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Scheme 1. Synthesis of 4,7-Substituted ACSMacr(Uetter£> pubs.acs.org/macroletters [2.2.2]Paracyclophane-Trienes 4a-c

[2.2.2]Paracyclophane-Trienes—Attractive Monomers for ROMP Dominic Maker, Christopher Maier, Kerstin Brodner, and Uwe H. F. Bunz*

Organisch-Chemisches Institut, Ruprecht-Karls-Universitat Heidelberg, INF 270, D-69120 Heidelberg, Germany

0 Supporting Information

ABSTRACT: Three derivatives of 4,7-substituted [2.2.2]- paracyclophane-trienes were synthesized and used in ring-opening metathesis polymerization (ROMP), resulting in well-soluble poly(para-phenylenevinylene)s (PPV). The paracyclophane-tri- enes were prepared using an iterative buildup of a phenylene— ethynylene backbone, followed by a cis selective Grignard reduction and an intramolecular McMurry reaction. The monomers were applied in ROMP to result in well-soluble PPV derivatives with an unusual substituent pattern. The PPVs were spin-coated into amorphous, highly fluorescent films. To the best of our knowledge, we are the first to synthesize 4,7- Figure 2. Photographs of (A) absorption and (B) emission under substituted [ 2.2.2 Jparacyclophane-trienes and use them as ROMP monomers. ultraviolet illumination (365 nm) of polymer and monomer solutions in CHClj. (C) Photographs of spin-coated films out of C6HSC1 under ultraviolet illumination (365 nm).

