A series of poly(cyclohexylethylene)-block-poly-(methyl methacrylate) (PCHE−PMMA) diblock copolymers with varying molar mass (4.9 kg/mol ≤ M n ≤ 30.6 kg/mol) and narrow molar mass distribution were synthesized through a combination of anionic and atom transfer radical polymerization (ATRP) techniques. Heterogeneous catalytic hydrogenation of α-(hydroxy)polystyrene (PS-OH) yielded α-(hydroxy)poly(cyclohexylethylene) (PCHE-OH) with little loss of hydroxyl functionality. PCHE-OH was reacted with α-bromoisobutyryl bromide (BiBB) to produce an ATRP macroinitiator used for the polymerization of methyl methacrylate. PCHE−PMMA is a glassy, thermally stable material with a large effective segment−segment interaction parameter, χ eff = (144.4 ± 6.2)/T − (0.162 ± 0.013), determined by meanfield analysis of order-to-disorder transition temperatures (T ODT ) measured by dynamic mechanical analysis and differential scanning calorimetry. Ordered lamellar domain pitches (9 ≤ D ≤ 33 nm) were identified by small-angle X-ray scattering from neat BCPs containing 43−52 vol % PCHE ( f PCHE ). Atomic force microscopy was used to show ∼7.5 nm lamellar features (D = 14.8 nm) which are some of the smallest observed to date. The lowest molar mass sample (M n = 4.9 kg/mol, f PCHE = 0.46) is characterized by T ODT = 173 ± 3 °C and sub-5 nm nanodomains, which together with the sacrificial properties of PMMA and the high overall thermal stability place this material at the forefront of "high-χ" systems for advanced nanopatterning applications.

This Perspective provides a current survey on the synthesis, self-assembly, and vast variety of applications possible from block copolymer (BCP) systems containing at least one segment of either poly­(2-vinylpyridine), P2VP, or poly­(4-vinylpyridine), P4VP. Strategies and insight toward P2VP or P4VP as the segment of choice and their influence on material properties, effective interaction parameters, and hosting a variety chemistries from small molecules to nanoparticles are provided. It is with hope that this work serves as a modern field guide to the versatility and utility of these chemistries for those new to or continuing research with these BCP systems.

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This Viewpoint highlights the viability and increasing variety of functionalized polypentenamers as unique and valuable materials created through enthalpydriven ring-opening metathesis polymerization (ROMP) of low ring strain cyclopentene monomers. The terms "low ring strain" and "enthalpy-driven" are typically conflicting ideologies for successful ROMP; however, these monomers possess a heightened sensitivity to reaction conditions, which may be leveraged in a number of ways to provide performance elastomers with good yield and precise functional topologies. Over the last several years, a rekindled interest in these systems has led to a renaissance of research aimed at improving their synthesis and exploring their potential. Their chemistry, applications, and future outlook are discussed. Figure 7. (a) Sulfonation of H 2 −P4PCP produces a precision polyelectrolyte similar to polystyrenesulfonate (PSS) but with precise 5-carbon distances along the chain. This material has an accessible T g (109°C) allowing it to be thermally molded (inset). Adapted with permission from ref 86. Copyright 2018 John Wiley and Sons. (b) Design of a polypentenamer bottlebrush polymer from ROMP of BIB-functionalized CP followed by ATRP. Block polymer grafts of PS and PMMA, monitored by SEC traces, produce a core−shell polymer visualized by AFM (inset). Adapted with permission from ref 57.

