Herein, we report the development of a scalable and synthetically robust building block based on norbornadiene (NBD) that can be broadly incorporated into a variety of macromolecular architectures using traditional living polymerization techniques. By taking advantage of a selective and rapid deprotection with tetrazine, highly reactive "masked" cyclopentadiene (Cp) functionalities can be introduced into synthetic polymers as chain-end groups in a quantitative and efficient manner. The orthogonality of this platform further enables a cascade "click" process where the "unmasked" Cp can rapidly react with dienophiles, such as maleimides, through a conventional Diels−Alder reaction. Coupling proceeds with quantitative conversions allowing high molecular weight star and dendritic block copolymers to be prepared in a single step under ambient conditions.

Molecular architecture plays a key role in the selfassembly of block copolymers, but few studies have systematically examined the influence of chain connectivity on tetrahedrally close-packed (TCP) sphere phases. Here, we report a versatile material platform comprising two blocks with substantial conformational asymmetry, A = poly(trifluoroethyl acrylate) and B = poly(dodecyl acrylate), and use it to compare the phase behavior of AB diblocks, ABA triblocks, and (AB) n radial star copolymers with n = 3 or 4. Each architecture forms TCP sphere phases at minority A block compositions (f A < 0.5), namely, σ and A15, but with differences in the location of order−order phase boundaries that are not anticipated by mean-field self-consistent field theory simulations. These results expand the palette of polymer architectures that readily self-assemble into complex TCP structures and suggest important design considerations when targeting specific phases of interest.

The hexagonally close-packed (HCP) sphere phase is predicted to be stable across a narrow region of linear block copolymer phase space, but the small free energy difference separating it from face-centered cubic spheres usually results in phase coexistence. Here, we report the discovery of pure HCP spheres in linear block copolymer melts with A = poly(2,2,2trifluoroethyl acrylate) ("F") and B = poly(2-dodecyl acrylate) ("2D") or poly(4-dodecyl acrylate) ("4D"). In 4DF diblocks and F4DF triblocks, the HCP phase emerges across a substantial range of A-block volume fractions (circa f A = 0.25−0.30), and in F4DF, it forms reversibly when subjected to various processing conditions which suggests an equilibrium state. The time scale associated with forming pure HCP upon quenching from a disordered liquid is intermediate to the ordering kinetics of the Frank−Kasper σ and A15 phases. However, unlike σ and A15, HCP nucleates directly from a supercooled liquid or soft solid without proceeding through an intermediate quasicrystal. Self-consistent field theory calculations indicate the stability of HCP is intimately tied to small amounts of molar mass dispersity (Đ); for example, an HCP-forming F4DF sample with f A = 0.27 has an experimentally measured Đ = 1.04. These insights challenge the conventional wisdom that pure HCP is difficult to access in linear block copolymer melts without the use of blending or other complex processing techniques.

Synthesis of a new class of conjugated polyenes containing N-heterocyclic six-membered rings was demonstrated via cyclopolymerization of N-containing 1,7-octadiyne derivatives using Grubbs catalysts. Successful cyclopolymerization was achieved by introducing protecting groups to the amines in the monomers. Moreover, a hydrazide-type monomer containing a di-tert-butyloxycarbonyl group ( 6) promoted the living cyclopolymerization to give poly(6) with a controlled molecular weight and narrow dispersity. This living polymerization allowed us to prepare various conjugated diblock copolymers using poly(6) as the first block.

Studies into the cyclopolymerization (CP) of diyne derivatives using metal carbenes have focused on the formation of five- and six-membered rings because these small rings can be easily synthesized while the preparation of medium-sized seven-membered rings are more difficult. For the first time, we achieved the CP forming challenging seven-membered rings as repeat units using Grubbs catalysts by novel design of 1,8-nonadiyne monomers. The key to the successful CP was the introduction of the appropriate aminal and acetal groups, which have short C–N and C–O bonds, and low rotational barriers, thus greatly enhancing the cyclization efficiency. During our mechanistic investigation, we directly observed an actual 14-electron Ru propagating carbene by 1H NMR spectroscopy for the first time during olefin metathesis reaction, presumably because the great steric hindrance from the propagating carbene containing a larger seven-membered ring than five- or six-membered ring retarded the coordination of ligands. We also observed decomposition of the catalysts to ruthenium hydrides during polymerization for the first time. Kinetic studies revealed three interesting features of this 1,8-nonadiyne CP: (i) in contrast to conventional polymerizations, the rate-determining step for the CP of 1,8-nonadiynes was the cyclization step; (ii) the intrinsic reactivity of the acetal monomers was higher than that of the aminal monomers; but (iii) the overall polymerization efficiency of the aminal monomers was higher than that of the acetal monomers because of the higher stability of their carbenes. Finally, we achieved a controlled CP of the aminal monomers using a fast-initiating third-generation Grubbs catalyst. This allowed the synthesis of not only the diblock copolymer containing five- and seven-membered rings but also the triblock copolymer containing five-, six-, and seven-membered rings.

An unsaturated polymer’s cis/trans-olefin content has a significant influence on its properties. For polymers obtained by ring-opening metathesis polymerization (ROMP), the cis/trans-olefin content can be tuned by using specific catalysts. However, cis-selective ROMP has suffered from narrow monomer scope and lack of control over the polymerization (giving polymers with broad molecular weight distributions and prohibiting the synthesis of block copolymers). Herein, we report the versatile cis-selective controlled living ROMP of various endo-tricyclo­[4.2.2.02,5]­deca-3,9-diene and various norbornene derivatives using a fast-initiating dithiolate-chelated Ru catalyst. Polymers with cis-olefin content as high as 99% could be obtained with high molecular weight (up to M n of 105.1 kDa) and narrow dispersity (<1.4). The living nature of the polymerization was also exploited to prepare block copolymers with high cis-olefin content for the first time. Furthermore, owing to the successful control over the stereochemistry and narrow dispersity, we could compare cis- and trans-rich polynorbornene and found the former to have enhanced resistance to shear degradation.

II. Experimental procedure for small molecule synthesis 1 1 , 2-a 2 , 3 3 , 4 4 , 5-a 4 , 6-a 1 , and 7 5 were synthesized according to the literature and their spectroscopic data were reported in the same literature. Scheme S1. Synthesis of 2 2: Propargyl bromide (80% wt. in toluene, 0.28 ml, 2.60 mmol) was added to DMF solution of 3-a (0.30 g, 1.18 mmol in 4.0 ml), then NaH (60% dispersion in mineral oil, 0.17 g, 2.60 mmol) was added to the mixture at 0 ˚C. After stirring for 5 h at rt, the reaction mixture was quenched with water and the product was extracted with diethyl ether. The organic layer was washed with brine, dried with MgSO4 and concentrated. The product was purified by column chromatography (EtOAc:Hexane=1:5) to afford the