Synthesis of Narrow-Distribution, High-Molecular-Weight ROMP

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Synthesis of Narrow-Distribution, High-Molecular-Weight ROMP Polycyclopentene via Suppression of Acyclic Metathesis Side Reactions William D. Mulhearn and Richard A. Register* Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, United States S Supporting Information *

ABSTRACT: Methods for the preparation of narrow-distribution ROMP polycyclopentene are developed to suppress the rate of acyclic metathesis: reaction between the active metal-carbene chain end and an acyclic olefin in the reaction medium. In particular, we investigate interchain metathesis, which generates linear polymers with “scrambled” chain lengths, and we demonstrate the formation of ring polymers by intrachain backbiting and quantify their content in the reaction product. By controlling the relative rates of propagation versus these side reactions, we prepare ROMP polycyclopentene with low dispersity to substantially higher molecular weights than have been reported previously. Polymerization kinetics are quantitatively described by a kinetic model, which accounts for the reversible binding of added trimethylphosphine to the active chain end.

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ing-opening metathesis polymerization (ROMP) of cyclopentene, followed by catalytic hydrogenation, is a valuable synthetic route to narrow-distribution, perfectly linear polyethylene.1−6 Many initiators can be used to prepare ROMP polycyclopentene, including Mo-based Schrock-type,3,7 Rubased Grubbs-type,8 and W-based catalysts.2,7 Cyclopentene is a commercially available and easily handled liquid, and since the ROMP of cyclopentene proceeds in a living fashion, this is one of the few available methods for incorporating linear polyethylene into a well-defined block copolymer. For example, sequential polymerization of norbornene,5,6 or substituted norbornene derivatives,3 followed by cyclopentene, yields narrow distribution diblock copolymers that can be hydrogenated to give the desired linear polyethylene block. Despite the synthetic value of ROMP polycyclopentene, it is difficult to prepare a high-molecular-weight polymer due to “acyclic metathesis” side reactions: olefin metathesis between the metal-carbene living end of a polycyclopentene chain and acyclic olefins within the reaction medium. The low ring strain of cyclopentene,9 relative to a highly strained cyclic olefin like norbornene,10 results in acyclic olefins having a substantial reactivity versus the monomer. Three categories of acyclic metathesis are possible, illustrated in Figure 1. First, acyclic olefin impurities commonly present in cyclopentene can act as chain transfer agents, with potentially high chain transfer coefficients due to the comparably low reactivities of the cyclopentene double bond and the double bonds found in acyclic species.7 Second, a living end can react with a double bond in the backbone of another polymer chain. This interchain mode has been studied previously,11 and results in the scrambling of chain length between the two interacting © XXXX American Chemical Society

Figure 1. Polycyclopentene acyclic metathesis: chain transfer to a small molecule (left), interchain metathesis (center), and intrachain backbiting (right). Ligands coordinated to the molybdenum active site are denoted Ln, R represents either the initiator fragment forming the chain end in a polycyclopentene homopolymer or previous blocks in a polycyclopentene-containing block copolymer, and R 1 and R 2 characterize the small molecule acyclic olefin.

chains, or a scrambling of block architecture if the chains are polycyclopentene-containing block copolymers. Third, a living Received: December 22, 2016 Accepted: January 18, 2017

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DOI: 10.1021/acsmacrolett.6b00969 ACS Macro Lett. 2017, 6, 112−116

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Figure 2. Polycyclopentene molecular weight distributions derived from gel permeation chromatography (GPC) on aliquots taken during four different polymerizations: (a) [CP]0/[I] = 2500, [CP]0 = 3 mol/L, [PMe3]/[I] = 5, maximum conversion αmax(at 2 h) = 26%; (b) [CP]0/[I] = 2500, [CP]0 = 3 mol/L, [PMe3]/[I] = 15, αmax(6 h) = 35%; (c) [CP]0/[I] = 25000, [CP]0 = 3 mol/L, [PMe3]/[I] = 15, αmax(6 h) = 16%; (d) [CP]0/[I] = 25000, [CP]0 = 10 mol/L, [PMe3]/[I] = 15, αmax(1.5 h) = 9%. Abscissa corresponds to the molecular weight of linear polycyclopentene eluting at a particular time point in the chromatogram.

