Importance of Macromonomer Quality in the Ring-Opening Metathesis

Aug 3, 2015 - Our surprising findings suggest the importance of MM quality and the absence of dinorbornenyl telechelic polymers in achieving the best ...
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Importance of Macromonomer Quality in the Ring-Opening Metathesis Polymerization of Macromonomers Yew Chin Teo and Yan Xia* Department of Chemistry, Stanford University, Stanford, California 94305, United States S Supporting Information *

ABSTRACT: Various macromonomers (MMs) were synthesized using controlled radical polymerization (CRP) by either directly growing from a norbornene-functionalized initiator or chain transfer agent (“direct-growth” or DG method) or coupling a norbornenyl group to preformed polymers (“growth-then-coupling” or GC method). The degree of control for the ring-opening metathesis polymerization (ROMP) of these MMs was found to be dependent on which synthetic method was used for the MMs. Narrowly dispersed brush polymers were consistently obtained from the GC-MMs. In contrast, the DG-MMs resulted in brush polymers with a small high-molecular weight (MW) shoulder or broader molecular weight distribution (MWD). Matrix-assisted laser desorption ionization time-of-flight mass spectrometry of DG-MMs showed the presence of a small amount of α,ω-dinorbornenyl telechelic species resulting from the biradical combination during polymerization. A control study further revealed that even the presence of 1 mol % α,ω-norbornenyl telechelic polymer in the MM resulted in broadening of MWD. Our surprising findings suggest the importance of MM quality and the absence of dinorbornenyl telechelic polymers in achieving the best control for high-MW brush polymers via efficient ROMP of MMs.

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INTRODUCTION In the past few years, ring-opening metathesis polymerization (ROMP) of macromonomers (MMs) has rapidly emerged as a powerful method for synthesizing bottlebrush polymers or molecular brushes, which have a very high density of side chains grafted to the backbone and adopt extended conformations.1−5 Efficient ROMP of MMs using the highly active Grubbs catalyst [(H2IMes)(pyr)2Cl2RuCHPh] 1 allowed modular, relatively simple syntheses of brush polymers, especially brush block copolymers (BCPs). The “graft-through” or MM approach guarantees complete grafting on each repeat unit and can provide precise control of both the side chain length and the main chain length, providing the synthesis of MMs and their ROMP are both well-controlled. A variety of brush (co)polymers have recently been synthesized using ROMP of (oxa)norbornenyl MMs prepared by atom transfer radical polymerization (ATRP),6−8 reversible addition−fragmentation chain transfer (RAFT) polymerization,9−16 other types of polymerizations,17−22 and dendrimer synthesis.23−26 Because of the unique architectures, physical properties, and assembly behaviors, these brush polymers and their BCPs represent a versatile macromolecular platform for a wide range of applications.1,3,4 Even if we consider only the ones prepared using ROMP of MMs, brush (co)polymers have already been widely used as soft photonic crystals,8,21,27 drug delivery vehicles,28−32 optoelectronic and lithographic materials,22,33,34 and surface or rheological modifiers.14,35,36 © XXXX American Chemical Society

Synthesis of monofunctional MMs is a critical step toward successful synthesis of brush polymers via ROMP. Two methods can be used to prepare MMs: (1) direct growth from a norbornenyl-functionalized initiator or chain transfer agent (CTA), which we annotate as “direct-growth” MM (DG-MM), and (2) premade polymer end-capped or coupled to a norbornenyl group, which we annotate as “growth-thencoupling” MM (GC-MM) (Figure 1). MMs from both methods have been used to produce brush polymers. For DG-MMs, one possible side reaction is the incorporation of norbornenyl groups during radical polymerizations, especially when acrylate monomer is used, which can give rise to a bimodal molecular weight distribution (MWD) in the resulting polymers.37,38 By

Figure 1. Two methods, (a) direct-growth (DG) and (b) growth-thencoupling (GC), for the MM synthesis result in seemingly the same MMs. Received: June 1, 2015 Revised: July 17, 2015

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indicating no or minimal incorporation of the norbornenyl groups.