ue to their spectacular optical and optc"'l'"-Trn"L6 ring-opened into PPV, whereas the (Z,Z,Z,Z)-isomer does not D^properties, conjugated polyrqe/s such as polyfluorene.1 have enough strain energy to be active (Figure 1)." hydrochloric acid, overnight in the dark. Phase separation, polythiophene, poly(phenyleneethynylene) (PPE),' or poly- filtration through a plug of silica, and evaporation of the solvent (para-phenylenevinylene) (PPV\ have been studied intensely.5 nder light exclusion give 3a-c with a nearly complete all-cis Their potential as semiconductors in applications such as onfiguration according to ]H NMR. The dialdehydes 3a—c organic field-effect transistors (OFET),6 organic photovoltaics Scheme 2. Polymerization of Monomers 4a-c to PPV vere directly applied in the pseudo high dilution McMurry (OPV),7 or organic light-emitting diodes (OLEDs)8 is Derivatives Sa—c yclization using a syringe pump, protected with aluminum foil, significant. 'he [2.2.2]paracyclophane-trienes 4a—c are obtained in good PPV mightjie^hp r^pct^y-imifted semiconducting polymer. ields (55-64%) after scaling up. This is higher than the yields UnsuSstitutedPPV has successfully been exploited ror ULhLJs* (Z.Z.Z.Z) (E.Z.E.Z) ublished for similar systems.24' 8'29 4a—c were isolated as pale and OPVs.10 Due to the parents' poor solubility, it is processed ellow-colored oils which show weak fluorescence in their pure Figure 1. Structures of the [2.2.2.2]paracyclophane-tctraenes. out of a precursor polymer by extrusion of leaving groups, as nd dissolved states.10 studied by Wessling et al., which gives the parent PPV.11 If Aromatic proton signals in 'H NMR spectra are high-field solubilizing groups are attached to the PPV backbone, Herein we report the synthesis of dodecyl-, dodccyloxy-, and lifted, overlapping with vinylic signals, reflecting the electronic 2-ethylhexyloxy substituted [2.2.2]paracyclophane-trienes 4a—c nteraction within the system. Surprisingly cyclophanes 4a-c processable PPVs with controlled morphology are accessible.12 ire oils, in contrast to their unsubstituted parent molecule11 or Dozens of different side chains were attached to the PPV as monomers of well-soluble PPVs with an unusual substituent pattern. We developed a new synthetic route for these icir ortho substituted derivatives.29 backbone, highlighting relationships between polymer struc- [2.2.2]paracyclophane-trienes and applied them in ROMP to In Scheme 2, ROMP of 4a-c into PPVs 5a-c is shown; ture, phototuminescence yield, and efficiency of electro- give the PPVs 5a-c. 'able 1 summarizes the properties of the resulting PPVs. luminescence. ll>14 Scheme 1 shows the key synthetic steps. In the original ilymerization of 4a at room temperature yields PPV 5a with a Substituted and soluble PPVs are synthesized by Gitch-type, synthesis of the parent 4, Tanner et al. used a Wittig reaction olydispersity of 2.8. Increasing the temperature lowers the Pd-catalyzed, and other methods.1^"1* Acyclic diene metathesis building up the cis double bonds. Trying the same with para- olydispersity of 5a to 1.9, assuming that higher temperatures (ADMET) or ring-opening metathesis polymerization substituted benzene derivatives led to a mixture of cis and trans nproves the initiation of ROMP of 4a. The alkoxy substituted (ROMP) of suitable precursors is an attractive alternative for isomers, hardly separable. Consequently, we developed an erivatives 4b and 4c do not show any reaction at room defect free PPVs.19 After Bazan et al. studied the ROMP of alternative route. Sonogashira-Hagihara coupling gives la—c, ;mperature or in refluxing THF. A change to the higher paracyclophenes into PPV precursor polymers,20 Turner et al. which are easily and cis selectively transformed into 2a—c - Ill aii inliAlllulauUl McMurry reaction, yielding the 4,7- oiling solvent toluene (110 °C reaction temperature) elegantly developed a direct ROMP-approach of substituted through a Grignard reduction.26 substituted [ 2.2.2 ]paracyclophane-trienes 4a—c. reduces PPVs 5b and Sc with polydispersities of 1.4 and 1.7. [2.2]paracyclophane-dienes giving PPV-homo-21 and -block- For cyclization we used the McMurry reaction but To avoid the problem of isomerization of 3a—C, we The higher reaction temperature is necessary due to the copolymers.22' * However, the paracyclophane-diene mono- encountered several problems. Deprotection of acetals 2a—c employed the configurationally stable acetals 2a-c in the oxygen in the side chains of 4b,c, which works as anchor group mers are difficult to synthesize, yet ROMP is a suitable and with hydrochloric acid in THF yields crude diatdehydes 3a—c, intramolecular McMurry reaction.27 Acceptable yields are for the catalyst, inducing an energy barrier for ROMP. 'H powerful method for the direct synthesis of soluble PPV which can be purified by column chromatography. Unfortu- obtained (100 mg ~40%) on small scales but yields drop NMR spectroscopy of polymers 5a—c (see Supporting derivatives with low polydispersities. nately, when dissolved, 3a—c isomerize rapidly. The yield of the upon scale-up, and the competing intermolecular polymer- Information) shows a trans/cis ratio about 59—70%, Inspired by Mullen et al and Oda et al." we contended desired all-cis isomers drop. Nevertheless, 3a—c can be coupled ization becomes prevalent. Reductive ring closure, yielding a respectively, indicating that the double bonds formed during that larger cyclophenes might have enough strain energy to yive _ single instead of a double bond, is another side reaction when ROMP are mostly trans-con figured and those that are not using 2a—c, complicating purification. ROMP-able monomers We found that unsubstituted [2.2.2]- Received: March S, 2014 opened by ROMP tend to isomerize easily (calculated trans/cis The solution to the problem of isomerization and low yields paracyclophane-triene follows ROMP into insoluble PPV. The Accepted: Apnl 8, 2014 ratio: 33%). Thermogravimetric analysis of the polymers during scale-up is the simple omission of the purification of the (E,Z,E,Z)-isomer" of [2.2.2.2]paracyciophane-tetraene is also indicates no weight loss up to 300 °C. As metathesis is quite dialdehydes 3a-c. Dioxolanes 2a-c are deprotected by diluted sensitive to steric effects, we assume that the double bond r ACS Publications ag/10.1031/miS0013Si I ACS M •oirtt 2014, ), 414-41*

