REPORT◥

POLYMERS

Combining polyethylene andpolypropylene: Enhanced performancewith PE/iPP multiblock polymersJames M. Eagan,1 Jun Xu,2 Rocco Di Girolamo,1* Christopher M. Thurber,2

Christopher W. Macosko,2 Anne M. LaPointe,1† Frank S. Bates,2† Geoffrey W. Coates1†

Polyethylene (PE) and isotactic polypropylene (iPP) constitute nearly two-thirds of the world’splastic. Despite their similar hydrocarbon makeup, the polymers are immiscible with oneanother. Thus, common grades of PE and iPP do not adhere or blend, creating challengesfor recycling these materials. We synthesized PE/iPP multiblock copolymers using anisoselective alkene polymerization initiator. These polymers can weld common grades ofcommercial PE and iPP together, depending on the molecular weights and architecture ofthe block copolymers. Interfacial compatibilization of phase-separated PE and iPP withtetrablock copolymers enables morphological control, transforming brittle materials intomechanically tough blends.

Polyethylene (PE) and isotactic polypropylene(iPP) are the twomost abundantly producedplastics worldwide. More than 70 millionand 50 million metric tons of PE and iPP,respectively, are produced annually (1). The

vast majority of PE and iPP are prepared usingheterogeneous chromium and titanium catalysts(2). Heterogeneous olefin polymerization catalystshave a multitude of active sites, each with theirown reactivity differences that give rise to poly-mers of different molecular weights (MW), MWdistributions, and microstructures (3). In the caseof PE and iPP, these differences and their phaseseparation inhibit interfacial adhesion and erodethe mechanical properties of melt blends (4).Roughly 5% of the value is retained when theseplastics are recycled, typically into lower-valueproducts as a result of sorting expenses and de-graded physical properties (5). Compatibilizersopen opportunities for upcycling recovered PE/iPP into equal- or higher-value materials withlower sorting costs (6, 7). Because PE and iPP areof great economic importance (more than ~$200billion in annual sales, worldwide), strategies tocombine these materials may have considerablepotential to affect sustainability and the economy.Single-site metallocene catalysts have been de-

veloped and commercialized to produce polyole-fins with a more uniform molecular composition(8). These polymers demonstrate distinct physical

properties from their heterogeneous counterparts,such as the ability to form interfacial welds (9).Many nonmetallocene single-site catalysts havealso been developed, some of which demonstrateliving character for olefin polymerization as wellas precise control over the MW, stereochemistry,and architecture of the resulting polymer (10, 11).Strategies to compatibilize iPP and PE rely on

the addition of large amounts (≥10%) of addi-tives, typically amorphous polymers (12–14). Webecame interested in howblock copolymers of PEand iPP would behave as additives in commer-cial heterogeneous-grade polyolefin materials.Despite the simplicity of this idea, we are awareof only a single example of a well-defined semi-crystalline polymer combining blocks of PE andiPP. Busico and co-workers used a modified C2-symmetric zirconium catalyst to synthesize a PE-b-iPP diblock copolymer (15, 16). The iPP blockwas shown to be unusually stereoregular [mmmmpentad content (8) (m4) = 99%; melting temper-ature (Tm) = 152°C] and polymerization was wellcontrolled [dispersity (Đ)= 1.3], butnumber-averageMWs were relatively low (Mn = 22 kg/mol) (16).Experimental results revealed that the catalystunderwent b-hydride elimination, resulting inlimited chain growth and accessible architec-tures. As part of our long-standing interest inliving olefin polymerization, we sought to devel-op improved catalysts with longer lifetimes ca-pable of producing higher-MW PE/iPP diblockand multiblock copolymers.Pyridylamidohafnium catalysts are active for

the high-temperature production of high-MW iPP(17) and chain-shuttling polymerization (18). Werecently described the optimization of a pyridyl-amidohafnium catalyst (1) for the controlled iso-selective polymerization of propylene (19). Wereport the use of the catalyst to produce high-

MW iPP-b-PE diblock and multiblock copoly-mers with precise control of block length (Fig. 1).The activation of precatalyst 1 with B(C6F5)3,

followed by the condensation of propylene intothe reaction, afforded isotactic polypropylene(Fig. 1). After complete consumption of propyl-ene, the introduction of ethylene to the reactionvessel resulted in the formation of a iPP-b-PE di-block copolymer. Mn values increased linearly asa function of monomer conversion and were ingood agreement with theoretical values, consistentwith the behavior of a living polymerization (20).The MW of the ethylene block was controlled

by varying reaction time (Table 1, entries 1 and 2)