tuning the polymerization conditions and quenching the reaction at low monomer conversions, the Wooley group was able to achieve monomodal MWD of various DG-MMs with low dispersities (ĐM).9−11,13 Functionalized norbornenes have also been used to initiate ring-opening polymerization (ROP) of various epoxides20 and lactones7,17,19,35,39 and polymerization of isocyanates21 to prepare DG-MMs. On the other hand, for GCMMs, successful preparation of MMs requires very high end group functionality of the premade polymer and nearly quantitative coupling between the functional chain end and the norbornenyl group. Grubbs and co-workers synthesized a series of GC-MMs by “click” coupling of ATRP polymers with alkynyl norbornene, which then produced brush homopolymers and brush BCPs with a high molecular weight (MW) (200−2600 kDa) and a very low ĐM of 95%), indicating excellent polymerization efficiency and end group functionality. DG-MMs from ATRP resulted in brush polymers with ĐM values around 1.1−1.2 (Table 2, entries 1−5); however, a high-MW shoulder was consistently observed adjacent to a very narrow main peak for all the formed brush polymers (Figure 2a−c). Furthermore, we observed broadening of MWD with an increase in the targeted DP. For example, for PnBA MM, the high-MW shoulder in the resulting brush polymer became more pronounced when the targeted DP was increased from 50 to 100, and when a DP of 200 was targeted, the shoulder has merged with the main peak to give rise to one broader peak (Figure 2b). Therefore, the high-MW shoulder and peak broadening at high DP indicate that certain intermacromolecular coupling may have occurred during ROMP. DG-MMs prepared using RAFT polymerization were

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RESULTS AND DISCUSSION Synthesis of MMs. DG-MMs can be synthesized via ATRP or RAFT (Scheme 1) using norbornenyl-functionalized ATRP Scheme 1. Synthesis of DG-MMs Using ATRP and RAFT

initiator 2 or RAFT CTA 3, respectively. For the ATRP synthesis of the MMs, CuBr/PMDETA catalytic systems were used for styrene and acrylate monomers, while the CuBr/TMEDA catalytic system was used for methacrylate monomers.41 Both ATRP and RAFT polymerization were stopped at relatively low conversions (30−55%) to prevent the incorporation of norbornenyl functionalities during radical polymerizations and to minimize chain end termination. Under low to moderate conversions, narrowly dispersed norbornenyl DG-MMs of styrene, n-butyl acrylate (nBA), and methyl methacrylate (MMA) were obtained via both ATRP and RAFT, with MWs ranging from 2 to 10 kDa and monomodal MWD (Table 1), B

DOI: 10.1021/acs.macromol.5b01176 Macromolecules XXXX, XXX, XXX−XXX

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be removed by simple precipitation into MeOH because of the large difference in MW between MM and brush polymer (Figure 2i, dashed line). Sensitivity to α,ω-Dinorbornenyl Telechelic Species for ROMP of MMs. To probe the origin of the consistent difference in the control of ROMP between DG-MMs and GCMMs, we decided to closely examine what process(es) during MM synthesis could result in different products depending on which synthetic method was used. It is well-known that biradical combination and disproportionation are the two major termination pathways in radical polymerizations. In CRPs, these termination processes are strongly suppressed but may not be completely eliminated.42 As outlined in Scheme 3, in the events of biradical combination, α,ω-dinorbornenyl telechelic MMs will be formed if the DG method is used, because functionalized norbornene is used as the initiator or the CTA and stays at the end of the MM chain, while in the GC method, the propagating chain end of the preformed polymer is derivatized and coupled to norbornene to produce the MMs. Any combination events would simply result in some dead chains that lack the functional end groups for subsequent coupling; therefore, no α,ω-dinorbornenyl species can be generated. In the disproportionation process, the propagating chain end functionality will be lost, generating saturated and unsaturated chain ends, but the same structures and populations of the disproportionated chain ends would exist regardless of whether the DG or GC method is used. Even if a small amount of the hypothetical unsaturated chain ends on MMs were to affect the control of subsequent ROMP, they would have affected ROMP to the same extent. Thus, disproportionation cannot be attributed to the contrast in the control of ROMP between DG-MMs and GC-MMs. We therefore hypothesized that a small amount of α,ωdinorbornenyl telechelic species may exist in DG-MMs, which broaden the MWD of the resulting brush polymers. We became curious about the tolerance of dinorbornenyl species for ROMP of MMs, which has not been investigated before but provides important information for the future development of the ROMP methodology. Therefore, we deliberately synthesized an α,ωdinorbornenyl polymer by ATRP using a difunctional initiator and subsequently coupling norbornene to both chain ends (NB-

Scheme 2. Synthesis of Brush Polymers via ROMP of (a) DGMM-ATRP and (b) DG-MM-RAFT

qualitatively similar to those prepared using ATRP in terms of their ROMP results (Table 2, entries 6−8). The resulting brush polymers from RAFT DG-MMs had one relatively broad peak with a ĐM of 1.15−1.3 (Figure 2d−f). Although these ĐM values are commonly considered relatively low, especially for the challenging synthesis of brush polymers, the resulting brush polymers from DG-MMs have noticeably broader MWDs compared to those from GC-MMs. In contrast to DG-MMs, the GC-MMs reproducibly yielded very narrowly dispersed brush polymers with a single sharp peak and a ĐM of 99 >99 >99 >99 96 97 95 97 97 >99 >99 70 67

MM:catalyst 1 ratio. bMn,theo = Mn,MALLS (MM) × [MM/C] × conversion. cDetermined by GPC in tetrahydrofuran on two PolyPore columns using RI and MALLS detectors. dConversion of MM to brush polymer is determined by comparing the peak areas of brush polymer and residual MM from GPC measurement of the crude ROMP product. a

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Figure 2. GPC traces of the MMs and respective resulting brush polymers for ROMP of (a−c) DG-MMs from ATRP, (d−f) DG-MMs from RAFT, and (g−i) GC-MMs.

