintrinsic viscosity measurement (Mv). The melting point (Tm)and heat of fusion (ΔH) of the polymer were determined bydifferential scanning calorimetry (DSC). As will be discussedlater, some of the PP-g-MAH polymers were furtherinterconverted into the corresponding PP-g-MAH−CH2−Si(CH3)3 polymers (compound IV′ in Scheme 2), allowingdetailed molecular structure characterization by a combinationof 1H NMR and GPC triple detector measurements.Infrared spectroscopy is the most commonly used method in

determining MAH content in the polymer.49 Figure 3 comparesFTIR spectra of the starting PP-co-p-BT (run B-6) and twocorresponding PP-g-MAH copolymers (runs B-6-MAH-1 andB-6-MAH-3). After the MAH graft-from reaction, two new

Figure 2. Mayo−Lewis plots for copolymerization between propyleneand p-BT comonomer by using (a) rac-Me2Si[2-Me-4-Ph-(Ind)]2ZrCl2/MAO and (b) TiCl3·AA/Et2AlCl catalysts.

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absorption peaks were observed at 1860 and 1780 cm−1,corresponding to the symmetric and asymmetric stretching ofthe two CO groups in the resulting succinic anhydride group(compound III′ in Scheme 2). The incorporated MAH contentin PP-g-MAH was calculated by the following equation: MAH(wt %) = K(A1780/d), where A1780 is the absorbance of thecarbonyl group at 1780 cm−1, d is the thickness (mm) of thefilm, and K is a constant (0.25) that is determined by acalibration curve of several commercial PP-g-MAH polymerswith known MAH contents. Overall, PP-co-p-BT copolymers

showed much higher MAH incorporation than the correspond-ing PP-co-BB copolymers and PP homopolymers, under thesame reaction conditions (Table 2). In two PP homopolymerruns (control-B-2-MAH-1 and control-B-2-MAH-2), theresulting PP-g-MAH polymers showed no detectable MAHcontent as well as little change in the PP molecular weight. Inaddition to the lack of reactive sites, high PP crystallinity, andthus less matrix swelling at 75 and 85 °C, may contribute to itschemical inefficiency. However, under the same suspension andrelatively low reaction temperature conditions, all PP-co-p-BTcopolymers show a systematic increase of MAH content withthe increase of the initiator concentration, temperature, andtime. In the B-6 set, using the starting PP-co-p-BT copolymerwith 1 mol % p-BT content, we can obtain PP-g-MAH with >1wt % MAH content without a major change in polymermolecular weight and distribution (discussed later). Evidently,most of the graft reactions were taken place in the p-BT sidechains located in the swollen amorphous phase under thissuspension reaction condition. Interestingly, only very lowMAH contents were observed in the comparative PP-co-BBcopolymer runs (C-3-MAH-1 and C-3-MAH-2). This copoly-mer, with only −CH2-φ moieties (without −φ-CH3 moiety),clearly shows very ineffective in free radical grafting reactivityunder this suspension reaction condition.Polymer viscosity (in melt or solution) is commonly used to

monitor polymer molecular weight during the MAH graftingreaction. As summarized in Table 2, the intrinsic viscosities ofall polymers were measured in decalin dilute solution at 135 °C,and the viscosity molecular weight (Mv) was calculated by theMark−Houwink equation: [η] = KMv

α where K = 1.05 × 10−4

dL/g and α = 0.80.48 Comparing all PP-g-MAH polymers in theB-3 set, the polymer molecular weight (Mv) seems to be quitesensitive to the reaction temperature and less dependent on the

Table 2. Summary of PP-g-MAH Polymers Prepared by Free Radical Mediated Grafting Reactions with Various PP Polymers

PP-g-MAH polymers

runa[comonomer]

(mol %)initiator(wt %)

temp/time(°C/h)

MAHb

(wt %)MAHc

(wt %)MAHc

(mol %)Mv

(kg/mol)Tm(°C)

ΔH(J/g)