RESEARCH

Eagan et al., Science 355, 814–816 (2017) 24 February 2017 1 of 3

1Department of Chemistry and Chemical Biology, BakerLaboratory, Cornell University, Ithaca, NY 14853, USA.2Department of Chemical Engineering and Materials Science,University of Minnesota, Minneapolis, MN 55455, USA.*Present address: Dipartimento di Scienze Chimiche, Universitàdi Napoli Federico II, Complesso Monte S. Angelo, Via Cintia,I-80126 Napoli, Italy. †Corresponding author. Email: lapointe@cornell.edu (A.M.L.); bates001@umn.edu (F.S.B.); coates@cornell.edu (G.W.C.)

Hf

N

N

CH3

CH3

H3C

B(C6F5)3

H3C

i Pr

i Pr 1

po

m n po

3

2

10

50

100

150

0 1 2 3 4

Ð (

Mw

/Mn)

Yield (g)

M n (th

eo.)

ÐMn of i PP-b -PEMn of i PP

Mn

(kg/

mol

)

switch toethylene

1)

2)

3)

4)

LHf+

LHf+

PE-block-i PP(PEkDaPPkDa)

PE-block-i PP-block-PE-block-i PP(tetrablock copolymer)

Fig. 1. Synthesis of PE/iPP block copolymers.The structure of isoselective pyridylamidohafniumcatalyst (1)/B(C6F5)3 and the controlled polym-erization of olefins to ethylene and propylene mul-tiblock copolymers are shown. After quenchingwith methanol, hafnium end groups were proto-nated. Number-average molecular weights (Mn) anddispersities (Đ = Mw/Mn) were determined usingsize-exclusion chromatography calibrated with poly-ethylene standards.Mw, weight-average molecularweight; theo., theoretical; iPr, isopropyl.

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under a constant ethylene feed, whereas propyl-ene MWs were tuned by the monomer:catalystratio and full conversion. The single melting endo-therms observed (fig. S2) are due to regio- andstereoerrors in the propylene block. These errorslower the Tm of the iPP homopolymers to 134°C(versus ~165°C for perfect iPP), which is verysimilar to the Tm of the PE block (135°C). Thiswas confirmed by quantitative 13C nuclear mag-netic resonance (NMR) spectroscopy (fig. S1),which showed high stereoselectivity for 1,2-insertion of polypropylene (m4 = 91%). Regio-errors previously observed with this class ofcatalysts were also detected (17). The NMRspectrum showed neither detectable vinylideneend groups, which would arise from b-hydrideelimination, nor peaks consistent with randomethylene-co-propylene segments; this confirmsthat there is minimal tapering in the materials.Consistent with this reactivity, catalyst 1/B(C6F5)3was capable of synthesizing PE/iPP tetrablockcopolymers (Table 1, entries 4 and 5). Gel per-meation chromatography analysis of aliquotstaken after complete consumption of the mono-mers showed that MWs increased after eachmonomer addition and molar mass dispersitiesremained low (fig. S3), although someMWbroad-ening was observed due to precipitation of theinsoluble, semicrystalline polymer.Owing to thermodynamic incompatibility,

weak van der Waals interactions, and the accu-mulation of amorphous polymer at the junctionbetween melt-molded laminates, most commer-cial grades of iPP and PE homopolymers dis-play poor interfacial adhesion (9). We used asimple peel test to evaluate adhesion betweenheterogeneous-grade PE and iPP laminates withand without the presence of the block copolymeras an adhesive layer (Fig. 2). Rectangular plaquesof two (PE/iPP) and three layers (with block co-polymer film) were compression-molded in themelt and then pulled apart while peel strength(S = force/sample width) was monitored. Thistest provided a facile method for comparing theinterfacial strength between themolded films.Wereport the results obtained from 100-mm-thickblock copolymer films, but note that no thick-ness dependence was observed down to 5-mmsolvent cast films (fig. S9).Figure 2 shows representative peel test results

obtained as a function of block copolymermolec-

ular architecture and MW. Laminates withoutblock copolymers peel apart easily (S < 0.5 N/mm).Incorporation of the PP24PE31 and PP73PE50 di-block copolymers increases the peel strength to S≈1 and 3 N/mm, respectively. Increasing the MW ofboth blocks beyond a threshold value leads to asubstantial change in the failuremechanism fromadhesive failure (low MWs) to cohesive failure(fracture, S > 6 N/mm) of the PE homopolymerfilm above ~75 kg/mol, as shown in Fig. 2 (seealso figs. S4 and S5). The interfacial strength be-tween the diblock and homopolymer films is de-pendent on the block sizes due to two factors.

