Note pubs.acs.org/Macromolecules
Tailor-Made Thermoplastic Elastomeric Stereoblock Polypropylenes by Modulation of Monomer Pressure Moshe Moshonov, Sinai Aharonovich, and Moris S. Eisen* Schulich Faculty of Chemistry, Technion − Israel Institute of Technology, Technion City 3200008, Israel S Supporting Information *
S
nonstereoselective monomer insertion (Bernoullian distribution), and a noticeable shoulder at 20.97−20.99 ppm, assigned to the mmmrmr heptad,18 hints the presence of considerable amounts of boundaries between isotactic and stereoirregular blocks19 (Scheme 2). To gain a better understanding of the unexpected microstructure of P1, we utilized a calculative method to isolate the contribution of polymer D to the observed pentad distribution of P1 (see Supporting Information). We have found that this distribution has ca. twice the frequency of the rr triad compared to the Bernoullian distribution, implying that the chain epimerization is slightly syndioselective. When the monomer pressure is periodically changed from 10 to 1 atm during a polymerization run, an additional elastomeric f raction is obtained, at the expense of the isotactic f raction, strongly indicating that the same catalytic species II produces both fractions. The polymerization data for the isotactic fraction obtained at constant pressure and the elastomers are presented in Table 1 (see Table SI3 in the Supporting Information for yield and molecular weights of the other fractions). As seen in Table 1, the isotacticity and transition temperatures are influenced by the pressure modulation regimes. The morphologies of polymers B, E3, and A were investigated using SEM, which showed decrease in the crystals size in the order iPP > sb-PP > irgPP, in accord with the decrease in the tacticity (see Supporting Information). AFM (Figure 1) showed that the iPP has a morphology of long interpenetrated and branched crystalline fibrils that interconnect amorphous regions. The elastomeric sample exhibits the development of spherulitic growth, or hedrites, appearing as circular elliptical features which radiate from central points.20 Interestingly, the mmmrmr heptad signal is present in the 13C NMR spectra of elastomers E, implying that block boundaries are present at a sufficient frequency to influence the observable pentad distribution. To better understand the dependence of the stereoblock lengths on the pressure regimes, we developed a method to calculate the polymerization degree of the blocks, using a stochastic model and the experimental parameters. For sb-PP produced by a two states catalyst, the following mass balances can be written21
ince the 1930s, polymer chemists in academia and industry have invested considerable attention and efforts in finding alternatives to thermoset elastomers.1,2 These materials attain key physicomechanical properties, such as elasticity and minimal creep,3 by curing, in which their macromolecules are covalently cross-linked. However, curing also severely limits the thermosets processability, causing recycling to rely on costly and polluting steps.4,5 As a result, for instance, 87% of the 4.4 × 106 tons of used tires produced in 2015 in the U.S. were either grounded, burned as fuel, or discarded,6 representing high environmental and economical losses. The discovery of thermoplastic elastomers such as atactic−isotactic stereoblock polypropylene (sb-PP),7 which can be processed by molding or extruding, has spurred a great interest in the study of the group IV based catalysts, which can produce these materials.8,9 In recent years, our group has studied the catalytic polymerization properties of octahedral group IV amidinate complexes.10 For zirconium, mono- and bis(amidinate) active catalytic species were identified (complexes I and II in Scheme 1, respectively), which produce a mixture of two distinct polymers with different tacticities, microstructures, and molecular weights.11−17 At high monomer pressure, the C1 symmetric complex (I) produces stereoirregular PP (irgPP), while predominantly isotactic PP (iPP) is obtained by the C2 symmetric complex (II). As the monomer pressure is reduced, the tacticity of the iPP produced by complex II decreases due to increased frequency of stereoerrors, which allows access to a range of materials: thermoplastic solids, elastomers, or oils (B, C, and D, respectively, in Scheme 1).12,13,16,17 These results were rationalized by a competition between site-controlled isotactic chain propagation, and chain racemization at the lastinserted monomer unit, which predominates at lower monomer concentrations.13,16 Herein, we use a pressure-modulated polymerization reactor system (see Supporting Information) to produce elastomeric stereoblock polypropylenes (sb-PP)s with a precise control over the stereoirregular block size (E, Scheme 1). We start the presentation with propylene polymerizations at constant low and high pressures and continue with polymerizations under periodic step functions of pressure in time. Polymerization at constant propylene pressure of 1 atm produces a blend of the oily, stereoirregular polymers A and D (P1), whereas polymerization at 10 atm yields after separation by fractional extraction a mixture of a stereoirregular and a largely isotactic fractions (polymers A and B), in accordance with the literature data.12,13,16,17 Interestingly, the frequencies of the isotactic (mmmm) and syndiotactic (rrrr) pentads in the 13C NMR spectrum of P1 are higher than the 6.25% expected for a © XXXX American Chemical Society
me ·P(1e) = mi ·P(1i) + ma ·P(1a) + mb ·P(1b)
(1)
me ·P(2e) = mi ·P(2i) + ma ·P(2a) + mb ·P(2b)
(2)
Received: August 24, 2016 Revised: November 8, 2016
A
DOI: 10.1021/acs.macromol.6b01825 Macromolecules XXXX, XXX, XXX−XXX
Note
Macromolecules Scheme 1. Various PP Microstructures Obtained by the p-Tolylamidinate Zr(IV) Complexesa
a
For simplicity, only purely isotactic, atactic, or stereoerrors repeating units are shown.