The depolymerization of bottlebrush (BB) polymers with varying lengths of polycyclopentene (PCP) backbone and polystyrene (PS) grafts is investigated. In all cases, ring closing metathesis (RCM) depolymerization of the PCP BB backbone appears to occur through an end-to-end depolymerization mechanism as evidenced by size exclusion chromatography. Investigation on the RCM depolymerization of linear PCP reveals a more random chain degradation process. Quantitative depolymerization occurs under thermodynamic conditions (higher temperature and dilution) that drives RCM into cyclopentenes (CPs), each bearing one of the original PS grafts from the BB. Catalyst screening reveals Grubbs’ third (G3) and second (G2) generation catalyst depolymerize BBs significantly faster than Grubbs’ first generation (G1) and Hoveyda–Grubbs’ second generation (HG2) catalyst under identical conditions while solvent (toluene versus CHCl3) plays a less significant role. The length of the BB backbone and PS side chains also play a minor role in depolymerization kinetics, which is discussed. The ability to completely deconstruct these BB architectures into linear grafts provides definitive insights toward the ATRP “grafting-from” mechanism originally used to construct the BBs. Core–shell BB block copolymers (BBCPs) are shown to quantitatively depolymerize into linear diblock polymer grafts. Finally, the complete depolymerization of BBs into α-cyclopentenyl-PS allows further transformation to other architectures, such as 3-arm stars, through thiol–ene coupling onto the CP end group. These unique materials open the door to stimuli-responsive reassembly of BBs and BBCPs into new morphologies driven by macromolecular metamorphosis.

A series of symmetric poly(4-tert-butylstyreneblock-methyl methacrylate) (PtBS-b-PMMA) diblock copolymers with varying molar masses and narrow molar mass distributions were prepared using sequential anionic polymerization. Order-to-disorder transition (ODT) temperatures were determined as a function of the overall degree of polymerization, N, using a combination of low-frequency dynamic mechanical spectroscopy (DMS) and variable temperature small-angle X-ray scattering (SAXS), leading to a mean-field expression for the segment−segment interaction parameter, χ = (41.2 ± 0.9)/T − (0.044 ± 0.002). This material is characterized by a larger value of χ, and much greater temperature sensitivity, than polystyrene-b-PMMA, providing access to tunable lamellar periods (pitch) down to 14 nm at technologically relevant temperatures. Nucleation and growth of lamellae, following cooling from the disordered state, was characterized by SAXS well above the glass transition temperature (T g ≈ 130 °C) and extrapolated to lower processing temperatures based on polymer chain relaxation times extracted from time− temperature superposed DMS data. These results qualify PtBS-b-PMMA as an attractive candidate for development as a new lithographic material.

Living-like conditions through ring-opening metathesis polymerization (ROMP) of low ring strain monomers were achieved through temperature variation during the reaction. For a variety of solvents and readily available ruthenium-based catalysts, warm initiation to low conversions followed by immediate thermal quenching and subsequent propagation to high conversions produced polycyclopentene with molar masses close to theoretical and with moderately low dispersities (Đ ≤ 1.3 ± 0.1). Optimization using Grubbs first-generation catalyst and THF as the solvent consistently resulted in narrow dispersities (<1.2) at various molar masses targeted which increased linearly with monomer-to-catalyst ratio. In all cases, intermolecular chain transfer reactions were suppressed at colder temperatures as evidenced by low dispersity for up to 300 min at 0 °C. This approach was extended to 3-cyclopentenol to emphasize the universality of the methodology to other low ring strain monomers with functional group tolerance specific to Ru-based catalysts.

Ring-opening metathesis polymerization of 4-phenylcyclopentene is investigated for the first time under various conditions. Thermodynamic analysis reveals a polymerization enthalpy and entropy sufficient for high molar mass and conversions at lower temperatures. In one example, neat polymerization using Hoveyda-Grubbs second generation catalyst at -15 °C yields 81% conversion to poly(4-phenylcyclopentene) (P4PCP) with a number average molar mass of 151 kg mol(-1) and dispersity of 1.77. Quantitative homogeneous hydrogenation of P4PCP results in a precision ethylene-styrene copolymer (H2 -P4PCP) with a phenyl branch at every fifth carbon along the backbone. This equates to a perfectly alternating trimethylene-styrene sequence with 71.2% w/w styrene content that is inaccessible through molecular catalyst copolymerization strategies. Differential scanning calorimetry confirms P4PCP and H2 -P4PCP are amorphous materials with similar glass transition temperatures (Tg ) of 17 ± 2 °C. Both materials present well-defined styrenic analogs for application in specialty materials or composites where lower softening temperatures may be desired.