In chain-growth polymerization, a chain grows continually to reach thousands of subunits. However, the real-time dynamics of chain growth remains unknown. Using magnetic tweezers, we visualized real-time polymer growth at the single-polymer level. Focusing on ring-opening metathesis polymerization, we found that the extension of a growing polymer under a pulling force does not increase continuously but exhibits wait-and-jump steps. These steps are attributable to the formation and unraveling of conformational entanglements from newly incorporated monomers, whose key features can be recapitulated with molecular dynamics simulations. The configurations of these entanglements appear to play a key role in determining the polymerization rates and the dispersion among individual polymers.

The tensile response to uniaxial deformation of polyethylene-based (Tetra-PE) and polyethylene glycol-based (Tetra-PEG) networks of various strand lengths with idealized diamond connectivity have been studied via atomistic molecular dynamics simulations. Tetra-PE and Tetra-PEG diamond networks with the same strand length show comparable maximum extensibility but the Young's moduli and tensile strength of the former are significantly lower than those of the latter, consistent with stronger intersegmental attractions in the amorphous Tetra-PEG networks. The stress-strain curves show that the stress in short-stranded networks increased rapidly and monotonically with strain while for long-stranded networks it increased very little at small strain, in a non-monotonic fashion at intermediate strains, and then very sharply as the limit of extensibility was approached. Spontaneous partial crystallization of a long-stranded Tetra-PE diamond network under supercooling was demonstrated, and the resulting system was used to: (1) Estimate its melting point as the temperature where any crystalline material disappeared abruptly, and (2) show that the presence of crystalline material in the undeformed state leads to higher stress responses upon deformation compared to amorphous samples, a result consistent with experimental observations. The spontaneous crystallization of Tetra-PEG networks at large supercooling was unsuccessful due to the slow motions of the network beads and the prohibitively long crystal nucleation times entailed.iii BIOGRAPHICAL SKETCHThe author, Endian Wang, born in Fujian, China, attended his middle school and high school in He studied and enjoyed sketching and oil painting during middle school and high school, and would like to continue painting in the near future.iv ACKNOWLEDGEMENTS

Coarse‐grained (CG) implicit‐solvent potentials are developed for tetra‐polyethylene glycol (PEG) at different water concentrations using the iterative Boltzmann inversion (IBI) technique. The resulting potentials are used to study the swelling and tensile properties of tetra‐PEG gels at various swelling degrees φm. Two types of network topologies are considered, one “ideal” with a defect‐free diamond connectivity and the other “realistic” as simulated from an experimentally based cross‐linking process. Equilibrium swelling results for the realistic Tetra‐PEG networks are consistent with available experimental data, while those for the ideal tetra‐PEG networks exhibit much larger swelling. The realistic networks have higher Young's modulus Em at the same φm than ideal networks due to the presence of trapped entanglements. Uniaxial deformation results of realistic networks show that Em increases with degree of swelling, in accord with experimental results. The Young's moduli of gels at different φm confirm that the CG potentials developed by IBI are most suited to predict swelling states commensurate with the φm values at which the potentials were calibrated. A more generic, coarser potential, based on matching the persistence length of atomistic PEG chains in water, is able to produce a similar swelling behavior of an ideal diamond network.