Model Branched Polymers: Synthesis and Characterization of

Apr 10, 2012 - Subsequent addition of s-BuLi and Bd monomer led to a living three arm asymmetric star 3. Another in-chain DPE 5 with two long arms was...
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Model Branched Polymers: Synthesis and Characterization of Asymmetric H-Shaped Polybutadienes M. Shahinur Rahman,† Hyojoon Lee,‡ Xue Chen,§ Taihyun Chang,‡ Ronald Larson,§ and Jimmy Mays*,† †

Department of Chemistry, University of Tennessee, Knoxville, Tennessee, 37966, United States Department of Chemistry and Division of Advance Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea § Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States ‡

S Supporting Information *

ABSTRACT: A new type of model branched polymer, asymmetric H-shaped polybutadienes, consisting of central crossbars having various combinations of short and long arms attached to the ends of the crossbars, was synthesized using living anionic polymerization and chlorosilane linking chemistry. The linking agent 4(dichloromethylsilyl)diphenylethylene provides selective reactivity to attach short or long arms on one side or both sides as desired. The samples were characterized thoroughly by size exclusion chromatography with light scattering detection (SECLS) and found to exhibit controlled molecular weights, as well as narrow polydispersity indices (PDIs of 1.01−1.06). Temperature gradient interaction chromatography, a method with far superior resolution as compared to SEC, also shows that these materials are well-defined, with minimal and identifiable impurities.

H

DCMSDPE was much more selective as compared to other synthetic routes, such as use of dichlorosilylmethylstyrene as linking agent, because the steric hindrance and inductively electron-donating character of DPE greatly diminished the reactivity of living polymers with the double bond of DPE relative to the chlorosilane group. The activation of the double bond of DPE and subsequent addition of monomers led to well-defined symmetric branched polymers. Herein, we report the controlled synthesis of four different asymmetric H-PBds (Figure 1) using DCMSPDE chemistry. These materials

-shaped polymers are the simplest example of a branched polymer having two branch points, where four “arms” are connected, two at each end, to a central backbone or “crossbar”. Because of their special structure, these polymers show unique rheological behavior in the melt and in solution1−6 and, thus, are useful as model materials in understanding the rheology of branched polymeric materials, such as low-density polyethylene (LDPE).7,8 Symmetric H-shaped polymers are those with four arms of equal length connected to a crossbar, each segment composed of the same polymer. Typical examples are polystyrene (PS), (PS)2-PS-(PS)2,9 polyisoprene (PI), (PI)2PI-(PI)2,10 and polybutadiene (PBd), (PBd)2-PBd-(PBd)211 synthesized by living anionic polymerization using chlorosilane linking chemistry. H-shaped copolymers where the backbone and arms are composed of two chemically different polymer segments can be achieved by using similar synthetic routes.12−14 The four arms of these H-shaped polymers and copolymers are equal in length. Asymmetric H-shaped polymers having arms of different lengths connected to a crossbar have not previously been synthesized due to the limited control in chlorosilane coupling chemistry, which often generates multiple side products.11 Thus, developing a route to asymmetric H-shaped polymers having arms of different lengths and placed in controlled positions as a model compound for rheological or dilute solution properties studies has heretofore remained a major synthetic challenge. We have recently reported an advantageous approach to synthesize symmetric H-shaped polybutadienes (H-PBd) using classical anionic polymerization and a multifunctional linking agent 4-(dichloromethylsilyl)diphenylethylene (DCMSDPE).15 The end-capping or coupling reaction of living PBd with © 2012 American Chemical Society

Figure 1. Structures of four different asymmetric H-PBds. S represents short arms and L represents long arms.

constitute a new class of model branched polymers for correlating structure with properties. We also report characterization of these materials using conventional (size exclusion chromatography with light scattering detection, SEC-LS) and Received: February 22, 2012 Accepted: April 4, 2012 Published: April 10, 2012 537

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the advanced chromatographic technique, temperature gradient interaction chromatography (TGIC). Four asymmetric H-PBds, namely, H(SSLL)B58, H(SLSL)B54, H(SSSL)B61, and H(SLLL)B60, where S, L, and B denotes short arm, long arm, and backbone, respectively, were synthesized using four different synthetic approaches. Scheme 1 Scheme 1. Synthesis of Asymmetric H(SSLL)B58

Figure 2. SEC-LS profile of SSLL and its precursors (a) short arm, (b) coupled short arms, (c) long arm, (d) coupled long arms, (e) asymmetric star, and (f) fractionated H(SSLL)b58.

Table 1. Molecular Characteristics of Asymmetric Hs and Their Precursors

shows the synthetic procedure for the preparation of H(SSLL)B58. A linear high 1,4-PBd 1 was synthesized using s-BuLi in benzene at room temperature. Living PBdLi was then added to DCMSDPE to replace two chlorines; the result is an in-chain DPE 2 with two short arms. Subsequent addition of sBuLi and Bd monomer led to a living three arm asymmetric star 3. Another in-chain DPE 5 with two long arms was synthesized separately and mixed with 3 in the presence of tetrahydrofuran (THF). A dark red color formed immediately, indicating the activation of DPE having two long arms. The reaction was terminated with degassed methanol to obtain the desired asymmetric H(SSLL)B58. Details of the synthesis are given in Supporting Information. Figure 2 shows SEC-LS profile of H(SSLL)B58 and its precursors. The SEC chromatogram recorded based on the light scattering signal at a 15° angle shows one dominant peak for short and long linear arm (Figure 2a and 2c), coupled arm with DPE (Figure 2b and 2d), star (Figure 2e) and fractionated H (Figure 2f). The absolute number-average molecular weights (Mn) determined from SEC-LS are summarized in Table 1 and are in good agreement with the target Mn of the H polymer and its precursors. The narrow PDI value (1.01) of the fractionated H(SSLL)B58 suggests that the sample is nearly monodisperse. The synthesis of H(SLSL)B54 with one short arm and long arm attached to each side of the crossbar was much more complicated than that of H(SSLL)B58. A short living PBdLi 1 was added carefully to DCMSDPE to replace one chlorine. Long living arm 4 was then added dropwise to replace the second chlorine and generate in-chain DPE 7 with one short arm and one long arm. The activation of the double bond of the

sample

Mn (S arm) kg/mol

Mn (L arm) kg/mol

Mn (star) kg/mol

Mn (backbone) kg/mol

Mn (H) kg/mol

Mw/ Mn (H)

