Designing delivery agents for therapeutics is an ongoing challenge. As treatments and desired cargoes become more complex, the need for improved delivery vehicles becomes critical. Excellent delivery vehicles must ensure the stability of the cargo, maintain the cargo’s solubility, and promote efficient delivery and release. In order to address these issues, many research groups have looked to nature for design inspiration. Proteins, such as HIV-1 TAT and Antennapedia homeodomain protein, are capable of crossing cellular membranes. However, due to the complexities of their structures, they are synthetically challenging to reproduce in the laboratory setting. Being able to incorporate the key features of these proteins that enable cell entry into simpler scaffolds opens up a wide range of opportunities for the development of new delivery reagents with improved performance. Herein we report the development of guanidinium-rich polymeric protein mimics using a ring-opening metathesis polymerization (ROMP)-based scaffold capable of interacting with cell membranes and facilitating the internalization of small interfering ribonucleic acids (siRNAs). These materials are referred to interchangeably as cell-penetrating peptide mimics (CPPMs) or protein transduction domain mimics (PTDMs), and derive inspiration from proteins and peptides with cellular internalization capabilities, capturing key features of these materials necessary for intracellular delivery, including cationic charge content in the form of guanidinium moieties and a segregated, hydrophobic component. This thesis documents the development of design principles for PTDMs with optimal membrane interactions and siRNA internalization and delivery. Chapter 2 documents the development of homopolymer CPPMs that contain aromatic rings with varying π-electronics. This study demonstrated that a wide range of functional groups could be incorporated into CPPMs without negatively impacting their ability to interact with cellular membranes. It is also suggested that other design parameters, such as cationic charge content and overall hydrophobic content, play more dominant roles in membrane interactions. This finding ultimately influenced the PTDM optimization performed in later chapters. Chapter 3 documents the development of homopolymer and block copolymer PTDMs with varying numbers of guanidinium moieties that were tested to assess the affect cationic charge content and the addition of a segregated, hydrophobic block had on siRNA delivery. This study demonstrated that there was a critical charge content necessary for internalization and established the importance of incorporating a hydrophobic block into PTDM structures. Furthermore, this platform demonstrated that bioactive siRNA could successfully be delivered into cells and affect the target gene. Chapter 4 documents the exploration of hydrophobic block incorporated into copolymer PTDMs in order to determine how the length of the hydrophobic block of the PTDMs as well as the hydrophobic block composition of the PTDMs impacted siRNA internalization. This study demonstrated that there was a critical hydrophobic content necessary for efficient siRNA internalization and that incorporation of additional hydrophobicity did not guarantee improved efficiencies.