Rational design of hydrogels and charged polymers is important for understanding the underlying principles that govern complex mechanical and phase behaviors of biomacromolecular systems. Mimicking and capturing such behaviors in synthetic polymer systems are highly desired for many biomedical and technical applications. Biological tissues exhibit complex dynamic mechanical properties. To capture and control dynamics in synthetic hydrogel scaffolds, I developed a hyaluronic acid (HA)-based dynamic hydrogel system crosslinked by dynamic covalent hydrazone bonds. The HA hydrogels and collagen I generated an interpenetrating network, which exhibited both well-controlled mechanical tunability and fibrillarity, thus providing an adaptable platform to study the cell-matrix interactions. A biocompatible catalyst that accelerates the exchange of hydrazones was then incorporated into the HA-hydrazone hydrogel to modulate the dynamics of crosslinks without affecting network structures. This approach was used to develop injectable hydrogels for cell delivery with high injectability and stability enabled by temporal catalyst controlled hydrogel dynamics. The catalyst control of network dynamics also enabled quantitative and unambiguous correlation between the network parameters and mechanical properties of dynamic polymer networks. In the second part, I investigated the fundamental study on liquid-like polyelectrolyte complexes (PECs) formed by oppositely charged polyelectrolytes. PECs are important in biological systems for membraneless compartments, however the understanding of the mechanism and structure-property relationship is limited. I developed a series of well-defined polymers with tunable structural parameters to modulate the microenvironment of PECs and clearly revealed the importance of local polarity and polyelectrolyte composition on PECs.