control-B-2 (PP) 253 156.7 106.3control-B-2-MAH-1 2.0 75/9 n.d.d n.d.d n.d.d 241 156.4 100.7control-B-2-MAH-2 2.0 85/3 n.d.d n.d.d n.d.d 235 156.2 98.1A-2 (PP-co-p-BT) 1.3 401 150.1 49.9A-2-MAH-1 1.3 2.0 75/3 0.5 0.37 0.16 377 149.6 42.8A-2-MAH-2 1.3 2.0 75/9 1.4 1.0 0.42 353 149.3 38.3A-2-MAH-3 1.3 2.0 85/3 1.5 1.1 0.48 342 148.9 32.9B-3 (PP-co-p-BT) 0.8 217 148.8 91.3B-3-MAH-1 0.8 1.0 75/3 0.1 0.07 0.03 209 148.6 85.2B-3-MAH-2 0.8 2.0 75/3 0.3 0.24 0.10 202 148.2 80.3B-3-MAH-3 0.8 2.0 75/9 0.8 0.55 0.24 191 148.0 76.8B-3-MAH-4 0.8 1.0 85/3 0.6 0.40 0.17 198 148.1 74.3B-3-MAH-5 0.8 2.0 85/3 1.0 0.76 0.32 179 147.7 70.2B-3-MAH-6 0.8 1.0 95/3 0.5 0.33 0.14 112 147.5 63.1B-3-MAH-7 0.8 2.0 95/3 0.9 0.67 0.28 73 146.9 55.2B-6 (PP-co-p-BT) 1.0 608 144.9 77.8B-6-MAH-1 1.0 2.0 75/3 0.4 0.29 0.12 581 144.7 70.3B-6-MAH-2 1.0 2.0 75/9 1.0 0.76 0.32 575 144.2 61.8B-6-MAH-3 1.0 2.0 85/3 1.2 0.90 0.38 541 144.0 58.3C-3 (PP-co-BB) 1.2 191 145.6 73.8C-3-MAH-1 1.2 2.0 75/9 0.2 0.17 0.07 165 145.2 65.2C-3-MAH-2 1.2 2.0 85/3 0.3 0.21 0.09 152 145.0 60.7

aReaction conditions: polymer fine powders 1.0 g, MAH 2.0 g, chlorobenzene 20.0 g, BPO as initiator. bEstimated by FTIR spectra. cEstimated by1H NMR spectra. dNot detectable.

Figure 3. FTIR spectra of (a) starting PP-co-p-BT (run B-6) and twocorresponding PP-g-MAH copolymers, including (b) run B-6-MAH-1and (c) run B-6-MAH-3.

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BPO initiator concentration and reaction time. The mostsuitable reaction temperature for this free radical induced MAHgrafting reaction on PP-co-p-BT copolymers is near 85 °C, inwhich the MAH grafting reaction is effective and the polymermaintains similar molecular weight. In addition to theheterogeneous reaction condition to minimize exposure of PPbackbone, the lower activation energy for H-extraction frombenzylic protons in the φ-CH3 moiety (vs that of tertiaryprotons in PP backbone) shall also contribute to the selectivity.Figure 4 compares DSC curves of the same reaction set (runs

B-6). The resulting melting temperature (Tm) and heat offusion (ΔH) data are summarized in Table 2. They areobtained in the second heating−cooling cycle at 10 °C/min,which removes any thermal history of each sample and providesreproducible results for side-by-side comparison. All DSCcurves show similar melting and crystallization temperatureswith a relatively narrow peak and a flat baseline, indicating auniform crystalline structure in semicrystalline morphologiesand narrow composition distribution along all copolymerchains. As discussed in Table 1, both Tm and ΔH values arestrongly dependent on the density of the comonomer (i.e.,branch density) in the PP-co-p-BT copolymer; the higher thedensity, the lower the Tm and ΔH. Even a small amount ofcomonomer incorporation (1 mol %) has a significant effect onthe crystallization of PP. On the other hand, the subsequentmodification of p-BT comonomer units in forming PP-g-MAHpolymers (Table 2), the change of melting temperature, andheat of fusion are very small. Both Tm and crystallinity seem tobe only governed by branch density and are independent ofcomonomer type, consistent with Flory’s prediction forsemicrystalline copolymers. Under the equilibrium condition,each polymer chain runs from one side of lamellae to the other,and the consecutive propylene units govern the thickness oflamellae. The comonomer units (side chains) are restricted inthe amorphous regions. It is interesting to note that thecommercial PP-g-MAH polymers usually show a lower andbroader melting peak, indicating inhomogeneous polymerstructures that are associated with polymer chain degradationduring the MAH modification.Structure Characterization of PP-g-MAH Polymers. To

understand the detailed molecular structure of PP-g-MAHpolymers, we have applied a combination of 1H NMR and

GPC-triple detectors techniques in conjunction with theimidization of succinic anhydride groups with (trimethylsily)-methylamine to form the corresponding PP-g-MAH−CH2−Si(CH3)3 polymer (compound IV′ in Scheme 2). For 1H NMRmeasurements, the new silane-capped MAH moieties reveal thetrue MAH grafting structure, especially answering single ormultiple MAH enchainment. On the other hand, the silane-capped MAH moieties in PP-g-MAH polymers preventpolymer chain interactions during GPC measurements, whichprovide fair side-by-side comparison of polymer molecularweights and distributions before and after MAH graftingreaction.Figure 5 compares 1H NMR spectra of two corresponding

