Macromolecules最新文献

The development of efficient catalyst to overcome the limitation of equilibrium ring-opening polymerization (ROP) of cyclosiloxanes represents the most appealing solution to resolve the back-biting side reaction and high energy consumption during the traditional polysiloxane production. In this contribution, equilibrium polymerization of octamethylcyclotetrasiloxane (D₄) catalyzed by KOH at high temperature was shifted to more efficient polymerization at ambient conditions via fast kinetics and back-biting suppressing by adding a catalytic amount of phosphazenium salt as cocatalyst. Polydimethylsiloxanes (PDMSs) were synthesized in high conversion (≥90%) by bulky phosphazenium salt P₅Cl/KOH catalyzed ROP of D₄ at room temperature in THF solution. The ratio of P₅Cl could be lowered to 0.001 mol % of D₄, and the back-biting reactions were negligible as evaluated by in situ ¹H NMR characterization of the polymerization system. Kinetics investigations suggested that the fast and controlled manner of the ROP was promoted by the P₅Cl/KOH combination. Furthermore, bulk ROP of D₄ and octaphenylcyclotetrasiloxane (P₄) catalyzed by activated phosphazenium salt P₅OMe conveniently afforded PDMSs with different molecular weights (up to 1616 kg mol^–1) and poly(dimethylsiloxane-co-diphenylsiloxane) (PMPS) copolymers with different diphenylsiloxane contents (8.4–63.8 mol %) at 100 °C. The good random characteristic of PMPS copolymers were thoroughly verified by ¹H/²⁹Si NMR and thermodynamic measurements.

The variations of side-chain architecture can significantly affect the thermoresponsive behaviors of graft polymers. This study is aimed at designing linear graft polymers with linear or cyclic pendants (LGPLs/LGPCs) to elucidate the distinct thermostability of polymer solutions. Three pairs of poly(N-isopropylacrylamide) (PNIPAM)-bearing graft polymers with a weight-average graft number (N_g) up to 16 are synthesized by the combination of the ring-first method, fractionation, and topology transformation. The chain architecture and graft number can affect the thermostability and thermosensitivity of polymer aqueous solutions, in which thermo-induced maximal variations of transmittance, light scattering intensity, and fluorescence intensity ratio related to amide–water hydrogen bonding drastically decrease with the incorporation of cyclic pendants and an increase of N_g. LGPLs with pronounced chain entanglement can self-assemble into thermostable lamellae, while other polymer assemblies are subjected to thermo-induced sphere-to-vesicle or vesicle-to-lamella transitions. This research affords a promising method to construct graft polymers with variable architectural parameters to achieve topology-dependent thermoresponsive behaviors.

Vitrimers are a class of polymer networks featuring dynamic covalent cross-links that can undergo associative bond exchange. There has been recent interest in these materials due to their promise as recyclable thermosets or self-healing polymers because of the ability of vitrimer networks to rearrange at the molecular level and undergo macroscopic flow. However, the practical use of these materials often occurs in the supercooled regime or glassy state, where the implications of dynamic bonds are complicated by the interplay between slow activated segmental dynamics, cross-link (i.e., bond-exchange) kinetics, and ultimately material properties. In this paper, we combine coarse-grained molecular dynamics simulation and microscopic statistical mechanical theory to understand how cross-linking kinetics affect material dynamics and how this couples to segmental relaxation of the polymeric network strands across a spectrum of length and time scales, especially in the supercooled regime. We characterize the Kuhn segmental alpha relaxation time and bond exchange time for vitrimer systems across various cross-link densities, temperatures, and bond exchange rates. Simulation and theory both exhibit a bending-up behavior for bond exchange time upon cooling, suggesting a coupling between bond exchange dynamics and segmental relaxation that intensifies with faster bond exchange kinetics. We also found bond exchange dynamics have an impact on Kuhn segment alpha relaxation time, which is most significant at higher cross-link densities. Both these effects are most prominent when the bond exchange time is similar to the Kuhn segment alpha relaxation time, and the resulting coupling of these two relaxation processes is tied to both the probability of a free end to find a bonded pair and the time scale of the constraints imposed by the dynamic cross-links. This relationship is reflected by a cross-link dependence of a theoretical parameter which represents the quantitative degree of coupling between bond exchange and segmental dynamics. Overall, the combination of simulation and theory clarifies the intricate interaction between bond kinetics and segmental relaxation and demonstrates the ability to provide molecular-level insights into vitrimer dynamics over a wide temperature range.

Accessing a facile pathway to prepare polyolefin-polar block copolymers with low dispersity and high control remains a challenge due to the distinct polymerization pathways of the composing blocks. This study utilized the polyolefin active ester exchange, the PACE approach, as a viable solution. The PACE approach, using palladium-catalyst-based coordination-insertion polymerization, was combined with SARA ATRP (supplemental activator/reducing agent atom transfer radical polymerization). A single-chain-end active ester functionalized polyethylene (PE) was produced from an α-diimine Pd(II) hexafluoroisopropyl ester chelate complex, which facilitated a living polymerization of ethylene. Transesterification with 2-hydroxyethyl α-bromoisobutyrate (HOBIB) or 2-hydroxyethyl α-bromoisobutyramide (HOBIBA) formed α-bromoisobutyrate or α-bromoisobutyramide chain-end-functionalized polyethylene. The approach resulted in controlled synthesis of polymers with low dispersity (Đ), high initiation efficiency, and high reproducibility. Both the amide-linked and ester-linked macroinitiators showed >90% initiation efficiency and Đ values of block copolymers as low as 1.05. This work demonstrated a successful combination of two living polymerization techniques, an insertion and controlled radical polymerization, unified in PACE-SARA ATRP, offering access to polyolefin-containing block copolymers with chemically distinct structures.