difference between the terminal and internal olefins or to the greater thermodynamic stability of the internal olefin or both. The remaining two modes of acyclic metathesis consist of reaction between a living chain end and a backbone double bond. Unlike chain transfer to a small molecule, which can be eliminated by removal of that impurity, interchain metathesis and intrachain backbiting always occur in principle, since backbone double bonds will always be present in the reaction medium, along with the reactive chain ends. As such, suppression of reaction with backbone double bonds is entirely kinetic; the relative rates of monomer propagation versus side reaction must be manipulated by the choice of reaction conditions. Figure 2 illustrates four sets of molecular weight distributions evolving over time during a particular polymerization, referred to below as PCP-a through PCP-d, each corresponding to a different initial reactor composition and each terminated at low conversion (9−35%). The reaction parameters manipulated are the starting monomer-to-initiator molar ratio (2500 or 25000), the starting monomer concentration (3 or 10 mol/L), and the molar ratio of a reversibly binding inhibitor (trimethylphosphine) to initiator (5 or 15). The role of trimethylphosphine is to reduce the rate of monomer propagation relative to initiation due to stronger binding to a propagating chain end than to unreacted initiator arising from the reduced steric bulk of the thin polycyclopentene chain cross-section versus the neophylidene ligand on the initiator.12 Increasing the trimethylphosphine concentration promotes narrow molecular weight distributions at low Mn, as illustrated by the reduced Đ in PCPb (15 equiv of trimethylphosphine) versus PCP-a (5 equiv) at comparable Mn, though it is not expected to influence the relative rates of propagation versus inter- or intrachain acyclic metathesis.

end can react with a double bond in its own backbone. Such intrachain backbiting generates two chains: a shortened, linear living chain and a dead “macrocycle”. In this work, we investigate the severity of these side reactions and develop strategies to suppress their effects. All polymerizations are performed in toluene using a Schrock-type initiator, 2,6-diisopropylphenylimidoneophylidenemolybdenum(VI) bis(t-butoxide), and due to the substantial monomer concentration at equilibrium (1.3 mol/L),3 the polymerizations are necessarily terminated at low conversion (2−35%). By optimization of the synthetic procedures, we are able to prepare polycyclopentene to Mn > 150 kg/mol while retaining a narrow molecular weight distribution, Đ ≡ Mw/Mn = 1.15. Preparation of well-defined, narrow-distribution polycyclopentene to high molecular weights requires suppressing each of the three major modes of acyclic metathesis illustrated in Figure 1. Of these, chain transfer to small-molecule acyclic olefins is the simplest to suppress since the cyclopentene can be separated from acyclic olefin impurities by sufficiently stringent distillation. However, it is useful to quantify the reactivities of the most abundant acyclic olefins in order to assess their potential impact on a polycyclopentene polymerization. Commercially available grades of cyclopentene monomer typically contain low but detectable concentrations of acyclic C5 and C6 olefins, generally in the range of 1000−2000 ppm. Chain transfer coefficients, CT, were determined for 1-pentene and mixed cis- and trans-2-pentene, the species with the highest concentrations (see Supporting Information). 1-Pentene was found to be highly reactive (CT,1 = 0.73), with a chain transfer coefficient more than an order of magnitude larger than that of the mixed 2-pentenes (CT,2 = 0.025). This difference may be attributed to either kinetic factors stemming from the steric 113

