Date of Award
Fall 2022
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemical and Environmental Engineering (ENAS)
First Advisor
Zhong, Mingjiang
Abstract
Branched polymers are a major class of architectural macromolecules that possess a 3- dimensional conformation, commonly categorized as hyperbranched, dendritic, bottlebrush-like, and star-shaped polymers. In addition to the two dictating parameters of all polymers – degree of polymerization and dispersity, rationally-designed intramolecular connection endows branched polymers with a broader structural tuning window to diversify their viscoelastic, optoelectronic, and self-assembly properties, providing superior functionalities for energy storage, environmental preservation, and biomedical applications. Attaining sophisticated understanding of structure- property relationships is critical for the design of precisely defined branched polymers with desirable properties. While recent development of advanced polymerization techniques has enabled controlled synthesis of linear polymers, producing well-defined branched polymers requires extensive efforts due to their structural complexity. In particular, hyperbranched polymers synthesized from traditional methods are usually characterized by their randomly-distributed branching junctions, uncontrolled molecular weight, and broad molecular weight distribution. This thesis aims to address the synthetic challenges in controlled (hyper)branching polymerization. After that, cutting-edge environmental and energy-related applications are achieved with branched polymers. Mixed-graft block copolymers (mGBCPs), with two distinct types of side chains grafted from a linear backbone, are employed in various applications that benefit from the ordered nanostructures formed from phase separation of the two side chains. The performance of mGBCPsdepends greatly on their structural parameters such as backbone lengths and side chain compositions. This thesis will first review the synthetic pathways toward hyperbranched polymers and bottlebrush-like polymers, as well as their unique properties and applications in environmental protection and energy storage. Next, I will introduce a novel synthetic strategy to achieve the controlled synthesis of hyperbranched polymers site-specifically initiated via atom transfer radical polymerization. A special branching monomer, inibramer = initiator +branching junction creator + monomer, was designed to enable a controlled/living chain-growth polymerization with branching junctions exclusively generated from an enchained initiating site. The copolymerization of inibramer with vinyl monomers produced branched polyacrylates and polystyrenes with controlled molecular weights, tunable degree of branching, and low dispersity. Hierarchically- branched architectures, including linear-block-(hyper)branched, (hyper)branched-on-brush, and (hyper)branched-on-star, were constructed by a facile grafting-from polymerization from different macroinitiators. To gain a deeper insight into the copolymerization behaviors and further optimize the polymerization conditions, kinetic simulations and mechanistic studies were carried out with quantitative evaluation of every elementary reaction. The results indicated that an accelerated biradical termination in the branching polymerization was the main cause of limited conversion. Therefore, a constantly low radical concentration was maintained to suppress radical termination through an activator regeneration process mediated by external reduction power. As a result, a broad library of vinyl monomers, ranging from highly-reactive acrylonitrile to chemically-inert acrylamides were successfully copolymerized with inibramer, to synthesize various branched polymers with elevated molecular weight and degree of branching. The subsequent part of this thesis focuses on the applications of mGBCPs. Firstly, mGBCPs with well-defined polyethylene (PE) and polystyrene (PS) side chains were synthesized by a grafting-to method to serve as the compatibilizers of high-density polyethylene (HDPE)/PS blends. The compatibilization performance was dictated by the backbone length, and the blends compatibilized with mGBCP of the longest backbone exhibited mechanical properties comparable with HDPE. Blends with compatibilizer content as low as 0.5 wt% displayed elongation of break almost 100-fold higher than that of pristine blends. The extraordinary performance was attributed to the strong co- crystallization and entanglements between side chains and corresponding homopolymers, an additive effect of multiple side chains in the same polymer, and a trapped entanglement conformation introduced by the backbone. Lastly, mGBCPs bearing short poly(ethylene oxide) (PEO) and polydimethylsiloxane (PDMS) side chains synthesized by a grafting-through method were employed as solid polymer electrolytes. The mGBCPs could phase-separate to form microstructures with PEO as the continuous lithium ion conductive phase. Due to the sub-room- temperature melting point of short PEO, this material maintained a high chain mobility and therefore a high ionic conductivity at ambient conditions. The superior performances of the two abovementioned materials both benefit from the unique architecture and self-assembly behavior of mGBCPs.
Recommended Citation
Cao, Mengxue, "Controlled Synthesis and Emerging Applications of Branched Polymers" (2022). Yale Graduate School of Arts and Sciences Dissertations. 865.
https://elischolar.library.yale.edu/gsas_dissertations/865
