UC Berkeley

Although alkene metathesis has found a wide range of applications since its discovery in the mid-1960s, alkyne metathesis has only recently become the focus of attention. Despite recent synthetic advances toward highly functionalized ring-strained alkynes, the application of ring-opening alkyne metathesis polymerization (ROAMP) to the field of polymer synthesis has remained limited due to the lack of commercially available well-behaved catalysts. Previous attempts at synthesizing polymers using ring-opening of strained alkynes showed polydispersities ranging from 1.1 to 7.0. Polymers resulting from these catalysts tend to have higher molecular weights than predicted on the basis of the monomer to catalyst loading.

The pseudo-octahedral molybdenum benzylidyne
complex [TolC≡Mo(ONO)(OR)]·KOR (R = CCH3(CF3)2),
featuring a stabilizing ONO pincer ligand, initiates the controlled
living polymerization of strained dibenzocyclooctynes at T > 60
°C to give high molecular weight polymers with exceptionally
low polydispersities (PDI ∼ 1.02). Kinetic analyses reveal that
the growing polymer chain attached to the propagating catalyst
efficiently limits the rate of propagation with respect to the rate
of initiation (kp/ki ∼ 10−3). The reversible coordination of KOCCH3(CF3)2 to the propagating catalyst prevents undesired chain- termination and -transfer processes. The ring-opening alkyne metathesis polymerization with 1 has all the characteristics of a living polymerization and enables, for the first time, the controlled synthesis of amphiphilic block copolymers via ROAMP.

Semiconducting π-conjugated polymers have been widely explored as functional materials in advanced electronic devices. Among these materials, poly(phenylene ethynylenes) (PPEs), a class of conjugated polymers featuring a pattern of alternating aromatic rings and triple bonds, have stood out for their stability, moderate fluorescence quantum yields, and readily tunable band gap. The classical syntheses of PPEs rely on step-growth polymerizations based on either transition-metal-catalyzed cross-coupling reactions or alkyne cross-metathesis (ACM). While these strategies benefit from readily accessible monomers, they lack the precise control over degree of polymerization (Xn), molecular weight, end-group functionality, and polydispersity index (PDI) unique to a controlled ring-opening alkyne metathesis polymerization (ROAMP) mechanism.

Molybdenum carbyne complexes [RC≡Mo(OC(CH3)(CF3)2)3] featuring a mesityl (R = Mes) or an ethyl (R = Et)
substituent initiate the living ring-opening alkyne metathesis
polymerization of the strained cyclic alkyne, 5,6,11,12-tetradehydrobenzo[a,e][8]annulene, to yield fully conjugated poly-
(o-phenylene ethynylene). The difference in the steric demand of
the polymer end-group (Mes vs Et) transferred during the initiation
step determines the topology of the resulting polymer chain. While
[MesC≡Mo(OC(CH3)(CF3)2)3] exclusively yields linear poly(o-phenylene ethynylene), polymerization initiated by [EtC≡Mo-
(OC(CH3)(CF3)2)3] results in cyclic polymers ranging in size from
n = 5 to 20 monomer units. Kinetic studies reveal that the
propagating species emerging from [EtC≡Mo(OC(CH3)(CF3)2)3] undergoes a highly selective intramolecular backbiting into the butynyl end-group.

As the field of ring-opening alkyne metathesis polymerization continues to grow, there is an increase in interest for controlling terminations, and understanding undesirable ones. Attaching a desirable end-group expands the potential applications of the polymerization by adding a new degree of functionalization. Addition of water to the alkylidynes studied herein results in the formation of free radicals, leading to a variety of products. In order to control the termination, ynamines may be used for the regioselective termination of molybdenum benzylidyne catalysts, such as [RC≡Mo(OC(CH3)(CF3)2)3] (R = Tol, Mes) and [TolC≡Mo(ONO)(OR)]·KOR (R = CCH3(CF3)2). Quantitative termination is accomplished and proper functionalization of the polymers is achieved.