First, the block copolymer acts as a surfactant,eliminating the thermodynamic driving force foramorphous materials to localize at the interfacebetween block copolymer and iPP and PE filmjunctions (9). In some respects, the block copoly-mer acts as a type of macromolecular weldingflux material. Second, increasing the overall blocksize enhances interpenetration and the number ofentanglements between the chemically identicalblocks and homopolymer chains in the melt state(21, 22). Moreover, we anticipate a threshold MWbeyond which the polymer block will be able tobridge the amorphous layers associated with the

Eagan et al., Science 355, 814–816 (2017) 24 February 2017 2 of 3

Table 1. Properties of PE/iPP block copolymers. Cat., catalyst; Pethylene, pressure of ethylene; trxn, reaction time;Mn, number-average molecular weight; theo.,

theoretical; tot., total; Mw, weight-average molecular weight; Đ, dispersity; Tm, melting temperature. See supplementary materials for experimental details.

Entry Product

(PPkDaPEkDa)

Cat.

(mmol)

C3H6

(g)

Pethylene

(atm)

trxn C2H4

(min)

Yield

(g)

Mn (theo.)

(kg/mol)

Mn (tot.)

(kg/mol)

Đ(Mw/Mn)

Tm(°C)

1 PP71PE137 30 2.3 2.7 10 6.2 207 208 1.29 133.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .

2 PP73PE50 30 2.3 2.7 5 3.9 130 123 1.29 131.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .

3 PP24PE31 75 1.5 2.0 3 3.3 44 55 1.32 132.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .

4 PP36PE20PP34PE24 25 1.0, 1.0 1.4, 1.4 4, 4 3.0 120 113 1.38 124.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .

5 PP60PE80PP75PE90 30 2.0, 2.0 2.7, 2.7 4, 4 8.5 283 306 1.29 126.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .

Fig. 2. Peel strengthof commercial PE/iPPwith various blockcopolymers. Rectangu-lar sheets (0.6 cm by6 cm, 340 mm thick) ofPE/iPP were laminatedin the melt at 180°C,with and without PE/iPPblock copolymer layers(100 mm thick), andwere pulled apart at10 mm/min. After test-ing, specimens wereinvestigated withscanning electronmicroscopy (fig. S6).Proposed models toexplain the adhesivedifferences between(A) tetrablock, (B) high-Mn diblock, and (C) low-Mn diblock copolymersare shown. Blockcopolymers are inmultiple lamellae (40 to70 nm) (fig. S17); thefirst layer is shown.Stars indicate that PEfilms break or deformrather than undergodelamination.

40-7

0 n

m

0 10 20 300

2

4

6 PE breaks/yieldsPP36PE20PP34PE24

PP71PE137

PP73PE50

PP24PE31

no block copolymer

Peel Displacement (mm)

Pee

l Str

engt

h (N

/mm

)

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lamellar morphology of semicrystalline polymerssuch as iPP and PE, leading to cocrystallizationalong the film interfaces, as shown in Fig. 2B andfig. S15 (21). Lower-MW blocks are less capableof reaching the homopolymers’ crystalline lamel-lae (Fig. 2C) and will be prone to chain pullout,resulting in lower adhesive strength. The weld-ing effect was observed in various polyolefinmate-rials (fig. S7) and only with semicrystalline blockpolymer adhesives (fig. S8).The PP36PE20PP34PE24 tetrablock copolymer

exhibits considerable adhesive strength evidencedby cohesive failure (Fig. 2A), which seeminglycontradicts the aforementioned arguments, asall of the blocks are well below the thresholdMW required for cohesive failure with diblocks.Thus, we invoke a different mechanism for thisresult. A tetrablock molecular architecture ensuresthat half of the iPP and PE blocks are flankedby thermodynamically incompatible counter-parts. This implies that interfacial mixing duringmelt compression produces entangled loops thateffectively stitch together the homopolymers andblock copolymer films upon crystallization whenthe laminates are cooled, as illustrated in Fig. 2A.Similar arguments account for the enhancedtoughness of bulk multiblock versus triblockpolymers (23, 24). Consistent with this line ofreasoning, the PP36PE20PP34PE24 tetrablock co-polymer is microphase-separated up to 260°C,as shown by rheological measurements (fig. S17).Challenges of recyclingmixed polyolefinmunic-

