Molecular Kinetic Study of âChain Shuttlingâ Olefin Copolymerization

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A MOLECULAR KINETIC STUDY OF ‘CHAIN SHUTTLING’ OLEFIN COPOLYMERIZATION Antonio Vittoria, Vincenzo Busico, Felicia Daniela Cannavacciuolo, and Roberta Cipullo ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00841 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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ACS Catalysis

A MOLECULAR KINETIC STUDY OF ‘CHAIN SHUTTLING’ OLEFIN COPOLYMERIZATION Antonio Vittoria, Vincenzo Busico, Felicia Daniela Cannavacciuolo, Roberta Cipullo* Department of Chemical Sciences, Federico II University, Via Cintia, 80126 Naples, Italy

ABSTRACT

Statistical olefin block copolymers (OBCs) with ‘hard’ and ‘soft’ Linear Low-Density Polyethylene (LLDPE) blocks can be synthesized by tandem catalysis under ‘Coordinative Chain Transfer Polymerization’ (CCTP) conditions. This process, disclosed in 2006 and commonly referred to as ‘Chain Shuttling Copolymerization’ (CSCP), is now exploited commercially by Dow Chemical, to produce thermoplastic elastomers with the InfuseTM tradename. Whereas the general kinetic principles of CSCP as well as the fundamental physical properties of the products are rather well-understood, the details are still poorly defined, to the point that even average block numbers and lengths of commercial InfuseTM grades are not available in the public domain. In this paper, we report the results of a molecular kinetic investigation in which High Throughput Experimentation tools and methods were employed to unravel the microstructure and architecture of these materials. The problem was factored in two parts. First, each of the two catalysts in the original Dow Chemical formulation was studied individually in ethene/1-hexene CCTP. Next, the two catalysts together were used in CSCP experiments under otherwise identical reaction conditions. The robust database thus obtained enabled us to disambiguate the interpretation of the results, and sort out system behavior as a function of the relevant variables.

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Plausibly, the process turned out to be governed by the relative probabilities of ‘self-shuttling’ versus ‘cross-shuttling’ (that is, of exchanging blocks of the same or different type). In particular, the synthesis of OBCs with long hard blocks and an excess of soft blocks, which are those featuring the most desirable application properties, requires a moderate chain shuttling rate, and an excess of the catalyst with the higher comonomer incorporation ability; as a result, at practical average molecular weight values, these products are characterized by a pronounced inter-chain disuniformity, with an abundant fraction of chains undergoing exclusively ‘selfshuttling’ at the aforementioned catalyst, and therefore consisting of just one soft block.

KEYWORDS: olefin block copolymers, trans-alkylation, coordinative chain transfer polymerization, chain shuttling, tandem catalysis, kinetics

INTRODUCTION The idea of polyolefin ‘chain shuttling’1,2 is almost as old as catalytic olefin polymerization itself. As a matter of fact, Natta noted the trans-alkylating ability of triethyl-Al and diethyl-Zn with early TiCl3-based polypropylene catalysts, and traced the stereoblock fraction found in the polymer to the repeated exchange of polymeryls growing at different surface sites with the mediation of the main group metal cocatalyst.3,4 Whereas this interpretation turned out to be incorrect at a later stage4-6, it had the merit to introduce the concept of reversible trans-alkylation between two or more diverse catalysts by means of a suitable ‘Chain Shuttling Agent’ (CSA) as a way to produce olefin stereoblock polymers5 and, by extension, olefin block copolymers (OBCs).2 Yet, it took several decades and the decisive contribution of High Throughput


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Experimentation (HTE) methods to ultimately identify well-working molecular catalyst formulations, which seem to be rare.1,2 From the kinetic standpoint, ‘Chain Shuttling Copolymerization’ (CSCP) represents a special case of ‘Coordinative Chain Transfer Polymerization’ (CCTP).7,8 An olefin CCTP process can be described as one in which fast and reversible trans-alkylation between a transition metal (tM) polymerization catalyst and a conveniently large excess of a main group metal (mgM) alkyl, such as e.g. diethyl-Zn (DEZ), generates a pool of ‘dormant’

mg

M-Polymeryls which undergo

intermittent growth when temporarily delivered to tM centers. The average chain growth time on the tM catalyst in the absence of the

mg

M species (tcg) is extended by a factor τ ≥ n[mgM]/[tM],

where n is the average number of polymeryls bound to each

mg

M center. As long as the

experiment time t for a (semi)batch process, or the average catalyst residence time for a continuous process, is (well) below τtcg, a linear relationship between polymer yield (Y) and average molecular weight (MW) holds. If, additionally, chain initiation is fast relative to propagation, the process mimics a living polymerization, and the molecular weight distribution (MWD) of the polymer produced in (semi)batch experiments approaches the Poisson function (PDI = Mw/Mn = 1.0).7-9 In the CSCP variant1,2,9, a combination of two tM catalysts is used along with a CSA to copolymerize ethene and a 1-alkene. When the catalysts differ in their 1-alkene incorporation ability, OBCs with an alternation of blocks with different compositions can be produced. At odds with the well-defined architectures achievable by controlled (‘living’) catalysis10, CSCP products have statistically distributed block numbers and lengths; on the other hand, a large excess of copolymer chains can be obtained with respect to the (usually expensive) tM species, which is a tremendous advantage for practical application. The CSCP route is now used commercially by