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Chapter 5 is devoted to a discussion of their preparation and characteri/ation. For now. only the terminology involved in their description concerns us. Three different situations can be distin- guished along a chain containing pseudoasymmetric carbons: I. l\ntuct'u-. All sLibsiiiuents lie on the same side of the extended chain. Alternatively, the stereoconfiguration at the asymmetric centers is the same. say. -DDDDDDDDD-. 1. Syndiotactic. Substituents on the fully extended chain lie on alternating sides of the backbone. This alternation ot 'configuration can be represented as DLDLDLDLDLDL-. 3. Alactic. Substituents are distributed at random along the chain, tor example. DDLDLLLDLDLL-. Figure U shows sections of polymer chains of these three types: the substituent X equals phenyl for polystyrene and methyl lor polypropylene. The general term for this stereoregularity is tiifiiiiiy. a term derived from the Greek word meaning "to put in order." Polymers of different tactieily have quite different properties, especially in the solid state. As we will see in Chapter H. one of the requirements for polymer cry stallmity is a high degree of microstructural regularity to enable the chains to pack in an orderly manner. Thus atactic polypropylene is a soft, tacky substance, whereas both isoiaclic and syndiotactic polypropylene are highly crystalline.

1.6.3 Geometrical Isomerism The final type of isomerism we take up in this section is nicely illustrated by the various possible structures that result from the polymerization of 1.3-diencs. Three important monomers of ihis type are 1.3-butadiene, 1.3-isoprene. and 1,3-chloroprene. Structure (I.X) through Structure (I.XII). respectively:

(I.X) ^ 204 Copolymers, Microstructure, and Stereoregularity

Solution Since the total numbers of dsads and triads always occur as ratios in Equation 5.9.3 and Equation 5.9.4, both the numerators and denominators of these ratios can be divided by the total number of (Kails or Iliads to convert these total numbers into fractions, i.e.,

Thus the fractions in Table 5.7 can he substituted for the i/s in Equation 5.9.3 and Equation 5.9.4. The values of n, and HS so calculated for the three polymers are:

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Atactic 1 .5') 5.64 Sviuliotaelie 1.32 6.45 Isoiaetic 9.14 1.37

This analysis adds nothing new to the picture already presented by the dyad and triad probabilities. It is somewhat easier to visuali/.e an average sequence, however, although it must be remembered that the latter implies nothing about the distribution of sequence lengths. We conclude this section via Figure 5.9. which introduces the use of ' C-NMR obtained at KM) MHz for the analysis of Stereoregularity in polypropylene. This spectrum shows the carbons on the pendant methyl groups for an atactic polymer. Individual peaks are resolved for all the possible pentad sequences. Polypropylene also serves as an excellent starting point for the next section, in which we examine some of the catalysts that are able to control Stereoregularity in such polymers. TTTTT rmrr m m r m r m r r Trrrr Trrrr m m r r m r m r m m m r