ipalwaste (typically 70:30PE:iPP) are partly due tophase separation and fragile interfaces, which re-sults in poor mechanical properties (5–7). Becausespecialty grades of PE and iPP can be blended toimprove impact and crack resistance, the effectivecompatibilization of heterogeneous-grade poly-

olefins may allow an upcycling of plastic wastesinto higher-valuematerials (12). Figure 3A showsthe morphology obtained from a heterogeneous-grade polyolefin blend containing 70 weight %(wt%) PE and 30wt% iPP, and Fig. 3B illustratesthe consequences of adding 5 wt % tetrablockPP60PE80PP75PE90 to this mixture. Interfacial ac-tivity of the block copolymer is evidenced by areduction in the average droplet size from 2.2 to0.55 mm with the addition of the tetrablock co-polymer; similar results were obtained with otherarchitectures (figs. S11 to S14).Individually, pure iPP and PE display ductility

and strain hardening when pulled in tension atroom temperature (Fig. 3C). Blending the twocomponents leads to a phase-separated mate-rial and a marked reduction in strain at thebreak point (eb = 12% versus 300 and 800% foriPP and PE, respectively). The addition of 5 wt %PP60PE80PP75PE90 raises eb to 600%, owing tothe combined effects of interfacial adhesion,reduced particle size, and efficient stress trans-fer between phases (fig. S16). With just 1% of thistetrablock copolymer, eb = 450%, whereas theaddition of 1 wt % of the corresponding diblockcopolymer (PP60PE80) leads to a modest improve-ment (eb = 90%). The low-MW tetrablock polymerPP36PE20PP34PE24 exhibited similar properties,as did other blends with different PE:iPP ratios(fig. S18).We have developed a catalyst system capable

of synthesizing semicrystalline PE/iPPmultiblockcopolymers with precise control over block lengthand architecture. These macromolecules formstrong interfaces with commercial PE and iPPwhen properly designed. Two molecular mech-anisms are proposed to explain the MW depen-dence of diblock copolymer adhesion and the

behavior of tetrablock copolymers with relativelyshort blocks. The interfacial strength translatesinto control over morphology and mechanicaltoughness in melt blends of commercial PE andiPP, blends that are otherwise brittle at a ratiotypically found in municipal waste streams.

REFERENCES AND NOTES

1. D. B. Malpass, E. I. Band, “Introduction to polymers ofpropylene,” in Introduction to Industrial Polypropylene:Properties, Catalysis, Processes (Scrivener, 2012), pp. 1–18.

2. M. Stürzel, S. Mihan, R. Mülhaupt, Chem. Rev. 116, 1398–1433(2016).

3. W. Kaminsky, Ed., Polyolefins: 50 Years After Ziegler and Natta I(Advances in Polymer Science Series, Springer, 2013).

4. D. Nwabunma, T. Kyu, Eds., Polyolefin Blends (Wiley, 2008).5. “The new plastics economy: Rethinking the future of plastics”

(Ellen MacArthur Foundation, 2016); www.ellenmacarthurfoundation.org/publications/the-new-plastics-economy-rethinking-the-future-of-plastics.

6. M. Chanda, S. K. Roy, Eds., in Plastics Fabrication and Recycling(Plastics Engineering Series, Taylor and Francis, 2009),pp. 2-1–2-41.

7. M. Xanthos, Science 337, 700–702 (2012).8. L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 100,

1253–1346 (2000).9. K. A. Chaffin, J. S. Knutsen, P. Brant, F. S. Bates, Science 288,

2187–2190 (2000).10. G. J. Domski, J. M. Rose, G. W. Coates, A. D. Bolig,

M. Brookhart, Prog. Polym. Sci. 32, 30–92 (2007).11. P. D. Hustad, Science 325, 704–707 (2009).12. J. W. Teh, A. Rudin, J. C. Keung, Adv. Polym. Technol. 13, 1–23

(1994).13. Y. Lin, V. Yakovleva, H. Chen, A. Hiltner, E. Baer, J. Appl. Polym.