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Dow Chemical to produce ethene/1-octene (E/O) OBCs under the InfuseTM tradename.11 Copolymerization occurs in a single reactor in the presence of a bis(phenoxyimine)Zr catalyst1,12 (e.g. CAT-1 of Figure 1) and a (pyridylamido)Hf catalyst1,13 (e.g. CAT-2 of Figure 1), with diethyl-Zn (DEZ) as the CSA.1,2,9 Notably, both catalysts are characterized by multiple active species: CAT-1 can speciate into several isomers12, whereas CAT-2 undergoes an in-situ ligand diversification by comonomer insertion into the strained ortho-metalated bond of the naphthyl fragment.14,15 CAT-1 is much less reactive towards O than CAT-21,2; therefore, at a given [E]/[O] feeding ratio, O-poor (‘hard’) and O-rich (‘soft’) copolymers are produced at CAT-1 and CAT-2, respectively. Fast and reversible trans-alkylation of the growing chains with the CSA results into statistically distributed hard-soft multiblock architectures.1,2 The relative amounts of hard and soft blocks, as well as their average numbers, lengths and compositions, can be tuned (within the constraints inherent in the nature of the catalyst pair16) by adjusting the relative amounts of the two catalysts and of the CSA, as well as monomer concentrations.2,9

Figure 1. The bis(phenoxyimine)Zr (CAT-1; left) and (pyridylamido)Hf (CAT-2; right) precatalysts. InfuseTM OBCs are unique in that they escape the long-standing correlation between density and melting temperature of conventional Linear Low-Density Poly(Ethylene) (LLDPE).1,2,11,17,18 Self-separation of the hard and soft blocks into semicrystalline and amorphous domains is typically observed18-20; grades with long hard blocks and an excess of soft blocks behave as


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thermoplastic elastomers.1,2,11,17,18 Whereas the general kinetic aspects of the polymerization process2,9,21 and the physical properties of the products18-20 are rather well understood, the details are still poorly defined. To the best of our knowledge, even average block numbers and lengths of commercial InfuseTM grades are not available in the public domain, which hampers a thorough elucidation of structure-properties relationships. We can only speculate on the reasons for this impasse, which is unusual considering that more than ten years have passed since the initial discovery. One is likely the technical complexity of the catalytic reaction. CCTP and CSCP can only occur in solution, because the mobility of the M-polymeryl species is a necessary condition. For chains with high-melting hard blocks, this requires high-temperature operation (e.g. 100°C or above); yet, the vast majority of the literature studies were carried out at moderate temperature7,8, probably because controlling olefin polymerization reactions under conditions at which catalytic activities are exceedingly high, all deactivation processes are very fast, and the reaction is over in few minutes is challenging even for industry, and very few academic laboratories are equipped to do that well enough. The microstructural and structural assessment of OBCs is also deceptive. Most polyolefin materials on the market are physical blends that can be separated into the different components prior to the characterizations. With block copolymers this approach is conceptually hampered, because the different blocks are chemically bound, and therefore inseparable. On the other hand, typical InfuseTM samples can be separated into comparable amounts of a completely amorphous fraction and a high-melting semicrystalline fraction22,23, which highlights an extensive interchain disuniformity of yet unclear origin(s). In the present paper we report the results of a novel HTE approach to the question. Rather than for catalyst discovery, we designed and optimized our HTE workflow for rapid and thorough


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explorations of the chemical and physical variables space in polyolefin catalysis.24 In the present investigation, the two catalysts of Figure 1 were first screened individually in ethene/1-hexene (E/H) CCTP, and then together in E/H CSCP, in both cases at 100°C, using a 48-cell reaction platform specifically engineered for high-temperature olefin polymerizations. All copolymers were characterized by means of an array of high-end techniques including rapid Gel Permeation Chromatography (GPC), analytical Crystallization Elution Fractionation (aCEF), and quantitative 1H and

13

C NMR, all operated in high throughput mode. The robust database

resulting from the screening enabled us to effectively factor the problem, and ultimately disambiguate data interpretation, ending up with a semi-quantitative description of OBC microstructure and architecture, and a mechanistic interpretation thereof. EXPERIMENTAL SECTION Materials The bis(phenoxyimine)Zr and (pyridylamide)Hf precatalysts were prepared according to the literature.25,26 The specifications of all other chemicals are provided in the Supporting Information (SI, Table S1). E/H copolymerization experiments All copolymerization experiments were carried out in a Freeslate (former Symyx) Parallel Pressure Reactor (PPR) HTE platform. This setup, integrally contained in a triple MBraun LabMaster glove-box operated under N2 and featuring 48 reaction cells (6.0 mL working volume each) arrayed in six 8-cell modules, has been thoroughly described elsewhere.24,27 The operating protocol was as follows. Prior to the execution of a 48-experiment library, the PPR modules undergo ‘bake-and-purge’ cycles overnight (8 h at 90-140°C with intermittent dry N2 flow), to remove any contaminants and left-overs from previous experiments. After cooling to glove-box