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a- 1 /Mr —P TYYm m mY -^ Ligand Controls Stereochemistry Isotactic are now available that can control the molecular cant, the mode of stereochemical regulation is referred weight, molecular weight distribution, comonomer__a~ forajr^gplymeT chain-enJcontrar. It should be noted incorporation, and both the relative and absolute th at in rare^SsEanEelTTnofeTnah one stereogenic stereochemistry of a polymer in a way that is often center of the polymer can play a significant role in impossible using conventional hetereogeneous cata- stereoregulation. If the ligand set is chiral and lysts. Although their commercial implementation in fiYfvrrjHps f>m influence of the polymer chain end, the the solution phase is often impractical, they can be mechanism of stereochemical direction is termed heterogenized for efficient gas-phase or flow-through ."gjIflf^pmorpVnV-sitp pnnt.rnl" (Scheme 1). In the reaction by attaching them to a solid support. Per- tbrmerTnep-hanism, a slereocnemical error is propa- haps most importantly, these defined molecular- gated^ while in the latter a correction occurs since based systems allow detailed structural and mecha- theTIgands direct the stereochemical events' nistic studies. Thus, through theoretical and empirical Scheme 1 introduces the parameters that are used studies scientists can rapidly evolve new and im- to describe the stereoselectivity of the monomer proved generations of catalysts. enchainment process. For chain-end control, the parameters Pm and Pr refer to the probability ofmeso B. Scope of Review anj__rarpmf'r placements,. r°pp<"'t-iwly (*hf> Rnvpy formalism is a convenient way to describe polymer This review covers the scientific literature from the tacticity, with a small "m" for meso, and a small "r" mid-1980s to the present concerning stereoselective for racemic relationships between adjacent stereo- polymerization by single-site transition metal and f-block metal complexes. Strategies for controlling the genic centers). A Pm equal to unity indicates isotac- relative configuration of main-chain stereogenic cen- ticity, while a Pr equal to unity signifies syndiotac- ters of chain-growth polyolefins are included; since ticity. For site-control mechanisms, the parameter a represents the degree of enantiotopic selectivity of the emphasis is on stereochemical control of polym- the enchainment. When a is either 1 or 0 an isotactic erization by the homogeneous catalyst, the polymer- polymer forms, while an a parameter of 0.5 produces ization of optically active monomers will not be an atactic polymer. Polymer architectures relevant covered. The review will concentrate on examining to this review are shown in Figure 1. state-of-the-art stereoselective polymerization cata- lysts and will focus on proposed mechanisms of There are several techniques for determining the Stereocontrol. Although the emphasis will be on type of tacticity and degree of stereoregularity of a polymer sample. Commonly used methods include stereochemical control by the catalyst, other impor- tant characteristics such as polymerization activity solubility, X-ray diffraction, IR spectroscopy, and and polymer properties will be included. thermal properties (melting point and glass-transi- tion temperature). In the case of chiral polymers, optical rotation can be used to determine the absolute C. Mechanisms, Nomenclature, and Quantification configuration as well as degree of enantiomeric purity of Stereoregularity when the optically pure polymer is available. How- Both the ligand set of a single-site catalyst and the ever the most useful method for determining a growing polymer chain influence the stereochemistry polymer's tacticity classification as well as quantify- of the polymerization reaction.13 It is interesting to ing its stereochemical purity is nuclear magnetic note that, unlike the catalytic synthesis of small resonance (NMR).14'15 In many cases the shifts for the molecules, during a chain-growth polymerization various polymer nuclei are sensitive to adjacent reaction a polymer chain remains bound to the active stereogenic centers, resulting in fine structure that metal center during monomer enchainment. Thus, can provide quantitative information about the poly- th_£ siereogenic centerfrom the last enchained mono- mer microstructure once the shifts identities are mer unit will have an influence on the stereochem- assigned. For example, the methyl region of a high- istry of monomer addition; it this influence is signjfi- resolution 13C NMR spectrum of atactic polypropyl- define chain shuttling as the passing of a grow- ing polymer chain between catalyst sites, such that portions of a single