Sci. 113, 1945–1952 (2009).14. A. Colbeaux, F. Fenouillot, J.-F. Gérard, M. Taha, H. Wautier,

J. Appl. Polym. Sci. 95, 312–320 (2005).15. V. Busico, R. Cipullo, N. Friederichs, S. Ronca, M. Togrou,

Macromolecules 36, 3806–3808 (2003).16. V. Busico et al., Macromolecules 37, 8201–8203 (2004).17. T. R. Boussie et al., Angew. Chem. Int. Ed. 45, 3278–3283

(2006).18. D. J. Arriola, E. M. Carnahan, P. D. Hustad, R. L. Kuhlman,

T. T. Wenzel, Science 312, 714–719 (2006).19. G. W. Coates, G. J. Domski, Patent WO 2008112133A2 (2008).20. O. W. Webster, Science 251, 887–893 (1991).21. J. J. Benkoski, P. Flores, E. J. Kramer, Macromolecules 36,

3289–3302 (2003).22. C. Laurens, C. Creton, L. Léger, Macromolecules 37, 6814–6822

(2004).23. A. Phatak, L. S. Lim, C. K. Reaves, F. S. Bates, Macromolecules

39, 6221–6228 (2006).24. F. S. Bates et al., Science 336, 434–440 (2012).

ACKNOWLEDGMENTS

Financial support for this work was provided by the Center forSustainable Polymers, an NSF Center for Chemical Innovation(grant CHE-1413862). The Cariplo Foundation (“CrystallineElastomers Project”) provided funding to R.D.G. We thankN. Chen, S. Hayrapetyan, and M. Silberstein for assistance withmelt-blend and tensile studies and I. Keresztes for assistance withNMR spectroscopy. J.M.E., A.M.L., and G.W.C. are inventors onU.S. patent applications 62/421,680 and 62/342,583, submittedby Cornell University, which cover welds and compatibilization ofpolyolefin materials. All data are available in the supplementarymaterials. Author contributions: G.W.C. conceived the project.G.W.C., J.M.E., R.D.G., and A.M.L. designed and conductedexperiments related to polymer synthesis. J.M.E., J.X., and C.M.T.designed and conducted experiments related to adhesion andblends. G.W.C., A.M.L., C.W.M., and F.S.B. designed experimentsand directed research. J.M.E., G.W.C., A.M.L., J.X., and F.S.B. wrotethe manuscript. All authors commented on the manuscript.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/355/6327/814/suppl/DC1Materials and MethodsFigs. S1 to S18References (25–27)

1 September 2016; accepted 25 January 201710.1126/science.aah5744

Eagan et al., Science 355, 814–816 (2017) 24 February 2017 3 of 3

Fig. 3. Uniaxial tensile elonga-tion of PE/iPP materialsand blends. (A and B) Transmis-sion electron microscopy imagesof PE/iPP blends show dropletmorphology, both without blockcopolymers (A) and with5 wt % tetrablock copolymers(B). (C) Materials were melt-blended at 190°C without blockcopolymers (black) or with 1 wt %diblock (green), 1 wt % tetrablock(orange), or 5 wt % tetrablockcopolymers (purple). Thesematerials were then compressionmolded into tensile specimens at180°C and strained at a rate of100%/min (fig. S10).

Str

ess

(MP

a)

Strain (%)

PE:i PP+1% PP60PE80

i PP

PE

PE:i PP+1% PP60PE80PP75PE90

PE:i PP+5% PP60PE80PP75PE90

5 µm 5 µm

0

10

20

30

40

0 200 400 600 800

PE:i PP (70:30)no block

copolymer

00

10

20

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(6327), 814-816. [doi: 10.1126/science.aah5744]355Science Bates and Geoffrey W. Coates (February 23, 2017) Thurber, Christopher W. Macosko, Anne M. LaPointe, Frank S. James M. Eagan, Jun Xu, Rocco Di Girolamo, Christopher M.

PP multiblock polymersiperformance with PE/Combining polyethylene and polypropylene: Enhanced

Editor's Summary

, this issue p. 814; see also p. 797SciencePP and PE and allow blending of the two polymers.ireinforce the interface between

polymer initiator (see the Perspective by Creton). The high-molecular-weight blocks could be used to PP and PE by using a selectivei report the synthesis of multiblock copolymers of et al.products. Eagan

together, which limits options for dealing with mixed waste and decreases the value of recycledplastics and thus make up a large fraction of the waste stream. However, the two plastics will not mix

PP) are the two most widely used commodityiPolyethylene (PE) and isotactic polypropylene (How to make opposites compatible

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