Oxidatively Stable Polyolefin Thermoplastics and Elastomers for

May 25, 2017 - b. Strain at break, εb, and stress at break, σb, were determined at fracture using uniaxial tensile test. c. The tensile bar slipped ...
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Oxidatively Stable Polyolefin Thermoplastics and Elastomers for Biomedical Applications Yanzhao Wang and Marc A. Hillmyer* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455-0431, United States S Supporting Information *

ABSTRACT: Statistical copolymers were prepared by the Ring Opening Metathesis coPolymerization (ROMP) of (Z)5,5-dimethylcyclooct-1-ene and cis-cyclooctene. Subsequent hydrogenation yielded poly(ethylene-co-isobutylene) (PEIB) materials. The feed ratio of the comonomers controls the degree of branching and resulting thermal and mechanical properties of the PEIB samples. Oxidative degradation studies, conducted under accelerated in vitro conditions were used to assess and predict their long-term biostability. Relative to commercial poly(ether urethanes) and a structurally similar polyolefin, poly(ethylene-co-1-butylene), the PEIB samples showed much better oxidative resistance. The facile synthesis, improved stability, and excellent mechanical performance of these PEIB materials bode well for their use in biomedical applications that require long-term biostability.

T

in vivo as such small molecules may leach from the polymer over time.12,13 The oxidative resistance of POs largely depends on the presence and frequency of tertiary carbon atoms in the polymer chain. For example, polypropylene (PP) oxidizes much faster than polyethylene (PE) under natural weathering condition,14 because the tertiary radicals that result from H atom abstraction from tertiary carbons are stabilized and have relatively long lifetimes. The formation of peroxide radicals ROO• at the tertiary carbon atom in PP is ∼20× faster than that of a secondary carbon atom in PE, and the H atom abstraction to generate polymeric hydroperoxides is ∼6× faster (Scheme S1).15−18 To enhance the resistance of branched and, thus, less crystalline POs toward oxidation, it is critical to access materials with only quaternary carbon centers at the branching sites. For example, many polyisobutylene(PIB)-based polymers offer excellent biostability and biocompatibility, due to the absence of tertiary carbon centers.19,20 Another option is poly(ethyleneco-isobutylene) (PEIB), which is also biocompatible and approved by the FDA for food-related applications.21 Moreover, if the composition of ethylene (E) and isobutylene (IB) could be reliably and easily controlled, PEIB would offer tunable thermal and mechanical properties and, potentially, broader applications. The synthesis of PEIB presents a daunting challenge, since 1,1-disubstituted α-olefins like IB do not readily copolymerize with un/monosubstituted olefins via insertion or coordination polymerization mechanisms.22 There have been some successes from research groups led by Kaminsky,23 Shaffer,24 Marks,25,26

he production of medical polymers has grown exponentially over the last 60 years, with the associated products valued at nearly 10 billion USD in 2013.1,2 The incredible success of polymers in the biomedical industry hinges on their versatility. For example, poly(ethylene terephthalate) is used in nonresorbable sutures, poly(methyl methacrylate) for intraocular lenses, polyurethanes for heart valves, and polycarbonates for renal dialysis cartridges.1,3 Low-cost polyolefins (POs) are also extremely important in the medical field.4,5 For example, ultrahigh-molecular-weight polyethylene (UHMWPE) serves as the wear-bearing surface of artificial hips due to its high strength, low wear, and low-friction surface. Long-term durability is essential for polymers used in permanent or semipermanent implants, for example, insulated pacemaker leads. Failures of implantable medical devices have been reported since 1980, and many have been attributed to undesired polymer degradation.6 POs are essentially inert toward hydrolysis and enzymatic degradation due to their hydrophobicity, high molar mass, and lack of functional groups, but they can degrade by oxidative processes.7,8 In the presence of molecular oxygen, unstabilized POs can undergo autooxidation that leads to polymer degradation, surface cracking, discoloration, and ultimately, the loss of mechanical integrity.9 The auto-oxidation mechanism is a classical free-radicalinitiated chain reaction that involves initiation, propagation, branching, and termination (Scheme S1).9−11 Hydrogen-atomdonating antioxidants, such as hindered phenols, are commonly used to protect against oxidative degradation during processing and in service. While effective, phenolic antioxidants are not suitable for every application. For example, butylated hydroxytoluene (BHT) is an antioxidant approved by the FDA, but it suffers from high-temperature volatility and oxidation promoted by metal residues yielding product discoloration. In addition, there are additional toxicity concerns © XXXX American Chemical Society