polymer molecule are /~r Catalytic Production of synthesized by at least two different catalysts. C. ^~ Likewise, a chain shuttling agent (CSA) is a component such as a metal alkyl complex that Olefin Block Copolymers via facilitates this transfer. This approach can thus be used to prepare block copolymers from a Chain Shuttling Polymerization common monomer environment by using a mix- ture of catalysts of different selectivities, namely Daniel ]. Arriola,1* Edmund M. Carnahan,2* Phillip D. Hustad,2* stereoselectivity or monomer selectivity. Under Roger L. Kuhlman,2* Timothy T. Wenzel1* the right conditions, efficient chain shuttling pro- duces a linear multiblock copolymer that features We report a catalytic system that produces olefin block copolymers with alternating semicrystajljne alternating hard and soft blocks. 3P(J amorphous spgrnent^ achieved by varying the ratio of a-olefin to ethylene in the two One key to forming differentiated block co- types of blocks. The system uses a chain shuttling agent to transfer growing chains between polymers via chain shuttling is finding a mono- twcMJistinct catalysts with different monomer selectivities in a single polymerization reactor. mer or combination of monomers that, on the The block copolymers simultaneously have high melting temperatures and low glass transition basis of their arrangement in the polymer chain, 00 temperatures, and therefore they maintain excellent elastomeric properties at high temperatures. can give rise to both hard and soft materials. o Furthermore, the materials are effectively produced in economically favorable, continuous Stereoblock PPs do not have the low glass CM polymerization processes. transition temperatures required for most elas- tomeric applications. On the other hand, uring the past 50 years, polyolefms have or radical (13-15) polymerization processes, ethylene-based polymers that incorporate vary- ing fractions of ct-olefm fit this criterion. become, by far, the highest volume can be used to achieve precise structural control f Dcommercial class of synthetic polymers. in block copolymer synthesis through sequen- Polyethylenes (PE) with low co-monomer o Olefin polymerization catalysts have evolved tial monomer addition strategies. However, content are semicrystalline (hard) materials S1 during this time from heterogeneous mixtures living polymerization processes are uneconom- with melting temperatures (T } approaching q 05 (1) to well-defined soluble molecules (2), al- ical because they produce only one polymer 1J5°C, whereas PEs with higlLje^eis—qf ro lowing chemists to understand and control the chain per catalyst molecule and operate in a comonomer are amorphous (soft) materials effects of catalyst structure on polymer compo- batch polymerization process. In addition, the with very low glass transition tempera!11"*'' (Tjf< I sition and microstructure. These advancements low reaction temperatures typically required to -40°C). We therefore focused on ethylene-based in catalysis have enabled the production of poly- achieve living behavior with these systems Block" copolymers with both hard and soft olefins with an exquisite degree of control over inhibit the synthesis of materials containing segments, with the hypothesis that the benefits stereochemistry (3) and macromolecular branch more than one semicrystalline block because of of both the high Tm and low Tg would be architecture (4), leading to new classes of poly- premature precipitation of the polymer. retained. To this end, we required a mixed mers with useful combinations of physical prop- To circumvent the problems associated with catalyst system capable of producing these 1 erties. However, the economical preparation of previous strategies for the preparation of poly- different types of polymer in a common reaction TD environment. A further important requirement is CD olefin block copolymers (5) having both "hard" olefm block copolymers, we pursued the syn- -o (semicrystalline or high glass transition tempera- thesis of these materials via a technique that we that each of the catalysts undergoes chain ro o ture) and "soft" (amorphous and low glass call "chain shuttling polymerization" (16). We shuttling with a common chain shuttling agent. transition temperature) segments remains one of the major challenges in the field of polym erization catalysis. Here we report a method fo: Can Call bearing Call bearing the preparation of linear ethylene-based block poor incorporates "HARD" polymer "SOFT" polymer copolymers with such properties by using chain chain shuttling shuttling polymerization. A few strategies for preparation of stereo- CSA bearing block polyolefms have been reported in the "SOFT" polymer