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ACS Catalysis

temperature, the stir tops are taken off, and the 48 cells are fitted with disposable glass inserts (pre-weighed in a Bohdan Balance Automator) and inert metal stir paddles. The stir tops are then put back in place, and the cells are loaded with aliquots of a mixed alkane diluent (ISOPAR-G, from now on ‘alkane’), 1-hexene (H), and a methylalumoxane (MAO) solution in toluene ([Al] = 0.10 M). The modules are pressurized with ethene (E) at 80 psi (5.5 bar) and thermostated at 100.0 °C, after which ethene pressure is raised to the operating value of 160 psi (11.0 bar). At this point, the catalyst system injection sequence is launched; aliquots of (i) an alkane ‘chaser’, (ii) an alkane solution of the catalyst(s), (iii) a toluene solution of N,N-dimethylanilinium tetrakis-perfluorophenylborate (AB), (iv) an alkane solution of DEZ, and finally (v) an alkane ‘buffer’ are loaded into the injection line, and transferred into the cells, thus starting the reaction. This is left to proceed under stirring (800 rpm) at constant temperature and pressure by feeding ethene on demand until the desired conversion, and then quenched with an over-pressure of dry air before H conversion reaches 20% (max). After quenching all cells, the modules are cooled to glove-box temperature and vented; the stir tops are removed, and the glass inserts containing the reaction phases are taken out, and transferred to a Genevac EZ2-Plus centrifugal evaporator, where all volatiles are distilled out and the copolymers are thoroughly dried overnight. Reaction yields are determined by robotically weighing the dry copolymers while still in the reaction vials (subtracting the pre-recorded tare). Copolymer sample homogenization The copolymer samples as obtained after the drying protocol can be macroscopically disuniform; therefore, a homogenization treatment was carried out prior to the characterizations. Each sample was dissolved in 5.0 mL of xylene containing 0.40 mg mL-1 of 2,6-di-tert-butylphenol (BHT) as a stabilizer. After 2 h at 135°C under gentle stirring, to ensure complete


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dissolution, the solutions were sequentially poured into an excess of acetone to coagulate the copolymers, which were then recovered by decantation and transferred to a Genevac EZ2-Plus centrifugal evaporator for final drying. Copolymer Characterizations Gel Permeation Chromatography (GPC) curves were recorded with a Freeslate Rapid-GPC setup, equipped with two mixed-bed Agilent PLgel 10 µm columns and a Polymer Char IR4 detector. The upper deck of the setup features a 48-sample dissolution station accommodating 10 mL magnetically stirred glass vials. With robotic operation, pre-weighed copolymer amounts (typically 2.0 mg) were dissolved in proper volumes of ortho-dichlorobenzene (ODCB) containing 0.40 mg mL-1 of BHT stabilizer, so as to obtain solutions at a concentration of 0.5 to 1.0 mg mL-1. After 2 h at 150°C under gentle stirring, to ensure complete dissolution, the sample array was transferred to a thermostated bay at 145°C, and the samples were sequentially injected into the column line, also at 145°C, with a flow rate of 1.0 mL min-1. In post-trigger delay operation mode, the analysis time was 12.5 min per sample. Calibration was carried out with the universal method, using 10 monodisperse polystyrene samples (Mn between 1.3 and 3700 kDa). Before and after each campaign, samples from a known polypropylene batch produced with an ansa-zirconocene catalyst were analyzed for benchmarking. Analytical Crystallization Elution Fractionation (aCEF) curves were collected with a Polymer Char setup, equipped with an autosampler (42 wells), an IR5 detector, a dual capillary viscometer detector and a column cooling unit. With robotic operation, pre-weighed copolymer samples (typically 8-16 mg) were dissolved in ODCB added with 0.40 mg mL-1 of BHT stabilizer, so as to achieve a concentration of 2.0 mg mL-1. After 90 min at 150°C under vortexing in sealed vials to ensure complete dissolution, the samples were sequentially charged


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ACS Catalysis

into the injection loop, where they were held at 95°C for 5 min and then moved into the column. The crystallization step entailed a 2.0°C min-1 cooling ramp down to -20°C at a flow rate of 0.065 mL min-1; sample elution was started 1 min after reaching -20°C, with a 4°C min-1 heating ramp up to 140°C at a flow rate of 1.0 mL min-1. The analysis time was 60 min per sample. The amount of material eluted at -20°C will be referred to as the Amorphous Fraction (AF). Elution peaks at higher temperature will be associated with the temperature at the maximum (Tel,max). NMR spectra were recorded with a Bruker Avance III 400 spectrometer equipped with a 5 mm high-temperature cryoprobe, and a robotic sample changer with pre-heated carousel (24 positions). The samples (~25 mg) were dissolved at 120°C in tetrachloroethane-1,2-d2 (0.7 mL) containing BHT (0.40 mg mL-1) as a stabilizer, and loaded in the carousel maintained at the same temperature. The spectra were taken sequentially with automated tuning, matching and shimming. Operating conditions were: [1H NMR] 90° pulse; 2.0 s acquisition time; 10 s relaxation delay; 16-32 transients. [13C NMR]: 45° pulse; 2.3 s acquisition time; 5.0 s relaxation delay; 1K to 3K transients. Broad-band proton decoupling was achieved with a modified WALTZ16 sequence (BI_WALTZ16_32 by Bruker). Resonance assignment was based on the literature. Copolymer compositions and comonomer sequence distributions at triad level were measured with literature methods.28,29 Statistical analyses of the triad distributions were carried out with the Copolstat software code.30 Preparative fractionations Two commercial InfuseTM samples11 (grades 9107 and 9507) were fractionated by exhaustive Kumagawa extraction with boiling hexane. The raw samples and fractions thereof were then characterized as previously described for the copolymers made in the PPR.


