1,4-Polybutadienes with Pendant Hydroxyl Functionalities by ROMP: Synthetic and Mechanistic Insights

Abstract

The reactivity of cis-3,4-bis(hydroxymethyl)cyclobutene derivatives bearing free and protected hydroxyl groups during ring-opening metathesis polymerization (ROMP) was investigated using ruthenium-based initiators. It was found that the ROMP of cis-4-benzyloxymethyl-3-hydroxymethylcyclobutene (1) using highly reactive initiators containing N-heterocyclic carbenes as nonlabile ligands leads to well-defined polymers while cis-3,4-bis(hydroxymethyl)cyclobutene (2) was reluctant to polymerize under the same conditions. Kinetic studies were performed to assess a number of critical reaction parameters: initiator structure, solvent, and temperature. The results demonstrate that Grubbs' second-and third-generation catalysts are the best initiators to prepare welldefined 1,4-polybutadienes containing simultaneously free and protected hydroxyl side groups with predictable molecular weights (up to 40 000 g mol −1 ) and narrow molecular weight distributions. Besides, low values of k p /k i (the ratio of the rate constant of propagation to the rate constant of initiation) were found for the ROMP of monomer 1 with Grubbs' second-generation catalyst in chloroform or THF as the solvent, demonstrating a living process. The so-obtained polymers having hydroxyl side groups are an ideal platform to prepare original well-defined graft copolymers through the grafting-from strategy. ■ INTRODUCTIONOver the past decade, ring-opening metathesis polymerization (ROMP) has emerged as a powerful tool to synthesize complex macromolecular architectures such as block and graft copolymers. 1,2 Thanks to ruthenium-based (Ru-based) catalysts, 3 a wide range of functionalized cyclic olefins can be polymerized by ROMP. 4 Norbornene and derivatives are the most common monomers studied in ROMP. 1,4 Norbornene has a sufficiently high ring strain (27.2 kcal mol −1 ) 5 to promote ROMP compared to inter-and intramolecular chain transfer reactions (secondary metathesis reactions). Furthermore, the steric hindrance around the double bonds of the growing polynorbornene chains allows to minimize the secondary metathesis reactions when high monomer conversion is reached, leading to polymers with controlled number-average molecular weights (M̅ n ) and low dispersities (Đ M ). ROMP of cyclobutene and functionalized derivativeswhich also present a high ring strain (30.6 kcal mol −1 ) 6 has been studied with a lesser extent compared to the ROMP of norbornene 1,2,4 and oxanorbornene 7 derivatives. However, this synthetic methodology constitutes the best strategy to synthesize strictly 1,4-polybutadiene. 8 Driven by our interest in developing efficient methodologies for the preparation of well-defined graft copolymers, our group previoulsy reported the ROMP of various cyclobutene-derived macromonomers that were prepared using various orthogonal processes such as atom transfer radical polymerization (ATRP), 9 reversible addition−fragmentation transfer (RAFT) polymerization, 10 click chemistry, 10,11 and organocatalyzed ring-opening polymerization. 12 The first study...

Ring opening metathesis polymerization (ROMP) of five‐ to eight‐membered cyclic olefins: Computational, thermodynamic, and experimental approach

“…Buchmeiser and coworkers prepared functional polyolefins by the ROMP of poly(cyclopentene) . Most recently, Fontaine and coworkers investigated the reactivity of a series of ruthenium complexes as initiators for the ROMP of hydroxyl functionalized cyclobutenes resulting in well‐defined polymers …”

Ring-opening metathesis polymerization of 1,2-disubstituted cyclopropenes

“…[7][8][9] Additionally, various cyclobutenes have also been investigated for ROMP. [10][11][12] Cyclopropenes (CPs) are the smallest and most strained cyclic olefins with strain energy nearly twice that of NBE (depending on the substitution). 13 A wide range of disubstituted 1,2-and 3,3-CPs have been synthesized, and are stable at ambient conditions.…”

Understanding the Microstructure of Poly(p-phenylenevinylene)s Prepared by Ring-Opening Metathesis Polymerization Using 13C-Labeled Paracyclophanediene Monomers

“…Ring-opening metathesis polymerization (ROMP) of strained π-conjugated monomers has enabled the preparation of PPVs with control of the polymer molecular weight, narrow Đ m , defined regioregularity, well-defined block copolymers, and potentially limited main chain defects. − Although the mechanism of ROMP for the polymerization of strained cyclic olefins has been widely investigated, ,,− the nature of the backbone defects is still not completely understood and well documented. In this study, PPVs selectively labeled with 13 C along the main chain were prepared by ROMP of the 13 C-labeled [2,2]­paracyclophane-1,9-diene ( M ), using Grubbs second generation ruthenium carbene complex ( G2 ).…”