recent literature (6-9). Despite the high melting HARD/SOFT temperatures exhibited by isotactic or syndio- Olefin Block Copolymers tactic polypropylenes (PPs), the relatively high CSA bearing glass transition temperatures of these materials "HARD" polymer (T ~ 0°C) limit their utility in elastomeric ap- plications. More recently, olefin-based block copolymers have been made using living co- chain shuttling ordination polymerization catalysts (10). These Cat2 Cat2 bearing Cat2 bearing catalysts, like living anionic (11), cationic (12), good incorporator "SOFT" polymer "HARD" polymer Fig. 1. Depiction of the likely chain shuttling mechanism in a single reactor, dual-catalyst approach. 'The Dow Chemical Company, Building 1702, Midland, Ml Catl (solid circles) and Cat2 (solid triangles) represent catalysts with high and low monomer 48674, USA. 2The Dow Chemical Company, 2301 Brazosport, Freeport, TX 77541, USA. selectivity, respectively, whereas the CSA (solid squares) facilitates the chain shuttling reaction. Catl produces a segment of hard polymer with low comonomer content. Shuttling occurs when this *To whom correspondence should be addressed. E-mail: [email protected] (D.J.A.); [email protected] (E.M.C.); segment is exchanged with the CSA bearing a soft copolymer of higher comonomer content. Further [email protected] (P.D.H.); [email protected] (R.I.K.); chain growth at Catl then extends the soft copolymer chain with a hard segment, thus giving a block [email protected] (T.T.W.) copolymer.