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RESULTS AND DISCUSSION Figure 2-A shows the aCEF trace of a representative commercial InfuseTM sample (grade 9107). The sample separated into an amorphous fraction (AF; 41 wt-%), and a semicrystalline fraction featuring a broad elution peak; as was noted above, these observations are consistent with the previous literature.22,23 Preparative fractionation by Kumagawa extraction with boiling hexane confirmed the aCEF results (Table 1 and Figure 2-B).31 For each fraction, the comonomer triad distribution was determined by

13

C NMR28,29, and subjected to statistical analysis.30 The hexane-soluble (C6-s)

fraction turned out to be a purely random E/O copolymer with a mole fraction of O units xO = 0.20 (SI, Table S2). In the hexane-insoluble (C6-i) semi-crystalline fraction, on the other hand, random E/O copolymer sequences and an excess of EEE triads were detected, which is compatible with an OBC nature. The triad distribution (SI, Table S3) was well-reproduced in the framework of a stochastic two-site model assuming the sample to be a physical blend of ‘hard’ polyethylene sequences (weight fraction, whard = 20%) and ‘soft’ E/O copolymer sequences with the same composition of the C6-s fraction (xO = 0.20).32 No improvement of the fit was obtained when adopting a Coleman-Fox version of the two-site model4-6 appropriate for block polymers; this means that the junctures between hard and soft blocks (if present) were scarce enough not to affect the triad distribution. On the other hand, the aCEF elution peak indicates that the hard sequences are polydisperse and comparatively short; to what extent this is due to low amounts of isolated H units or to the bonding to soft blocks is not possible to say based on the available information. An analogous picture was obtained for a sample of InfuseTM grade 9507 (Table 1 and Figure 2-C).


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Table 1. Results of the analytical and preparative fractionation of two commercial InfuseTM samples (see text). Sample/ Fraction

wt%

xO

Raw s-C6 i-C6

41.1 58.9

0.16 0.20 0.14

Raw s-C6 i-C6

53.0 47.0

0.17 0.20 0.14

whard (%)

20

20

Mn Mw (KDa) (KDa) Grade 9107 63 156 43 113 90 188 Grade 9507 38 101 35 89 49 117

AF (wt%)

Tel(max) (°C)

2.5 2.6 2.1

41.0

94.1 ; 107.0

2.7 2.5 2.4

46.1

84.3 ; 104.7

PDI

Figure 2. aCEF curves of two commercial InfuseTM samples (grade 9107 (A) and 9507 (C)), and of the C6-i fraction of the former (B); see text. Notwithstanding the multiple characterizations, we were not able to determine unambiguously chain architecture for the two materials, nor to clarify the origin of the amorphous, purely random E/O copolymer fraction. It is also worthy to note that the raw samples and the two individual fractions thereof gave rather similar GPC traces (SI, Figure S1), which implies that drawing conclusions on a possible (dis)uniformity of these and related samples based on MWD data may not be trivial.


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To disambiguate the problem, we first studied the two catalysts of Figure 1 individually in random E/H copolymerization with and without DEZ, and then together in E/H CSCP. All experiments were carried out in alkane solution at 100°C. The catalyst system formulation included the precatalyst(s), N,N-dimethylanilinium tetrakis-perfluorophenylborate (AB), methylalumoxane (MAO), and DEZ as the CSA where applicable. In preliminary multipleactivation studies, said formulation was identified as the one ensuring the best reaction control in our setup. Full reaction details are provided in the Experimental Section and the SI (Tables S4 and S5). The results of E/H random copolymerization (at [E]/[H] = 0.6033) in the absence of DEZ are summarized at entries 1-2 (CAT-1) and 25-26 (CAT-2) of Table 2. Both catalysts yielded copolymers with PDI > 2.0, which is consistent with their aforementioned non-single-center nature. The mole fraction of H units in the copolymers, as measured by

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C NMR28, was xH =

0.0036 for CAT-1, xH = 0.135 for CAT-2. In the former case, the aCEF curves (SI, Figure S2-A) showed sharp elution peaks, which suggests that all catalytic species had a very similar comonomer incorporation ability. The aCEF curves of copolymers made with CAT-2 (SI, Figure S2-B), on the other hand, revealed the co-presence of an AF and a weakly crystalline fraction with a broad elution peak. The multi-sited nature of the catalyst, and the presence in the copolymers of non-negligible amounts of crystallizable (E)n sequences (according to copolymerization statistics, strands of 10 or more methylenes represent ≈10% of the copolymer mass), can both account for this observation. The copolymers produced with CAT-1 featured a comparatively low average MW; 1H NMR chain end analysis34 demonstrated that this can be entirely traced to β-H elimination (SI, Figure S3).


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Table 2. E/H copolymerization results with CAT-1 and CAT-2 (n.d.= not determined). Catalyst

CAT-1

CAT-2

Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

[Zn]/[tM] 0

100

0

100

Y (mg) 23 28 28 41 54 59 74 80 83 87 91 104 105 117 117 121 122 134 136 141 168 170 209 222 35 41 50 72 84 92 120 121 122 126 141 164 171 190 215 234 236 250 256 271

Mn (KDa) 109 108 6 10 11 14 17 16 15 16 19 20 23 20 25 19 23 22 26 24 30 29 29 38 734 843 13 19 20 20 25 25 23 28 26 38 33 35 51 45 44 49 45 50

Mw (KDa) 629 638 11 17 18 21 26 27 25 26 30 31 37 32 39 31 37 34 41 37 45 45 47 60 1.6×103 1.9×103 22 34 35 36 44 43 41 46 45 63 54 58 80 72 71 80 74 79

Mw/Mn

xH

5.8 5.9 1.8 1.6 1.6 1.6 1.6 1.7 1.6 1.6 1.6 1.5 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.5 1.5 1.6 1.6 1.6 2.2 2.3 1.8 1.8 1.7 1.8 1.7 1.6 1.7 1.6 1.7 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6