DOI: 10.1021/acsmacrolett.6b00969 ACS Macro Lett. 2017, 6, 112−116

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< 20 kg/mol and Mw/Mn < 1.10, as well as the high-molecularweight “shoulders” (Mw > 100 kg/mol) of the longest-reactiontime polymers from PCP-b, PCP-c, and PCP-d (see Supporting Information). These values are close to those for the chemically similar species high-1,4 polybutadiene in THF at 25 °C, KPB = 0.0457 mL/g and aPB = 0.693.14 For reference, the [η] versus Mw relationship for ring polymer is also included in Figure 3, following the θ-solvent prediction [η]ring ≈ 0.645 [η]linear.15 The intrinsic viscosities are consistent with linear polymer at the high-molecular-weight end of each distribution but approach the scaling expected for ring polymer in the low-molecularweight tail. For example, 9.3 mol % of all chains in the 2 h sample from PCP-a are rings, with the ring population highly enriched at the low-molecular-weight end of the distribution (calculations in the Supporting Information). Unlike interchain acyclic metathesis, simple dilution cannot suppress intrachain backbiting since a living end cannot be isolated from its own backbone. Instead, we enhance the rate of propagation relative to chain backbiting by increasing the monomer concentration: from 3 mol/L in PCP-c to 10 mol/L in PCP-d (close to bulk cyclopentene, 11.3 mol/L at 23 °C). In PCP-d, the side-reaction products do not begin to substantially impact the molecular weight distribution until much higher average chain lengths are achieved. Furthermore, the total ring polymer content is dramatically suppressed in the case of PCPd. While PCP-c contains 7.7 mol % ring polymers upon reaching Mn = 150 kg/mol, PCP-d contains only 2.4% ring polymers at this chain length. The above synthetic procedures involve manipulation of reaction conditions over a fairly wide range, and so it is necessary to employ a kinetic model to predict the reaction times required to reach the conversion corresponding to a targeted polymer molecular weight. We begin with the mechanism described by Trzaska and colleagues,3 in which trimethylphosphine (PMe3) reversibly binds in a 1:1 complex to the polymer living ends (P*) and exists in dynamic equilibrium between the free (f) and bound (b) states:

At higher molecular weights (higher conversions, hence higher polymer segment concentrations), the rates of inter- and intrachain backbone acyclic metathesis become significant relative to the rate of monomer addition, and the polycyclopentene begins to develop pronounced low- and high-molecular weight populations relative to the main population. This distribution broadening is apparent under each set of reaction conditions in Figure 2. The high-molecular-weight shoulders consist entirely of material formed by acyclic metathesis between two distinct chains. (Chain coupling by oxygen can also produce material with an elevated molecular weight relative to the target population,13 but oxygen is carefully excluded from the reaction environment so this side reaction is expected to be minimal.) Diluting the living end concentration and, thus, the polymer chain concentration at a given polymer molecular weight, can mitigate this interchain metathesis.11 To illustrate, PCP-c was carried out under the same initial reaction conditions as PCP-b except that the monomer-to-initiator ratio was increased by a factor of 10 in PCP-c. Since each active initiator molecule gives rise to a single living chain end, this dilution reduces the concentration of living ends as well as the concentration of backbone double bonds for a given degree of polymerization by a factor of 10 each. As a result, PCP-c does not develop a pronounced high-molecular-weight shoulder until a much greater chain length than PCP-b. Interchain acyclic metathesis is also partially responsible for the low-molecular-weight tail of the distribution, since a lowmolecular-weight chain accompanies each high-molecularweight chain generated via this pathway, such that Mn is not altered. However, ring polymers formed by intrachain backbiting can also be detected in the low-molecular-weight tail via measurements of intrinsic viscosity. Figure 3 reports the

Keq =

[PMe3]b [PMe3]f [P*]f

(1)

The free and bound states of trimethylphosphine and polymer chain ends must also obey a mass balance. The total concentration of trimethylphosphine is denoted [PMe3], and the concentration of bound plus free living ends must equal the concentration of initiator charged to the reactor, [I], and for a 1:1 complex, [P*]b = [PMe3]b.

Figure 3. Intrinsic viscosities across the molecular weight distribution. Data points correspond to absolute Mw (from light scattering) and [η] for 0.3 min “slices” in elution time taken from the GPC chromatogram.

[PMe3]b + [PMe3]f = [PMe3]

(2)

[P*]b + [P*]f = [I]

(3)

We can write an equation for the rate of consumption of cyclopentene: −

intrinsic viscosity, [η], versus Mw across the molecular weight distributions of the longest reaction time samples from the series PCP-a through PCP-d. Each data point corresponds to an elution time slice of 0.3 min during gel permeation chromatography. Mark−Houwink−Sakurada constants of K = 0.0480 mL/g and a = 0.702 were determined for linear polycyclopentene in THF at 25 °C by a best-fit to five samples expected to contain a negligible fraction of ring polymers: the two shortest-reaction-time polymers from PCP-b, both with Mw

d[CP] = ([CP] − [CP]eq )(k b[P*]b + k f [P*]f ) dt

(4)

