Alternating Copolymerizations Using a Grubbs‐Type Initiator with an Unsymmetrical, Chiral N‐Heterocyclic Carbene Ligand

Abstract

Reports on alternating copolymers prepared by metathesis copolymerization of two different monomers are relatively rare. [1][2][3][4][5][6][7] In 2005, Chen et al. reported on the mechanismbased design of a ROMP (ring-opening metathesis polymerization) catalyst for sequence-selective copolymerization. [8,9] Here, we report on an alternative system for the synthesis of alternating copolymers based on Grubbs-type initiators containing an unsymmetrical, chiral N-heterocyclic carbene (NHC) ligand.As part of our on ongoing research on ruthenium metathesis catalysts with saturated unsymmetrical NHC ligands [10,11] initiator 1 was prepared from [RuCl 2 (PCy 3 ) 2 -(CHPh)] and 1-mesityl-3-((1'R)-1-phenylethyl)-4,5-dihydroimidazolium tetrafluoroborate in 90 % yield. This initiator was used for the copolymerization of norbornene (NBE) and cyclooctene (COE) using a stoichiometry of 1/NBE/COE of 1:2000:100 000. Reactions kinetics recorded for this copolymerization in CH 2 Cl 2 revealed that NBE was consumed in less than 15 min. Terminating the copolymerization after 60 min in fact provided an almost perfectly alternating copolymer of NBE and COE. Unfortunately, the polymer was only partially soluble in standard solvents such as CHCl 3 . However, the soluble fraction showed a unimodal molecular weight distribution (PDI = 2.19), and M n = 40 400 g mol À1 was found. Longer reaction times led to the formation of substantial amounts of a poly(COE) homopolymer block attached to the alternating copolymer. This homopolymer block is easily identified by NMR spectroscopy.The 13 C NMR spectrum of the alternating copolymer (97 % alternating units, d = 128-136 ppm) is shown in Figure 1. Hardly any signals for poly(NBE) or poly(COE) can be detected. [2,12] The signals for the alternating copolymer are observed around d = 135.0 and 128.5 ppm. The first set of signals around d = 135.0 ppm is assigned to the C = C À CH 2 carbon atoms and consists of eight different signals at d = 135. 28, 135.26, 135.14, 135.00, 134.98, 134.95, 134.83, and 134.80 ppm, which we attribute, without specific assignment, to the ccc, tcc, ctc, cct, ttc, tct, ctt, and ttt triads. The fact that all eight signals display roughly the same intensity is indicative for a cis content of about 50 % and is in accordance with the 1 H NMR spectrum, which shows signals of the cis and trans double bonds in a 1:1 ratio at d = 5.36 and 5.28 ppm, respectively. Such a high cis content is rather unusual for polymers prepared by Grubbs-type initiators; most display a cis/trans ratio of roughly 25:75. The less well-resolved signals around 128.5 ppm are assigned to the C=CÀCH 2 carbon atoms and correspond to the same triads. DSC measurements of the alternating copolymer revealed a single glass transition at T g = À50.9 8C (Dc p = 0.32 J g À1 )), which is in between the glass transition temperatures of poly(COE) (À78 8C, 50 % trans) [13] and various poly(NBE)s (40-64 8C).[14]We next turned our attention to the initiation efficiency of 1. Second-generation Grubbs-type initiators having ...

Depolymerization of 1,4-polybutadiene by metathesis: high yield of large macrocyclic oligo(butadiene)s by ligand selectivity control

“…Creating steric ligand environments by synthesis of unsymmetrical NHC ligands has for instance been pronounced as a strategy to control product selectivity in several ROMP, RO-RCM and RCM reactions. 19,39,[59][60][61][62][63][64][65] The influence of catalyst type on the macrocycles distribution, as observed here, is not because of steric demands (due to dissymmetry), since symmetrical catalysts with different NHC ligands show a similar yield of C16-C44 macrocycles (see Table 1). Therefore, dissimilarities of the electronic structure between the firstand second-generation catalysts, which strongly impact the energetics and kinetics of the elementary steps in the mechanistic cycle, 66-69 more likely determine the macrocycles distribution.…”

Recent advances in ring-opening metathesis polymerization, and application to synthesis of functional materials

“…Appropriately designed dualsite Ru-carbene complexes catalyze the alternating copolymerization of norbornene and a large excess of cyclooctene, wherein one site of the complex shows chemoselectivity while the other site does not. [81][82][83] The combination of polar 2,3-difunctionalized 7-oxanorbornene derivatives and nonpolar cyclic olefins, including cyclooctene, also works satisfactorily. 84,85 The alternating copolymers form well-controlled micrometer-scale aggregates by complementary noncovalent interactions when diaminopyridine and thymine side chains are introduced.…”

Relevance of cis- and trans-dichloride Ru intermediates in Grubbs-II olefin metathesis catalysis (H2IMesCl2RuCHR)

“…12 This has led to several reports considering the side-bound pathway as highly significant, if not the most likely operative pathway. 3,6 We consider that this lack of a clear understanding of the catalytic pathway (side vs. bottom) is hindering the development of selective metathesis catalysts. To provide a basis for assessing the side-and bottom-bound metathesis pathways, we applied DFT methods to investigate the relative energies and the expected E : Z olefin product ratio of the cis-and trans-dichloro Ru pathways for the metathesis of E-and Z-2-butene with the Grubbs-II benzylidene catalyst (Fig.…”