0.0037 0.0035 0.0036 0.0035 0.0036 0.0034 n.d. n.d. n.d. n.d. n.d. 0.0036 0.0036 n.d. n.d. n.d. n.d. n.d. 0.0036 n.d. n.d. n.d. 0.0036 0.0035 0.130 0.140 0.129 0.128 0.146 0.147 0.124 0.136 0.140 0.134 0.152 0.149 0.155 0.142 0.153 0.166 0.148 0.220 0.187 0.207


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ACS Catalysis

80

CAT-1 CAT-2

70 60 50

Mn (KDa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 30 20 10 0 0

50

100

150

200

250

300

Y (mg)

Figure 3 . Mn vs Y for random E/H copolymers prepared with CAT-1 and CAT-2 in the presence of DEZ ([Zn]/[tM] = 100). Data from Table 2. Upon addition of DEZ (entries 3-24 and 27-44 of Table 2), copolymer MWD narrowed (to PDI 80% (Table 1). At each catalyst ratio, the [Zn]/[tM] ratio was set at 0 (physical blends), 50 and 100 ([tM]=[CAT-1]+[CAT-2]). A summary of the copolymerization and copolymer characterization results is reported in Table 3 (for full reaction details see SI, Table S6). The GPC and aCEF curves are shown in Figures 6 and 7. For all products, the

13

C

NMR triad distributions (SI, Table S7) were subjected to statistical analysis in the framework of


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the two-site stochastic model described before for the commercial InfuseTM samples, in order to calculate the composition of the soft chains (blocks) and their weight fraction (xH,soft and wsoft, respectively). Table 3. Main results of the E/H CSCP experiments (see text). In all cases, n(tM) = 20 nmol. Entry/ Sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Catalyst(s)

[Zn]/[tM]

CAT-1 CAT-2

0 0 0 50 100 0 50 100 0 50 100 0 50 100

[CAT-1]/[CAT-2] = 1:1 ; xCAT-2 = 0.50 [CAT-1]/[CAT-2] = 1:2; xCAT-2 = 0.67 [CAT-1]/[CAT-2] = 1:3 ; xCAT-2 = 0.75 [CAT-1]/[CAT-2] = 1:4 ; xCAT-2 = 0.80

Y (mg) 61 122 137 145 165 68 127 129 97 154 173 94 121 182

Mn (KDa) 61 357 86 36 26 158 35 28 269 45 29 299 38 32

Mw (KDa) 283 1.7×103 758 68 45 1.5×103 71 50 1.7×103 90 53 1.9×103 82 58

Mw/Mn

xH

wH,soft (%)

xH,soft

AF (wt%)

4.6 4.8 8.8 1.9 1.7 9.6 2.1 1.8 6.1 2.0 1.8 6.5 2.1 1.8

0.007 0.238 0.087 0.079 0.059 0.157 0.127 0.129 0.162 0.160 0.153 0.221 0.172 0.165

Tel(max) (°C) 109.4

39.6 42.6 33.0 72.6 66.4 67.7 77.1 76.7 79.0 86.7 83.4 81.3

0.277 0.223 0.220 0.241 0.212 0.210 0.229 0.224 0.210 0.270 0.218 0.213

16.1 1.0 0.4 49.6 6.4 5.0 67.3 25.8 21.0 86.9 39.1 28.3

108.6 107.5 108.1 108.7 105.8 95.4 108.7 99.6 94.0 107.9 94.8 91.2

Figure 6. GPC traces of the E/H copolymers of Table 3. The used [CAT-1]/[CAT-2] ratio, and the PDI values for the individual MWDs, are indicated in each graph.


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ACS Catalysis

Figure 7. aCEF profiles of the E/H copolymers of Table 3. The used [CAT-1]/[CAT-2] ratio is indicated in each graph.

Ironically, the most problematic experiments to carry out and interpret were those with CAT-2 (either alone or in combination with CAT-1) and no DEZ in the catalyst formulation, because the very high average MW of the soft copolymer (component) made it difficult to prevent the aforementioned ethene mass transfer limitation issues. As a matter of fact, an E/H random copolymer sample produced with CAT-2 alone (entry 2 of Table 3) featured xH = 0.238, and the soft component of the physical blends (entries 3, 6, 9, 12 of Table 3) was even richer in H (xH,soft up to 0.28). Moreover, partial co-crystallization of the hard and soft chains prevented their


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Page 20 of 33

complete aCEF separation (AF < wsoft). The most valuable piece of information obtained from these experiments concerned the hard copolymer component: for the E/H random copolymer sample produced with CAT-1 alone (entry 1 of Table 3) we measured xH = 0.007 by 13C NMR, and Tel(max) = 109.4°C by aCEF. For the physical blends (entries 3, 6, 9, 12 of Table 3), the Tel(max) values were similarly high (108.5±0.6°C); therefore, in the statistical analyses of all

13

C

NMR triad distributions (SI, Table S7) we set xH,hard = 0.007. The CSCP experiments (entries 4-5, 7-8, 10-11, 13-14 of Table 3), on the other hand, highlighted consistent and clear trends. The following main facts should be noted: i) The composition of the soft sequences, calculated by statistical analysis of the