Here [CP]eq is the equilibrium concentration of cyclopentene in the presence of living polymer, measured to be 1.3 mol/L.3 While Trzaska et al. assumed that only free living ends can propagate (kb = 0) and that trimethylphosphine binding is so favorable that nearly all chain ends are bound, we test these assumptions by allowing both kf and kb to remain nonzero and by making no simplifications to the expression for [P*]b. 114

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of toluene and cyclopentene. As such, kf and especially Keq, as defined in eq 1, might be expected to take on different values in the different reaction media, due to differences in the activity coefficients. Although there is insufficient data to obtain Keq and kf independently for PCP-d, we empirically find that the PCP-d series is described by parameters obeying kb = 0 and kf = 0.97Keq + 180, with kf in units of L/mol·hr and Keq in units of L/mol (dashed curve in Figure 4). The correlation between these two parameters implies that either Keq is elevated relative to 3 mol/L cyclopentene conditions (i.e., trimethylphosphine binding is more favorable in cyclopentene versus toluene) or kfree is decreased when the reaction medium is enriched in cyclopentene or both. The two modes of acyclic metathesis involving polymer, interchain reaction, intrachain backbiting, and macrocycle formation, cannot be completely eliminated since backbone double bonds have substantial reactivity relative to the cyclopentene monomer. However, the rates of these acyclic metathesis reactions can be suppressed by a careful choice of reaction conditions to favor propagation by diluting the living ends and increasing the monomer concentration.

Integrating eq 4 and relating monomer consumption to polymer molecular weight yields [CP]0 − [CP]eq Mn = (1 − exp( −(k b[P*]b + k f [P*]f )t )) m0 [I] (5)

Here, mo is the molecular weight of the monomer (68.11 g/ mol) and [CP]0 is the initial monomer concentration. The concentration of bound living ends is given by solving the quadratic equation: [P*]2b Keq + [P*]b ( − 1 − Keq([I] + [PMe3])) + Keq[I][PMe3] = 0

(6)

The concentration of free living ends is then given by eq 3. The parameters Keq, kf, and kb are found by a best fit to the experimental data, consisting of the four polymerizations in Figure 2 as well as an additional experiment with a very high trimethylphosphine-to-initiator ratio ([CP]0/[I] = 2570, [CP]0 = 3 mol/L, [PMe3]/[I] = 100, which yielded Mn = 4270 g/mol after 2 h, 2% conversion) to sensitively probe kb in the limit of very few unbound living ends. All polymerizations performed in 3 mol/L cyclopentene (PCP-a, PCP-b, PCP-c, and the hightrimethylphosphine experiment described above) are well described by the kinetic parameters (±1 standard deviation, room temperature): Keq = 350 ± 70 L/mol, kf = 690 ± 80 L/ mol·hr, and kb = 3 ± 4 L/mol·hr, shown in Figure 4. It is worth

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00969. Materials, synthesis, and characterization methods, details of chain transfer experiments, determination of Mark−Houwink-Sakurada constants, and calculations for content of ring polymer (PDF).

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +1 609 258 4691. Fax: +1 609 258 0211. E-mail: [email protected]. ORCID

Richard A. Register: 0000-0002-5223-4306 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was generously supported by the National Science Foundation, Polymers Program (DMR-1402180).

Figure 4. Model fit (curves) to molecular weight vs time data (symbols). Solid curves represent parameters fit to experiments in 3 mol/L cyclopentene, dashed curve represents parameters fit to PCP-d only (10 mol/L cyclopentene).

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noting that the propagation rate constant for trimethylphosphine-bound chain ends is not statistically different from zero, supporting the assumption that only unbound chain ends can add monomer. However, the fitted value of Keq = 350 L/mol is small enough that we cannot assume that nearly all chain ends are bound by trimethylphosphine; under the experimental conditions represented in Figure 2, the percentage of unbound chain ends varies from ∼14% (PCP-b) to ∼62% (PCP-c). This set of fit parameters modestly overestimates the molecular weight for PCP-d, where [CP]0 = 10 mol/L instead of 3 mol/L. We attribute this mismatch to the fact that the reaction medium in PCP-d differs from that of the other experiments: PCP-d was prepared in nearly bulk cyclopentene, while all other experiments were conducted in a 2.7:1 mixture 115

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