714 5 MAY' SCIENCE www.sciencemag.org RESEARCH ARTICLE I This process (Fig. 1) requires one catalyst, occurs via a usual mechanism such as hydrogen- of the relative rates of chain growth and shuttling, Call, with high ethylene selectivity to form hard olysis. A statistical analysis of this chain shuttling can be controlled simply by adjusting the ratio of phenomenon reveals multiblock copolymers with concentrations of CSA and monomer ([CSA]/ " polymer chains. Meanwhile, a good incor- porator of comonomer, Cat2, grows soft amor- a most probable distribution of block lengths and [C2HJ)- phous chains in the same reactor because of its number of blocks per chain. This microstructure Selection of CSA and catalysts. It is well es- dramatically different monomer selectivity. In is distinctly different from materials made with tablished in olefin polymerization that growing^ the absence of chain shuttling, a polymer blend living polymerization techniques, which ideally chains can be transferred from the catalyst to aiT is produced with none of the advantageous prop- have a Poisson distribution of block lengths and added main-group metal in exchange for an erties of block copolymer architecture. In the a precise number of blocks. Furthermore, the alkyjjgoup (i.e., chain transfer to metal) (1,17). presence of an effective CSA, however, polymer synthesis of these olefin block copolymers is "frustransfer is most often irreversible, leading chains are swapped between catalysts before the not stoichiometrically limited by Call, Cat2, or to the termination of the growing chain and the chains terminate. To understand this swapping CSA. initiation of a new polymer chain. However, phenomenon, it is instructive to follow the life- Furthermore, this approach enables precise the synthesis of block copolymers via chain time of a representative polymer chain. The chain control over polymer microstructure. Despite the snuffling requires this polymer chainjraiis-fgrjo may begin growing on Call as a hard polymer. differences in monomer selectivity between the be reversible. The main-group centers cannot The chain is then exchanged onto a CSA, where it two catalysts, fast rates of chain shuttling and act as a final repository for "dead" polymer is held for some period of time without growing. judicious selection of process variables produce chains; instead, they must serve as a reservoir of The dormant polymer chain may then return to copolymers with homogeneous molecular weight "live" chains that are intermittently reattached to another molecule of the same catalyst and and composition distributions. The overall com- catalyst centers for further growth. Chien (7) and ro lengthen the hard segment or it may shuttle to a position, i.e., the hard-to-soft polymer ratio, can Brintzinger (8) have independently claimed o molecule of Cat2, from which subsequent chain be easily controlled by the relative amount of the preparations of stereoblock PPs using reversible CN growth results in formation of a polymer chain catalysts used. The comonomer content of the chain transfer between two catalyst centers with C\ with both soft and hard blocks. The process may individual hard and soft blocks can be tailored by different stereoselectivities. However, polymer be repeated any number of times during the reactor feed or catalyst modifications. Finally, the fractionation revealed that the samples were lifetime of the chain before chain termination average length of the blocks, which is a function largely blends of isotactic and atactic PPs, with I at most a small fraction of block copolymer. o Primary Screening For single-catalyst systems, this process has E1 Hit Criteria been used to prepare long-chain metal alkyls o 1) Efficiency 0-: (18-22) and has more recently been described CD 2) Mn Supression E00EEEE 3) Mw/Mn Lowering as "catalyzed chain growth" (23, 24). Gibson et al. have discussed the effects of catalyzed Potential ZJ DOC Catalysts ^ vmmammmm chain growth on molecular weight distribution, A 0E00EEEE AA W\7 reporting that a Poisson distribution of molec- Catalyst ular weights (MJMn = 1, where Mw is the A "Hits- weight-average molecular weight and A/n is the rrvrrrr Shuttling Criteria number-average molecular weight) is expected Potential CSAs Narrow Mw/»n under these conditions instead of the Schulz- from 2 catalysts • CSA Flory distribution (MJMn = 2), observed when chain termination occurs (24). This behavior -0o> provides an easy means of probing the capa- CO o bilities of a catalyst system for chain shuttling c polymerization. Given the multitude of olefin polymeriza- tion catalysts, it was daunting to identify a pair of catalysts with substantially different mono- mer selectivities that are also capable of chain shuttling. Furthermore, the chosen system also Carl a R = isobutyl Cat! b R = 2-methylcyclohexyl needs to operate at a high solution-reaction Bn = benzyl temperature (T> 120°C) to prevent undesired polymer precipitation. We therefore adopted a high-throughput method to expedite this dis- covery process^!he technique uses a parallel screen of the effects of metal alkyl reagents on the molecular weight and molecular weight dis- 5.0 5.5 tributions of polyethylenes produced by catalyst/ logMw CSA combinations. These criteria provide a sim- ple test for finding catalyst/CSA combinations Fig. 2. High-throughput screening protocol and selected data from the chain shuttling screen. suitable for use in our dual-catalyst system Polymerizations are conducted in a parallel grid of individual computer-controlled reactors with robotic addition ofjsagents_4nd real-time monitoring, coupled with high-throughput characterization tecr\ (Fig. 2). niqueyfrTmary screening involves a broad screen of several catalysts in combination with many po\f agentsT o(^nmbinatinn begin the sselectio that resuln processt in goo, dw efficiencye first se,- lower molecular weight (Mn). and\r weight distribution (/H.../yMAjre considered hits. These hits are then subjected_tg_a lected representative examples from a broad va- riety of catalyst structure types known to have individual catalyst and dual-catalyst/CSA combinations. Structures of high polymerization rates. Two examples are and the CSA are depicted. Shuttling for this trio is demonstrated by the shown in Fig. 2. Ethylene polymerizations were coalescence of the bimodal molecular weight distribution by adding Et,Zn to the dual-catalyst system. hen carried out with these catalysts, in combi-