13

C NMR

triad distributions as previously described, was xH,soft = 0.216±0.008, i.e. always very close to the target (xH,soft = 0.20); ii) The 13C NMR calculated weight fraction of the soft component (wsoft) was in all cases close to the mole fraction of CAT-2 (xCAT-2). As a matter of fact, the two catalysts featured similar polymerization rates when used individually in E/H CCTP experiments (Table 2 and Figure 3). iii) All GPC traces (Figure 6) were narrow and very symmetrical, and the PDI values were close to those measured with the individual catalysts in E/H CCTP (Table 2). iv) The aCEF traces (Figure 7) revealed a progressive decrease of AF and a corresponding increase of Tel(max) with decreasing xCAT-2. At xCAT-2 = 0.5, practically no AF was observed, and Tel(max) ultimately reached the value for the random copolymer produced with CAT-1 (entries 1, 3, 6, 9, 12 of Table 3) within the error bar. v) At a given xCAT-2, rising [Zn]/[tM] from 50 to 100 resulted into a decrease of Tel(max), and of average MW too (due to the fact that nP ≈ 2nZn).


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To interpret the above facts, it is worth recalling that a chain shuttling event can exchange two chains that underwent their last extension period at the same (a) or at different (b) catalyst types; we will refer to case (a) as ‘self-shuttling’, and to case (b) as ‘cross-shuttling’.20,40 Then, the number average length of a block of type-i (i = 1 or 2) can be BLi = n(ELi) (n = 1, 2, 3…), where ELi is the number average chain extension length at CAT-i (i.e. the average polymerization degree of the chain segment grown during one extension period at said catalyst under the given experimental conditions). Let us now make the following simplifying assumptions (in view of point ii above): i) For each catalyst, nominal and active catalyst concentration coincide ii) kp1 = kp2 = kp (kp = specific chain propagation rate) iii) kcs-ii = kcs-ij = kcs (i.e., the specific rates of all chain shuttling events are the same) Then, the following simple relationships should hold: BL1/BL2 ≈ [CAT-1]/[CAT-2]

(at a given [Zn])

EL1 = EL2 = EL EL ∝ 1/[Zn]

Rel.1 Rel.2

(at given [CAT-1] or [CAT-2])

BLi → EL for xCAT-i → 0

Rel.3 Rel.4

Of course, BL1 = BLhard ; BL2 = BLsoft ; Mn,i = BLiM0i (where M0i is the reduced monomer mass of a block of type-i: in our conditions, M0hard ≈ 28 Da, M0soft ≈ 39 Da). Furthermore, we will assume that Tel(max) in the aCEF profile of an OBC sample is determined solely by the average length of the ethene homosequences in the hard blocks (made at CAT-1). Then, we should observe Tel(max) ≈ 109°C when BLhard > 1/0.007; otherwise, for BLhard ≤ 1/0.007, the value of Tel(max) should be close to that for an E/H random copolymer with 1/xH = BLhard (corrected for the average length of the ethene homosequences in the soft blocks in case BLhard is


21

ACS Catalysis

not much greater than 1/xH,soft). An experimental {1/xH, Tel(max)} correlation plot for a series of E/H random copolymers prepared with a molecular catalyst is shown in Figure 8. Based on the above, for all E/H CSCP products of Table 3 we estimated BLhard from Tel(max) (Table 3), and then BLsoft according to Rel.1. The results are given in Table 4. In view of Rels. 2 and 4, for the samples produced at the lowest xCAT-1 (= 1-xCAT2; entries 13 and 14 of Table 3) we suggest that BLhard ≈ EL, i.e. ca. 40 (20) monomeric units at [Zn]/[tM] = 50 (100). 130 120 110 100 90

Tel,max (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 70 60 50 40 30 20 10 0 0

20

40

60

80

100

120 140 160

180 200 220

1/xH

Figure 8. Tel(max) vs 1/xH for a series of E/H random copolymers (see text). Table 4. Estimated values of BLhard and BLsoft for the E/H CSCP samples of Table 3 (see text). Entry/ Sample

xCAT-2

[Zn]/[tM]

BLhard (monom. units)

BLsoft (monom. units)

Mn,hard (KDa)

Mn,soft (KDa)

4

0.50

50

>1.4×102

>1.4×102

>3.9

>5.5

2

2

≥3.9

≥5.5

5

0.50

100

≥1.4×10

7

0.67

50

90

180

2.5

7.0

8

0.67

100

40

80

1.1

3.1

10

0.75

50

55

165

1.5

6.4

11

0.75

100

30*

90

0.84

3.5

13

0.80

50

40

160

1.1

6.2

14

0.80

100

20*

80

0.56

3.1

≥1.4×10

*Corrected for the number average length of the ethene homosequences in the soft blocks (4 monomeric units; see text)


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ACS Catalysis

Despite of the very rough approximations and within the large experimental uncertainties, the values in Table 4 seem rather consistent, internally and with respect to Rel.3. Therefore, we will take them as a plausible semi-quantitative basis to describe sample architecture in the produced copolymers. Our first conclusion is that all samples are true OBCs; the pronounced inter-chain disuniformity observed for most of them is the result of a low number of blocks per chain (between 1 and 10, indicatively).40 In particular, the AF is made of purely random copolymer chains which underwent exclusively ‘self-shuttling’ at CAT-2, and as such only contain one soft block. In line with this interpretation, the AF turned out to decrease with decreasing xCAT-2, and was practically absent at xCAT-2 = 0.5. Concerning the semicrystalline fraction, we trace the peculiar bimodal shape of the aCEF elution peak to the presence of chains in which the longest hard block had BLhard = EL (that were eluted at the Tel(max) corresponding to the absolute peak maximum), or BLhard = 2EL (eluted at the Tel(max) corresponding to the shoulder at the higher temperature side of the peak); the position of said shoulder is consistent with the correlation plot of Figure 8. Simulation models in the literature are in line with this interpretation.41