Received: April 12, 2017 Accepted: May 17, 2017

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DOI: 10.1021/acsmacrolett.7b00277 ACS Macro Lett. 2017, 6, 613−618

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ACS Macro Letters

ultimate crystallinity of the final POs, the degree of branching (DB), defined here as the number of branched carbons per 100 backbone carbons along the polymer, was tuned by varying the comonomer feed ratios (Table S1). Upon hydrogenation (>99%), statistical PEIB copolymers with high molar masses were achieved (Table 1). The final products were isolated as colorless, viscous materials for amorphous PEIBs, or as white solids for semicrystalline variants, in >90% yield over two steps. To precisely compare the effect of a gem-dimethyl branch and an isomeric ethyl branch, POs with the same DB, PEIB-4.2, and PEB-4.2 (where the number indicates the DB of the polymer) were studied. The molecular characteristics of the PEIB, PEB copolymers and unsaturated precursors are given in Tables 1 and S1. DSC analysis (Figure 1) indicates that saturated homopolymer PEIB-12.5 is completely amorphous with a low glass

Nomura,27 and Shino.28 Many of these systems hold promise for tuning comonomer content and ultimate polymer properties, but there are still formidable hurdles to overcome. Alternatively, PEIB copolymers have been obtained by ringopening metathesis polymerization (ROMP)29 or acyclic diene metathesis (ADMET)30 followed by subsequent hydrogenation. In an early example, Wu and Grubbs prepared an alternating PEIB via a highly regioselective and stereoselective ROMP of 3,3-dimethylcyclobutene.31 Wagener et al. later reported the synthesis of PEIBs via ADMET, which allowed the creation of gem-dimethyl branching centers precisely positioned along the polymer backbone.32 In subsequent works, the syntheses of hydroxy-telechelic PEIBs were reported using ADMET33 and ROMP.34 Here, we describe the synthesis of PEIB thermoplastics and elastomers via ROMP of cyclooctene monomers, and evaluate their potential in biomedical applications from the perspective of mechanical properties and oxidative resistance under accelerated in vitro conditions. gem-Dimethyl-branched PEIBs and ethyl-branched poly(ethylene-co-1-butylene) (PEB) were prepared according to Scheme 1. Homo/copolymerization of cis-cyclooctene (COE) Scheme 1. Synthesis of PEIBs and PEB

Figure 1. DSC thermograms of selective PEIBs and PEB.

transition temperature (Tg) of −43 °C. When the DB is 9.5% or lower the resulting PEIBs are semicrystalline; PEIB-9.5 is characterized by a low, broad melting temperature (Tm), crystallization temperature (Tc), and low level of crystallinity (X) (Tc = −15 °C, X = 2%), and PEIB-1.5 has the highest Tm, Tc, and X among all the PEIB copolymers prepared in this work (Tm = 110 °C, Tc = 98 °C, X = 30%). The melting transitions of PEIB-9.5 and PEIB-6.3 are broad due to the nature of the statistical copolymer resulting from simultaneous cross meta-

and (Z)-5,5-dimethylcyclooct-1-ene (Me2COE) or (Z)-3-ethylcyclooct-1-ene (EtCOE) was performed using Grubbs secondgeneration catalyst (G2).35 The total monomer conversion was quantitative giving polymers with comonomer contents consistent with the feed ratio in all cases. To control the Table 1. Characterization Data of PEIBs and PEB polymer

HP(Me2COE) 1 (PEIB-12.5) HP(COE1-s-Me2COE3) 2 (PEIB-9.5) HP(COE1-s-Me2COE1) 3 (PEIB-6.3) HP(COE4-s-Me2COE3) 4 (PEIB-5.3) HP(COE2-s-Me2COE1) 5 (PEIB-4.2) HP(COE3-s-Me2COE1) 6 (PEIB-3.1) HP(COE4-s-Me2COE1) 7 (PEIB-2.6) HP(COE8-s-Me2COE1) 8 (PEIB-1.5) HP(COE2-s-EtCOE1) 9 (PEB-4.2)

Mw (kg mol−1)

Mn (kg mol−1)

c

Đ (Mw/Mn)

Tg

Tm

Tc

ΔHm

X

Td

(mol %)

(%)