www.sciencemag.org SCIENCE VOL 312 5 MAY 2006 715 RESEARCH ARTICLE I systematically varied to study the effects of the gen was added to give a polymer with A/w = weight distribution (A/w/Mn = 1.97) is obtained ratio on polymer microstructure. 137,300 g/mol (sample 3), which was a simple (sample 6). This narrow molecular weight distri- The set of experiments was begun with only blend of hard and soft PE made independently bution is normally associated with a single Cat2 to produce a copolymer with a density of by the two catalysts. Et^n was then added to catalytic species and is indicative of the multi- 0.862 g/cm3 by adjusting monomer feed rates induce chain shuttling between the two cata- block nature of the copolymer, because several and catalyst and cocatalyst flows (26). Molec- lysts. Products were produced at three differ- shuttling events are required to generate such a ular weight control was achieved with a mixture ent levels of blockiness, controlled by the homogeneous molecular weight distribution. of EtjZn and hydrogen, which were adjusted to ratio of concentrations of Et2Zn to ethylene This molecular weight response clearly in- reach a Mw of 110,000 g/mol (sample 1). Cat2 ([Zn]/[C2H4]). Sample 6 was made with the dicates that chain-shuttled ethylene-octene block feed was then stopped, and Catlb was intro- highest EtjZn level possible, while still achiev- copolymers, rather than blends, are formed duced to the reactor under identical reactor ing the desired molecular weight. upon introduction of Et2Zn. The Mn can also conditions. The higher ethylene selectivity of Characterization of block copolymers. Dur- be used in conjunction with the Et2Zn feed and this catalyst resulted in an increase in the mea- ing this set of experiments, it was apparent by polymerization rate to calculate the number of sured polymer density to 0.936 g/cm3. The large eye that this dual-catalyst chain shuttling sys- chains produced per Zn molecule. The low difference in comonomer content between these tem was producing desirable block copolymers Et2Zn level of sample 4 results in the production two copolymers, made under similar reaction at higher CSA levels. Physical blends of high- of approximately 12 chains per Zn. However, conditions, demonstrates the substantial differ- density and linear low-density PE are opaque the reaction is practically stoichiometric at ence in the monomer selectivity of the two because of the large high-density PE crystal- higher Et^n (no H2), with the production of catalysts. The molecular weight of this hard co- lites and the immiscibility of the two copoly- sample 6 resulting in 1.9 chains per Zn (or ~1 ro monomer-poor material was also much lower, mers. However, the copolymers made at higher chain per Zn-alkyl moiety). This result indicates o indicative of faster chain termination (primarily [Zn]/[C2H4] ratios are surprisingly transparent, that almost every polymer chain exits the CM by reaction with H2) for this catalyst system. despite having essentially identical octene con- reactor bound to the CSA with very little chain EtjZn feed was maintained, but some hydrogen tent (Fig. 3). This difference is a clear indica- termination, demonstrating the efficiency of the CM was removed to give a polymer with A/w = tion that the copolymers produced with Et2Zn chain shuttling reaction. Q_ 65,000 g/mol (sample 2). have a very different microstructure than the Despite the stoichiometric nature of the These two baseline polymerizations pro- physical blend of sample 3. We attribute the reaction with the CSA, a similar calculation of o vided an estimate of the catalyst ratio necessary enhanced clarity to a decrease in crystallite size the number of chains per catalyst molecule re- S> to achieve the desired composition for the dual- of the high-density blocks when the average veals that the polymerization is highly catalytic o cr catalyst product; an overall density of ~0.88 block length is shorter. in the hafnium and zirconium species. Block CO g/cm3 was targeted to give the desired co- From gel permeation chromatography (GPC), copolymers produced with living polymeriza- 03 polymer composed of 30% high-density mate- we found that the copolymer prepared without tion techniques are inherently expensive, be- o rial (33). As a control, a mixture of Catlb and EtjZn was clearly bimodal, with MJMn = 13.8 cause the living nature of the polymerization •CgD r Cat2 was added to the reactor under the same (Fig. 4). The GPC trace was deconvoluted in- makes it necessary to use one molecule of in process conditions with no Et2Zn, giving a re- to components of A/w ~ 240,000 and -9600 catalyst for each chain produced. In contrast, actor blend of the two component copolymers g/mol, with the high-molecular weight, low- the chain shuttling methodology is capable of with an overall density of 0.89 g/cm3. Hydro- density copolymer making up 64 wt % of the generating hundreds to thousands of olefin overall material. This large molecular weight block copolymer chains per catalyst. For o split reflects the differing propensities for example, the synthesis of sample 6 resulted in hydrogen-induced termination between the two the formation of ~260 chains per total catalyst. Sample 3 Sample 4 •o catalysts. The molecular weight distribution This feature allows these olefin block copolym- 03 Blend Low CSA O narrows as Et2Zn is added, as expected for an ers to be produced far more cheaply than efficient chain shuttling polymerization. At the materials available from living polymerization -highest ELjZfTlevel, a most probable molecular techniques.

Fig. 4. Characterization of copolymers produced with the dual-catalyst Sample 5 chain shuttling system Mid CSA in a continuous process. Samples 3 (Catlb + Cat2) and 6 (Catlb + £at2 + CSA) are de- picted. (A) GPC reveals a ' bimodal molecular weight distribution in the absence of CSA, whereas adding CSA homogenizes the Fig. 3. Image of comprgssipn-rnoldeoUsafnples' copolymer to a most prob- (thickness = 0.35 mm), illustrating Trie effect of able distribution (MJMn = chain shuttling on clarity. Sample 3 is a phys- 2). (B) Crystallinity dis- ical blend of high- and low-density polymer and tributions as revealed is opaque. Adding Et.,Zn during polymerization by crystallization anal- - results in a block copolymer microstructurejarith ysis fractionation (CRYSTAF). Sample 3, prepared in the absence of CSA, displays a bimodal intimately rrn^rt interchain h.?"* anH tnftgj- composition distribution with a peak around 78°C from Catlb and an amorphous soluble tracflon rjients, resultingjn the increased transjaienty from CatZ. Adding CSA gives a copolymer that crystallize's from solution at a much lower of sampTes 4 to S. temperature, with no indication of highly crystalline material

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