Interestingly, ROMP of MM was found to be not sensitive to SXL. Upon addition of 5 mol % SXL to the MM, the resulting brush polymer peak remained almost unchanged with only a small low-MW tail noticeable (Figure 3). In contrast, even adding 1 mol % PXL to the MM resulted in the appearance of at least two high-MW shoulders and an increase in ĐM to 1.15 for the resulting brush polymer. Increasing the amount of PXL added to the MM to 5 mol % resulted in more significant high-MW shoulders and a further increased ĐM of 1.32 (Figure 3). Our rationale for the different effect of MWD broadening between small molecular and polymeric cross-linkers is outlined in Figure 4. In the case of SXL, when one of the norbornenes reacts during ROMP, the other norbornene is quickly buried in the backbone of a brush polymer surrounded by a high density of crowding side chains (potentially the other norbornene may also react to cyclize the SXL, providing a linker of optimal length is used). Because all the active catalytic species remain at the propagating brush polymer chain ends, their approach to the backbone norbornene is very much sterically hindered (Figure 4a). On the other hand, when a much longer PXL is present, the other norbornene will be at the end of a side chain, which is much more accessible for a propagating catalyst from another brush polymer chain, leading to intermacromolecular coupling (Figure 4b). Our results

Scheme 3. Possible Termination Pathways, Combination and Disproportionation, during Controlled Radical Polymerizations Used for DG-MM Synthesis, and Their Resulting Polymeric Species

PBA-NB; Mn = 6 kDa). This telechelic polymer would function as a polymeric cross-linker (PXL). In comparison, we also synthesized a small molecule bis-norbornene cross-linker (SXL). To probe the effect of both PXL and SXL on ROMP of MMs, we blended a predetermined amount of the PXL or the SXL with a GC-MM (NB-PBA; Mn = 6 kDa) that formed narrowly dispersed brush polymers from its ROMP. D

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separated by m/z 104 and 128, corresponding to the mass of a repeat unit of styrene and butyl acrylate, respectively. For the NB-PS MM, three sets of peaks were observed. The most intense peak series A corresponded to PS with one norbornene end group and one terminal double bond formed by loss of HBr. The loss of labile bromide to give a terminal double bond is known to occur during MALDI ionization process.43,44 A very lowintensity set of peaks corresponded to the coupled telechelic species B. For instance, a representative mass peak at m/z 1928.49 corresponds to the 13-mer of C15H18NO4-(C8H8)13C15H18NO4·Na+ with a calculated mass of m/z 1928.03 (Figure 5). Because a high laser power had to be used to clearly observe the weak peak series B, a third set of peaks C at median intensity was also observed, which possibly resulted from a fragmented species during the MALDI ionization process. We assigned the peak series C to a population corresponding to species A with a norbornene-dicarboximide group eliminated at the other chain end [for example, C6H9O2(C8H8)16-C8H7·Na+ with a calculated mass of m/z 1904.08]. The MALDI mass spectrum of NB-PBA showed a major peak series D, corresponding to PBA with one norbornene end group and one bromide end group. In addition, a low-intensity set of peaks corresponded to the coupled telechelic products E. For instance, the mass peak at m/z 5702.4 corresponds to a coupled 40-mer of C15H18NO4-(C7H12O2)40C15H18NO4·Na+ with a calculated mass of m/z 5702.43. We also performed MALDI-TOF MS on DG-MM-PMMA and did not observe the masses for coupled telechelic species. Combination for methacrylate chain ends should be disfavored, but DG-MMPMMA still resulted in a small high-MW shoulder or broader MWD for their brush polymers.

Figure 3. Effect of dinorbornenyl species (SXL and PXL) on ROMP of MMs. GPC traces of the brush polymers obtained at an MM:1 ratio of 100 using pure GC-MM (red), upon addition of 5 mol % SXL (black dashed), addition of 1 mol % PXL (blue), and addition of 5 mol % PXL (green).

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CONCLUSIONS We report a consistent difference in the control of ROMP between GC-MMs and DG-MMs prepared by ATRP and RAFT. ROMP of DG-MMs resulted in a broader MWD and a higher ĐM, which was attributed to the presence of a small amount of α,ω-dinorbornenyl telechelic polymer in DG-MMs. In contrast, the GC method, which does not give rise to dinorbornenyl species, produced well-defined brush polymers with a ĐM of