CONCLUSIONS The scope and objectives of HTE42 as applied to polyolefin catalysis have been changing with time. Initially introduced for catalyst discovery purposes43 with remarkable results (among which CAT-2 is an outstanding example13), later on HTE tools and methods proved to be well-suited to screen complex catalyst formulations for desired applications.24,27,44,45 The one of interest here is an exemplary case history1: its identification at Dow Chemical as one of the rare cases enabling ethene/1-octene CSCP was like finding a needle in the haystack (as a matter of fact, decades of previous searches with conventional methods failed2). In the present investigation, we made use of HTE with yet another purpose, that is the rapid semi-quantitative exploration of the variable


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space for a complex catalytic process, so as to highlight its molecular kinetics and mechanistic features. We gave experimental evidence that the molecular architecture of statistical ethene/1-hexene OBCs produced by CSCP is governed by the relative probabilities of ‘self-shuttling’ and ‘crossshuttling’. We concluded in particular that, with the original catalyst formulation disclosed in ref 1, OBCs featuring long hard blocks and an excess of soft blocks (which are those with the most desirable application properties1,2,11,17-20) are necessarily characterized by a pronounced interchain disuniformity.22,23,40,41 As a matter of fact, the excess of CAT-2 over CAT-1 and the rather low [Zn]/[tM] ratio required for the purpose result in the formation of a comparatively large amount of random copolymer chains which underwent exclusively ‘self-shuttling’ events at CAT-2, and therefore consist of only one soft block. We are fully aware that running CSCP in HTE semibatch mini-reactors is a very delicate and difficult exercise, and therefore our results can only be regarded as semi-quantitative. This notwithstanding, we were able to closely reproduce the features of commercial InfuseTM OBCs11, as the aCEF profile overlay in Figure 9 and a comparative examination of the results in Tables 1 and 3 suggest. The fundamental understanding provided by the present study can be useful for further product development. Using homologues of CAT-1 with a lower propensity to β-H elimination is a first obvious improvement, that indeed has already been reported in more recent Dow Chemical papers.16 Homologues of CAT-2 with a higher activity compared with their counterpart in the catalyst pair, in turn, would help reducing inter-chain disuniformity, if desired. On the other hand, we believe that the HTE strategy introduced in this investigation can find wider application, and has the potential to become a paradigm for elucidating other complex


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ACS Catalysis

catalytic processes and products where drawing mechanistic conclusions can be completely nontrivial without a properly designed, robust experimental database.

Figure 9. Overlay of the aCEF profiles of a commercial InfuseTM sample (Grade 9107; black trace) and the OBC sample at entry 13 of Table 3 (red trace).

Supporting Information. Chemicals specifications. Experimental and calculated triad distributions for the commercial InfuseTM OBC samples. Additional information on the E/H copolymerization experiments of Table 2. Additional information on the E/H copolymerization experiments of Table 3. Experimental and calculated triad distributions for the E/H copolymer samples in Table 3. Additional GPC, aCEF and NMR data for some E/H (block) copolymer samples. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The Authors declare no competing financial interest.


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(16) Kuhlman, R. L.; Klosin, J. Tuning block compositions of polyethylene multi-block copolymers by catalyst selection. Macromolecules 2010, 43, 7903-7904. (17) Hustad, P. D. Frontiers in olefin polymerization: reinventing the world's most common synthetic polymers. Science 2009, 325, 704-707. (18) Liu, G. M.; Guan, Y.; Wen, T.; Wang, X. W.; Zhang, X. Q.; Wang, D. J.; Li, X. H.; Loos, J.; Chen, H. Y.; Walton, K.; Marchand, G. Effect of mesophase separation and crystallization on the elastomeric behavior of olefin multi-block copolymers. Polymer 2011, 52, 5221-5230. (19) Park, H. E.; Dealy, J. M.; Marchand, G. R.; Wang, J. A.; Li, S.; Register, R. A. Rheology and structure of molten, olefin multiblock copolymers. Macromolecules 2010, 43, 6789-6799. (20) Li, S.; Register, R. A.; Weinhold, J. D.; Landes, B. G. Melt and solid-state structures of polydisperse polyolefin multiblock copolymers. Macromolecules 2012, 45, 5773-5781. (21) Saeb, M. R.; Khorasani, M. M.; Ahmadi, M.; Mohammadi, Y.; Stadler, F. J. A unified picture of hard-soft segmental development along olefin chain shuttling copolymerization. Polymer 2015, 76, 245-253. (22) Shan, C. L. P.; Hazlitt, L. G. Block index for characterizing olefin block copolymers. Macromol. Symp. 2007, 257, 80-93. (23) Zhou, Z.; Miller, M. D.; Lee, D.; Cong, R.; Klinker, C.; Huang, T.; Li Pi Shan, C.; Winniford, B.; deGroot, A. W.; Fan, L.; Karjala, T.; Beshas, K. NMR study of the separation mechanism of polyethylene–octene block copolymer by HT-LC with graphite. Macromolecules 2015, 48, 7727-7732.