SSLL SLSL SSSL SLLL

11.7 11.7 11.7 11.7

28.9 28.9 28.9 28.9

82.0 65.0 82.0 117.0

58.0 54.0 61.0 60.0

141.0 135.0 125.0 158.0

1.01 1.04 1.03 1.06

DPE moiety and subsequent addition of Bd monomer results a living star 8. Coupling of 8 with dimethyldichlorosilane produced a novel asymmetric H(SLSL)B54 (Scheme 2, see Supporting Information for details). H(SLSL)B54 and its precursors were thoroughly characterized at every step using SEC-LS, and the results summarized in Table 1 confirm controlled synthesis with targeted Mn and narrow PDI. H(SSSL)B61 was synthesized by mixing 8 and 2 in the presence of THF, similar to the final step of the synthesis of H(SSLL)B58. The reaction mixture turned dark red immediately, indicating the activation of DPE with two short arms connected to it. The reaction was terminated with degassed methanol, yielding the asymmetric H(SSSL)B61 where three short arms and one long arm are connected to a crossbar (Scheme S1; see Supporting Information for details). The Mn obtained by SEC-LS (Table 1) was in good agreement with target Mn and shows a narrow PDI, indicating that the synthesis of asymmetric H(SSSL)B61 was also successful. For the synthesis of asymmetric H(SLLL)B, in-chain DPE 5 with two long arms was first synthesized by slow addition of living PBdLi 4 to DCMSDPE. The subsequent activation of 5 and addition of fresh Bd monomer resulted in the three-arm asymmetric star 9 (Scheme S2). The mixing of 9 and 7 in the presence of THF produced asymmetric H(SLLL)B60 with one short arm and three long arms connected to the crossbar. HA(SLLL)B60 (see Supporting Information for details) and its 538

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using SEC-LS. Furthermore, TGIC of fractionated sample shows the presence of minimal impurities. More importantly, TGIC can be used to determine the structure and percentage of H contents and other byproducts based on the reaction mechanism1,18 and Mn information obtained from each step of the synthesis (see Supporting Information for expected structures). The detailed step-by-step analysis of these materials using TGIC, their possible structures, and their rheological behavior will be reported elsewhere. In summary, four well-defined asymmetric H-PBds having different arms lengths and controllable arm placement were synthesized for the first time. These materials constitute a new class of macromolecular model compound for rheological and dilute solution studies. SEC-LS results suggest that these materials are nearly monodisperse. Higher resolution TGIC characterization shows that these materials contain more than 80% of desired product, which is higher than for previously reported symmetric H-PBds that have been characterized by TGIC. Thus, the synthetic methodology reported herein can be successfully used for the synthesis of various asymmetric Hshaped polymers, copolymers, terpolymers, and so on, where the arms could be chemically different and/or of different lengths. The advances in controlled synthesis and advanced characterization reported herein will facilitate fundamental understanding of the effect of long chain branching on solution and rheological and mechanical properties of polymers and copolymers.

Scheme 2. Synthesis of Asymmetric H(SLSL)B54

precursors were thoroughly characterized using SEC-LS and results summarized in Table 1 show controlled Mn with narrow PDI. The controlled Mn and narrow PDI values obtained from SEC-LS for samples H(SSLL)B58, H(SLSL)B54, H(SSSL)B61, and H(SLLL)B60 are similar to or better than those obtained for symmetric H-shaped polymers synthesized so far using living anionic polymerization techniques.9−11 However, this does not necessarily mean that these materials are monodispersed and free of any byproducts. Specifically, it has been demonstrated that a small amount of side products with different degrees of branching cannot be readily detected with conventional SEC-LS.11 This is an expected behavior in SEC, which separates the polymer molecules based on their hydrodynamic volume and exhibits high band broadening. Thus, a more precise and advanced characterization technique is required to rigorously characterize these materials and their structures. We have thus used temperature gradient interaction chromatography (TGIC) for further characterization of these materials. TGIC is an interaction chromatography technique and has much higher resolution than SEC with much less band broadening.16−19 TGIC chromatograms shown in Figure 3a,b correspond to unfractionated H(SSLL)B58 and fractionated H(SSLL)B58. The Mn corresponding to the dominant peaks for both unfractionated (144k) and fractionated (150k) samples are in good agreement with observed Mn (141k)

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ASSOCIATED CONTENT

S Supporting Information *

Details of the synthesis and characterization of asymmetric H polymers. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from NSF under Grants DMR 0906893 and DMR 0906587. We are also grateful to the Army Research Office for an instrumentation grant that allowed the purchase of a TGIC system at UT (Agreement No. W911NF10-1-0282). Any opinions, findings, conclusions, or recommendations expressed in these materials are those of the authors and do not necessarily reflect the views of the National Science Foundation (NSF). T.C. acknowledges the support from NRF via NRL (R0A-2007-000-20125-0), SRC (R112008-052-03002), and WCU (R31-2008-000-10059-0) programs.

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REFERENCES

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Figure 3. TGIC profile of H(SSLL)B58: (a) unfractionated sample; (b) fractionated sample. 539

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