PP-g-MAH and PP-g-MAH−CH2−Si(CH3)3 polymers and thestarting PP-co-p-BT copolymer. In Figure 5b, in addition tothree major chemical shifts at 0.95, 1.35, and 1.65 ppm,corresponding to the methine, methylene, and methyl groups inthe PP chain, and three smaller chemical shifts around 2.35,2.65, and 7.1 ppm, corresponding to φ-CH3, −CH2-φ-CH3,and aromatic protons in p-BT comonomer units, there is anadditional new chemical shift at 2.78 ppm, corresponding to−φ-CH2-MAH. It is difficult to directly observe the resonanceof methylene and methane protons of the graft MAH units,possibly due to the dipolar broadening of resonance near thegraft points that have restricted mobility. Overall, the intensityof this new −φ-CH2-MAH chemical shift systematicallyincreases while the peak intensity for φ-CH3 group propor-tionally decreases. On the other hand, the peak intensity for−CH2-φ-CH3 maintains almost constant throughout the MAHmodification steps. The combination strongly suggests that theselective MAH grafting reaction at φ-CH3 active sites andinternal −CH2-φ- sites with similar benzyl protons but loweraccessibility are not effective. The MAH grafting efficiency atthe active φ-CH3 sites can be directly estimated by the peakintensity ratio between −CH2-φ-CH3 and −φ-CH2-MAH. Thedetailed MAH contents in the resulting PP-g-MAH polymersare summarized in Table 2, which are slightly lower than theresults obtained from FTIR studies. In general, the MAHincorporation increases with the initiator concentration,reaction time, and temperature. Up to 40% of φ-CH3 activesites in the PP-co-p-BT copolymer were grafted with MAHgroups without significant changes in the PP molecular weight

Figure 4. DSC plots of (left) heating and (right) cooling cycles for (a) starting PP-p-BT (run B-6) and three corresponding PP-g-MAH copolymers,including (b) run B-6-MAH-1, (c) run B-6-MAH-2, and (d) run B-6-MAH-3.

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(discussed later) under this low-temperature reaction con-dition.Figure 5c shows 1H NMR spectrum of the corresponding

PP-g-MAH−CH2−Si(CH3)3 polymer. There are two newchemical shifts at 2.95 and 0.32 ppm, corresponding to

MAH−CH2−Si(CH3)3, respectively. It is interesting to notethe peak intensity ratio (2:2:9) between three types of protonsin the resulting −CH2−MAH−CH2−Si(CH3)3 moiety, whichstrongly indicates a clean immidization reaction and confirms aimportant fact: each grafting point only contains one MAH

Figure 5. 1H NMR spectra of (a) the starting PP-co-p-BT copolymer with 1.0 mol % p-BT units (run B-6 in Table 1), (b) the resulting PP-g-MAHcontaining 0.76 wt % MAH (B-6-MAH-2 in Table 2), and (c) the associated silane modified PP-g-MAH-Si(CH3)3 polymer.

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unit. In other words, there was no oligomerization of MAHmonomers during this low-temperature free radical MAHgrafting reaction of PP-co-p-BT copolymer to form white PP-g-MAH product, which is quite different from the browncommercial PP-g-MAH polymers that also exhibit low polymermolecular weight.Figure 6 compares absolute GPC (LS) curves of two reaction

sets A-2 and B-6 (Table 2), using light scattering (LS) detectorto determine the absolute polymer molecular weight. Figure 6(left) compares the starting PP-co-p-BT copolymer (with 1 mol% p-BT reactive comonomer units) prepared by thehomogeneous metallocene catalyst and three correspondingPP-g-MAH polymers (with 0.29, 0.76, and 0.90 wt % MAHcontents determined by 1H NMR). Figure 6 (right) comparesthe starting PP-co-p-BT copolymer (with 1.3 mol % p-BTreactive comonomer units) prepared by the heterogeneousZiegler−Natta catalyst and three corresponding PP-g-MAHpolymers (with 0.37, 1.0, and 1.1 wt % MAH contentsdetermined by 1H NMR). The calculated polymer molecularweight information is summarized in Table 3. As discussed,before GPC measurements, the MAH groups in all PP-g-MAHpolymers were also converted to MAH−CH2−Si(CH3)3 groupsto prevent polymer chain interactions.All polymers in set B-6, including the starting PP-co-p-BT

copolymer prepared by the homogeneous metallocene catalystand three corresponding PP-g-MAH polymers with varyingdegrees of MAH grafting reactions, show very high molecularweights with narrow molecular weight distributions (PDI ∼