Scheme 2. Example of Boundary Pentads in sb-PPa
a
For clarity, the repeating unit are shown as 100% isotactic or stereoirregular.
me = mi + ma + mb
(3)
To solve eqs 1−3 for the block masses, the stereoblock boundary pentad distribution was first calculated using a twostate probability model for the insertion processes. The insertion at 1 atm was modeled by a first-order Markov process (chain end control),22 as supported by the partial syndioselectivity of polymer D (vide supra) and previous examples for Zr12 and Ti23 amidinates. At 10 atm, a zero-order Markov process (enantiomorphic site control) was used, in accord with the observed spectrum of polymer B and the literature data.13 For the boundary distribution calculations (see Supporting Information), the m diad probabilities of either of the states was derived from the pentad distribution of polymers B and D using eq 4.
p(m) = p(mmmm)1/4
Figure 1. AFM images of the elastomeric polymer E (right) and isotactic polymer B (left) at 5 μm.
Solving the set of linear eqs 1−3 using the calculated pentad distributions of the boundaries and polymer D, and the observed pentad distributions of polymers B and E, gave only one solution for the blocks masses. With these masses, we used eqs 5 and 6 to calculated the polymerization degree of the isotactic (PDi) and stereoirregular (PDa) blocks for elastomers E (see Supporting Information), which are presented in Table 1.
(4)
Table 1. Selected Polymerization Data for the Elastomeric and Isotactic Fractionsa label
th/tlb
mc
Mwd
MWDe
mmmmf
PDig
PDah
Tgi/Tmj
B E1 E2 E3
180/0 0.5/2 1/4 1/8
147.2 32.1 29.7 100.9
123 304 283 414
5 7.8 8.4 6.7
74.8 58.1 56.1 27.4
585 77 101 32
0 17 31 51
ni/148.4 −10.4/140.7 n/n −8.8/136.8
Polymerizations were performed in toluene (10 mL) at 24 °C, with catalyst and MAO loadings of 5 mg and 1:250 Zr:Al mol ratio, respectively. Time at 10 atm [min]/time at 1 atm [min] in a pressure modulation cycle. cPolymer yield [mg]. dWeight-average molecular weight [kg/mol]. e Polydispersity. fPentad frequency [%]. gPolymerization degree of the isotactic blocks. hPolymerization degree of the stereoirregular blocks. iGlass transition temperature [°C]. jMelting point [°C]. kNot applicable/performed. a b
B
DOI: 10.1021/acs.macromol.6b01825 Macromolecules XXXX, XXX, XXX−XXX
Macromolecules PDi =
(
0.5me 1 +
PDa =
■
PD(mi + 0.5mb) PDmb 8me
PD
(
0.5 1 +
PDmb 8me
)
)
Note
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01825. Experimental details, reactor design, NMR spectra of the polymers, DSC of polymers, polymerization results at constant pressures, calculative modeling of the polymerization degrees of the stereoblocks (PDF)
(5)
− PDi (6)
As shown in Figure 2, PDa increases linearly as the low pressure period in a cycle (tl) increases from 0 to 8 min. It is noteworthy
■
AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (M.S.E.). ORCID
Moris S. Eisen: 0000-0001-8915-0256 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS M.S.E., M.M., and S.A. thank the Israel Science Foundation administered by the Israel Academy of Science and Humanities under Contract No. 78/14.
Figure 2. Linear dependence of PDa on tl.
■
to point out that since our calculative methods use the numberaverage molecular weight of the polymers, the calculated block numbers per chain, and as a result, the block sizes, are averages. Hence, chain termini are not counted as block boundaries. Thus, propylene pressure can serve as an easy to use tool for the production of sb-PP’s with a precise control of their block size. Further, the ability to apply programmable pressure changes (i.e, by PLC valves) can give access to unique stereoblock microstructures, inaccessible by other methods. In these polymers the block sizes would comply to various pressure functions, such as sine wave, exponential increase/ decay and so on. Support for the dependence of the block sizes on the pressure regimes is also independently given by the experimental transition temperatures of the polymers (Table 1). The glass transition temperature (Tg) of elastomer E1, which also has the lowest PDa, is lower than the Tg of elastomer E3, or the Tg’s of polymers A and P1 (see Supporting Information), in accord with the Flory−Fox equations, considering the shorter amorphous chain segments of the former.24 The melting temperatures of the polymers increase with PDi in the order (A, P1) ≪ E3 < E1 < B (logarithmically for the solid polymers) corroborating the increase of the isotactic block size.25 Interestingly, logarithmic dependence of Tm on PD also can be derived from the data of Natta26 for isotactic PP’s19 (see Supporting Information). In summary, isotactic−stereoirregular stereoblock polypropylenes are produced using the bis(p-tolylamidinate)zirconium catalyst by rapid changes of the monomer pressure during the polymerization. We have shown that the stereoirregular blocks lengths depend linearly on the low pressure duration in a cycle, which in turn affect the physicomechanical properties of the polymers. This ability to tune the microstructure of the polymer by pressure, an easily controlled process parameter, enables the preparation of unique materials, which are inaccessible by other polymerization methods. Additionally, we illuminated some details of the syndioselectivity observed at low propylene pressures in zirconium amidinate catalysts.
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D
DOI: 10.1021/acs.macromol.6b01825 Macromolecules XXXX, XXX, XXX−XXX