SEC

SECc

SECc

(°C)e

(°C)f

(°C)f

(J g−1)e

(%)i

(°C)j

25.0 19.0 12.5 10.5 8.3 6.2 5.2 2.9 8.4k

12.5 9.5 6.3 5.3 4.2 3.1 2.6 1.5 4.2

158 190 226 258d 266d 300d 308d 358d 295d

90 106 120 125d 131d 132d 159d 173d 133d

1.75 1.79 1.88 2.06d 2.03d 2.27d 1.94d 2.07d 2.22d

−43 −43 −37 −31 −26

brdh 50h 61h 75 89 97 110 70

−15 35 50 64 77 85 98 60

6 26 31 45 56 67 84 40

2 9 11 16 20 24 30 14

411 396 391 390 422 406 407 410 413

IB%b a

DBb

g g g

−39

a

Hydrogenation >99% achieved. The number in the name represents the DB of the polymer. bThe IB content and DB were determined using 1H NMR spectroscopy. cMeasured using an RI detector versus polystyrene standards on an SEC instrument with chloroform at 35 °C. dMeasured using an RI detector on an SEC instrument with 1,2,4-trichlorobenzene at 135 °C. eDetermined by DSC (second heating cycle) at 2 °C min−1. f Determined by DSC (second heating cycle) at 10 °C min−1. gNot determined. hBroad melting transition. iCrystallinity was determined based on ΔHm = 277 J g−1 for HDPE. j5% mass loss determined by TGA at 20 °C min−1 under N2. k1-Butylene content. 614

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applied strain, and εr is the strain at zero load after the applied strain in the “nth” cycle. SR1 was calculated to be 76%, SR2 to be 72%, SR10 to be 68%, and SR20 to be 67% for PEIB-6.3. After a considerable change in the stress response occurred between the first cycle and the second cycle, minimal nonrecoverable deformation was observed on subsequent cycles. Elastomeric behavior of related olefin thermoplastic elastomers of 1-octene and ethylene were comprehensively studied by Hiltner and Chum et al.40 In their work, the SR1 and SR10 values for block polymer variants were 75% and 65%, respectively, and 60% and 53% for statistical copolymers. PEIB6.3 shows higher elastic recovery (SR1 = 76%, SR10 = 68%) than both materials. The shore hardness (SA or SD) of PEIBs is estimated based on the relationship between the EY and the ASTM D2240 hardness.38 As DB decreases, the hardness of PEIB increases from SA of 68 to SD of 66 (Table 2). Since PEIB-4.2 contains an intermediate level of branching in the PEIB series, it was chosen as a representative sample to evaluate the oxidative resistance of PEIB, and compared to PEB-4.2, a sample that contains the same amount of branching sites as PEIB-4.2 but includes tertiary carbon centers (see Scheme 1).41 We also compared poly(ether urethane) (PEU) Elasthane 80A (E80A), a conventional and widely used biomedical material. The samples were treated in vitro with the 20% hydrogen peroxide/0.1 M cobalt(II) chloride solution at 37 °C, which generates hydroxyl radicals and mimics the in vivo oxidative degradation at an accelerated rate.42 Size exclusion chromatography (SEC) and scanning electron microscopy (SEM) were used to examine the degradation of the samples after 2, 4, 7, and 12 weeks. The SEC curves of PEIB-4.2, PEB-4.2 and E80A before and after oxidative treatment are shown in Figure 3A and summarized in Table S2. After 2 weeks, weight-average molar mass (Mw) and number-average molar mass (Mn) values of PEB-4.2 and E80A started to decrease. Four weeks later, the diminution of Mw and Mn continued for PEB-4.2 and E80A, and the small peak around a retention time of 30 min observed in the SEC curve of E80A was attributed to the low Mn degradation products. After 7 weeks, the Mw of PEB-4.2 and E80A decreased by 23% and 36%, and the Mn decreased by 31% and 91%, respectively (Table S2). In contrast, the Mw and Mn of PEIB-4.2 remained essentially constant during this experiment; SEC curves were almost identical over 7 weeks, indicating only minimal change. At week 12, SEC curves of PEB-4.2 and E80A shifted dramatically and the changes of Mw and Mn values imply that their mechanical properties would be significantly compromised.43,44 Conversely, the SEC curve of PEIB-4.2 was only minutely impacted after 12 weeks under this aggressive treatment. These observations strongly support that PEIB offers superior oxidative resistance over PEB and E80A. SEM images of the samples also support these findings (Figure 3B). The surface of PEB-4.2 appeared to be roughened after 2 weeks and small pits started to appear and proliferate after 4 weeks. Eventually, those large shallow pits widened to ∼30 μm in diameter, indicating severe damage to the PEB surface. For E80A, no obvious change on the surface was observed after 2 weeks, but surface cracking occurred at week four, and the cracks grew bigger and deeper with longer exposure to the oxidative solution. Surface microcracking of PEU in vivo is well documented in the literature, and our observations were consistent with those results.45−47 On the other hand, the surface of PEIB-4.2 remained relatively unchanged within the first 7 weeks, and only became slightly