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(24) Busico, V.; Cipullo, R.; Mingione, A.; Rongo, L. Accelerating the research approach to Ziegler-Natta catalysts. Ind. Eng. Chem. Res. 2016, 55, 2686-2695. (25) Arriola, D. J.; Carnahan, E. M.; Cheung, Y. W.; Devore, D. D.; Graf, D. D.; Hustad, P. D.; Kuhlman, R. L.; Li Pi Shan, C.; Roof, G. R.; Stevens, J. C.; Stirn, P. J.; Wenzel, T. T. Catalyst composition comprising shuttling agent for ethylene multi-block copolymer formation. (Dow Chemical Company) WO2005/090427 A2, September 29, 2005. (26) Frazier, K. A.; Boone, H.; Vosejpka, P. C.; Stevens, J. C. High activity olefin polymerization catalyst and process. (Dow Chemical Company) US2004/0220050 A1, November 4, 2004. (27) Busico, V.; Pellecchia, R.; Cutillo, F.; Cipullo, R. High‐throughput screening in olefin‐ polymerization catalysis: from serendipitous discovery towards rational understanding. Macromol. Rapid Commun. 2009, 30, 1697-1708. (28) Randall, J. C. A review of high-resolution liquid carbon-13 nuclear magnetic resonance characterizations of ethylene-based polymers. Macromol. Chem. Phys. 1989, 2-3, 201-317. (29) Liu, W.; Rinaldi, P. L.; McIntosh, L. H.; Quirk, R. P. Poly(ethylene-co-1-octene) characterization by high-temperature multidimensional NMR at 750 MHz. Macromolecules 2001, 34, 4757-4767. (30) Vacatello, M., University of Naples “Federico II” (Italy). (31) For a quantitative aCEF analysis of the copolymer samples discussed in the present investigation with a Polymer Char setup, the version equipped with a column cooling device is necessary.


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(32) Tong, Z. Z.; Huang, J.; Zhou, B.; Xu, J. T.; Fan, Z. Q. Chain microstructure, crystallization, and morphology of olefinic blocky copolymers. Macromol. Chem. Phys. 2013, 214, 605-616. (33) Kissin, Y. V. Isospecific Polymerization of Olefins; Springer-Verlag: New York, 1985; p 3. (34) Busico, V.; Cipullo, R.; Friederichs, N.; Linssen, H.; Segre, A.; Van Axel Castelli, V.; van der Velden, G. 1H NMR analysis of chain unsaturations in ethene/1-octene copolymers prepared with metallocene catalysts at high temperature. Macromolecules 2005, 38, 6988-6996. (35) Kuhlman, R. L.; Wenzel, T. T. Investigations of chain shuttling olefin polymerization using deuterium labeling. Macromolecules 2008, 41, 4090-4094. (36) Alfano, F.; Boone, H. W.; Busico, V.; Cipullo, R.; Stevens, J. C. Polypropylene "Chain Shuttling" at Enantiomorphous and Enantiopure Catalytic Species: Direct and Quantitative Evidence from Polymer Microstructure. Macromolecules 2007, 40, 7736-7738. (37) Cueny, E. S.; Johnson, H. C.; Anding, B. J.; Landis, C. R. Mechanistic studies of hafniumpyridyl amido-catalyzed 1-octene polymerization and chain transfer using quench-labeling methods. J. Am. Chem. Soc. 2017, 139, 11903-11912. (38) Cipullo, R.; Mellino, S.; Busico, V. Identification and count of the active sites in olefin polymerization catalysis by oxygen quench. Macromol. Chem. Phys. 2014, 215, 1728-1734. (39) Maury, J.; Feray, L.; Bazin, S.; Clement, J.-L.; Marque, S. R. A.; Siri, D.; Bertrand, M. P. Spin-trapping evidence for the formation of alkyl, alkoxyl, and alkylperoxyl radicals in the reactions of dialkylzincs with oxygen. Chem. Eur. J. 2011, 17, 1586-1595.


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(40) Tongtummachat, T.; Anantawaraskul, S.; Soares, J. B. P. Understanding the formation of linear olefin block copolymers with dynamic Monte Carlo simulation. Macromol. React. Eng. 2016, 10, 535-550. (41) Anantawaraskul, S.; Suwicha, S.; Siriwongsarn, E.; Soares, J. B. P. Chemical composition distribution and Temperature Rising Elution Fractionation of linear olefin block copolymers. Macromol. Symp. 2013, 330, 123-131. (42) High-Throughput Screening in Chemical Catalysis, Hagemeyer, A.; Strasser, P.; Volpe, A. F. Jr., Eds., Wiley-VCH, Weinheim 2004. (43) Boussie, T. R.; Diamond, G. M.; Goh, C.; Hall, K. A.; LaPointe, A. M.; Leclerc, M.; Lund, C.; Murphy, V.; Shoemaker, J. A. W.; Tracht, U.; Turner, H.; Zhang, J.; Uno, T.; Rosen, R. K.; Stevens, J. C. A fully integrated high-throughput screening methodology for the discovery of new polyolefin catalysts: discovery of a new class of high temperature single-site group (IV) copolymerization catalysts. J. Am. Chem. Soc. 2003, 125, 4306-4317. (44) Vittoria, A.; Meppelder, A.; Friederichs, N.; Busico, V.; Cipullo, R. Demystifying ZieglerNatta catalysts: the origin of stereoselectivity. ACS Catal. 2017, 7, 4509-4518. (45) Peil, K. P.; Neithamer, D. R.; Patrick, D. W.; Wilson, B. E.; Tucker, C. J. Applications of high throughput research at the Dow Chemical Company. Macromol. Rapid Commun. 2004, 25, 119-126.


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Molecular Kinetic Study of âChain Shuttlingâ Olefin Copolymerization

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