2.1); the molecular weight difference before and after the MAHmodification is extremely minimal. We also note a slight retreatin the B-6-MAH-3 sample, implying the up limit for theselective free radical MAH grafting reaction condition (85 °Cfor 3 h) without significant side reactions. As shown in set B-3(Table 2), a reduction of polymer molecular weight becomesmore apparent for PP-g-MAH polymers (runs B-3-MAH-6 andB-3-MAH-7) that were operated at a slightly higher reactiontemperature (95 °C). Similarly well-controlled MAH graftingreactions were also observed in set A-2, with the exception of abroad molecular weight distribution (PDI > 4.8) with a lowmolecular weight shoulder shown in all polymers, which areindicative of the starting PP-co-p-BT copolymer being preparedby the heterogeneous Ziegler−Natta catalyst. It is important tonote that these polymers in set A-2 also exhibit a broadcomposition distribution. Most of the reactive p-BT groups andthe subsequent incorporated MAH moieties are located in lowmolecular weight polymers that have highly undesirablemolecular structures serving as the interfacial agents.

■ CONCLUSION

This paper addresses a long-standing scientific challenge andindustrially important issue in the free radical maleic anhydridegrafting reaction of polypropylene to prepare a desirable PP-g-MAH molecular structure with high molecular weight, narrowmolecular weight and composition distributions, single MAHgrafting unit, and a controlled amount of MAH content. Thisnew chemical strategy is started with the design of poly-(propylene-co-p-(3-butenyl)toluene) copolymer that can beeffectively prepared by metallocene-mediated propylene co-polymerization to form a broad composition range of highmolecular weight PP-co-p-BT copolymers with narrowmolecular weight and composition distribution. The bulky p-BT comonomer units containing the reactive φ-CH3 moietiesare largely located in the amorphous domains with good sidechain flexibility that facilitates the selective MAH graftingreaction at φ-CH3 moieties in suspension reaction conditions.Despite the proton concentrations of secondary CH2 andtertiary CH in the PP backbone, which are much higher thanthat of the fewer φ-CH3 in the PP-co-p-BT copolymer, weobserved a remarkable selectivity of the free radical MAH

Figure 6. Absolute GPC (LS) curves for (left) starting PP-co-p-BT (run B-6) and three corresponding PP-g-MAH copolymers (runs B-6-MAH-1, B-6-MAH-2, and B-6-MAH-3) and (right) starting PP-co-p-BT (run A-2) and three corresponding PP-g-MAH copolymers (runs A-2-MAH-1, A-2-MAH-2, and A-2-MAH-3).

Table 3. Summary of Absolute GPC (LS) Results for TwoComparative Sets Shown in Figure 6

sample Mn (g/mol) Mw (g/mol) Mz (g/mol) Mw/Mn Mz/Mw

B-6 262 900 559 400 946 300 2.12 1.69B-6-MAH-1 255 300 531 400 885 800 2.08 1.67B-6-MAH-2 245 100 526 700 878 500 2.14 1.67B-6-MAH-3 226 000 466 200 773 700 2.06 1.66A-2 60 100 369 200 1 030 300 6.14 2.79A-2-MAH-1 64 900 357 900 1 005 800 5.52 2.81A-2-MAH-2 57 100 336 300 916 900 5.89 2.73A-2-MAH-3 59 700 291 400 738 000 4.88 2.53

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grafting reaction on φ-CH3 moieties. It appears that thecombination of reactivity sequence of φ-CH3 > −CH2-φ ≫CH (backbone) and CH2 (backbone), intact PP semicrystallinestructure under suspension reaction condition, and low reactiontemperature (<85 °C) allows this free radical MAH graftingreaction to avoid nearly all undesirable side reactions, includingPP backbone degradation and MAH oligomerization. Theresulting PP-g-MAH polymers essentially keep their similarmolecular weight as the starting PP-co-p-BT copolymer, and thecontent of the single MAH unit is controlled by p-BTcomonomer concentration, the free radical initiator, andreaction time. Overall, this well-controlled MAH graftingreaction is very different from those in the commercial MAHgrafting reaction of PP homopolymers, in which the MAHcontent is reversely proportional to the polymer molecularweight (Scheme 1). In addition, the suspension reaction allowsfor the use of a high concentration of the PP-co-p-BT reactantand simple filtration and washing of the resulting PP-g-MAHproduct. Overall, it also provides an economic reaction process.Lastly, these new PP-g-MAH polymers shall effectively increasethe needed PP interactive properties in many applications,including PP blends and composites, which are currently underinvestigation.

■ ASSOCIATED CONTENT*S Supporting InformationPropylene/p-BT copolymerization reactions with low mono-mer conversions using rac-Me2Si[2-Me-4-Ph(Ind)]2ZrCl2/MAO and TiCl3·AA/AlEt2Cl catalysts. This material is availablefree of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: chung@ems.psu.edu (T.C.M.C.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors at Penn State University thank the NationalScience Foundation for the financial support. The authors alsothank Dow Chemical Company for the kind assistance in thehigh-temperature GPC triple detector measurements.

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