thesis.36 As the COE content increases, the run length of ethylene segments increases. Accordingly, the melting transition increases in temperature and sharpens. No distinct glass transitions were observed by DSC for PEIBs 6−8, possibly due to the overlapping of the glass and broad melting transitions. Thermal gravimetric analysis (TGA) of the PEIBs revealed high thermal stability toward mass loss with temperatures of degradative decomposition (Td, 5% mass loss) from 390−422 °C under nitrogen (Figure S3). Figure 2A shows representative tensile data (Table 2) for three PEIB copolymers with varying DB (see SI for sample

Figure 2. Evaluation of mechanical performance: (A) representative tensile strength curves of selective PEIB samples; (B) plot of hysteresis experiment for PEIB-6.3.

Table 2. Mechanical Data for Select PEIBs and PEB EYa polymer

MPa

PEIB-6.3 PEIB-5.3 PEIB-4.2 PEIB-3.1 PEIB-2.6 PEIB-1.5 PEB-4.2

9.2 ± 0.2 19 ± 1 36 ± 2 56 ± 2 74 ± 4 122 ± 5 33 ± 1

σbb MPa 12 33 36 36 35 37 35

± ± ± ± ± ± ±

1 1 2 2 2 2 1

εbb

S Dd

%

calcd

1290 ± 70c 880 ± 40 840 ± 30 790 ± 40 780 ± 40 760 ± 50 790 ± 30

68e 32 44 52 57 66 42

a

Young’s modulus, EY, is the initial slope of the nominal stress vs nominal strain curve in the linear region and was calculated from the average of at least five monotonic curves. bStrain at break, εb, and stress at break, σb, were determined at fracture using uniaxial tensile test. cThe tensile bar slipped out the grip at the average strain of 1290%. dThe shore hardness, SD and SA was estimated according to the relationship between the EY and the ASTM D2240 hardness S: log EY = 0.0235S − 0.6403, where S = SA when 20A < S < 80A and S = SD + 50 when 30D < S < 85D.38 eSA.

preparation details). These samples exhibited high strain at break (εb) values. The mechanical performance is highly dependent on DB and X:37 the Young’s modulus (EY) increases nearly an order of magnitude when the DB decreases from 6.3% to 1.5% (from 9.2 MPa for flexible PEIB-6.3 (X = 9%) to 122 MPa for stiff PEIB-1.5 (X = 30%)). A stiff initial response is observed for PEIB-1.5, followed by a yield point and subsequent strain hardening. At small strains, the behavior of PEIB-4.2 is similar to that of PEIB-6.3 but with a higher yield stress, while at large strains it stiffens with increasing strain. Hysteresis testing was performed on the elastomeric PEIB6.3 samples by extending to 300% strain over 20 cycles and the elastic strain recovery (SRn) for the sample was determined (Figure 2B). SR n was calculated using the following equation:39,40 SRn (%) = [(εa − εr)/εa]100, where εa is the 615

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Figure 3. Evaluation of oxidative resistance of PEB-4.2, PEIB-4.2 and E80A under accelerated in vitro conditions: (A) SEC curves before and after oxidation;48 (B) SEM images before and after oxidation.

rough after 12 weeks. This in vitro oxidation experiment strongly suggests that PEIB indeed provides superior oxidative resistance. In conclusion, we prepared a robust PO by ROMP and subsequent hydrogenation that can behave as either a thermoplastic or an elastomer. The control of DB as well as the resulting thermal and mechanical properties was accomplished via copolymerization of Me2COE with COE. Longterm biostability of PEIB was evaluated under accelerated in vitro conditions, and PEIB showed much better oxidative resistance compared with its isomer PEB as well as conventional biomaterial E80A. Remarkably, PEIB offers tunable and robust mechanical properties, and superior biostability, especially toward oxidation, which is the main biodegradation pathway of POs. Based on this study, we posit that PEIB is a very promising material for long-term biomedical applications.

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Experimental details and supporting figures, tables, and schemes, as well as NMR spectra (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Marc A. Hillmyer: 0000-0001-8255-3853 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Medtronic, Inc. and the Petroleum Institute in Abu Dhabi for financial support of this research. We also thank Dylan Loomis for the help with some SEM images.

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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.7b00277.

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