# Nucleobase-Containing Polymers: Structure, Synthesis, and Applications
## 3 Synthetic Nucleobase-Containing Polymers
### Synthesizing Nucleobase Polymers by ROMP  
As a variation of the olefin metathesis reaction, ring-opening metathesis polymerization (ROMP) [92; 93] represents another widely used method to synthesize polymer materials with  
Figure 5: Sequence-controlled hetero-functional, topological polymers (R\({}_{1}\) and R\({}_{2}\) denote two different functional groups and S represents a thioether bond, which can be transformed into a sulfone bond by oxidation) and the synthetic route to a DNA-mimetic nucleobase-containing polymer [39]. Reproduced with permission. Copyright Springer Nature, 2014. Figure 6: Schematic illustration of water-soluble CNA polymer synthesis [40]. Reproduced with permission. Copyright Royal Society of Chemistry, 2017.  
tunable sizes, shapes, and functions, especially in the preparation of nucleobase-containing polymers. [PERSON] et al. [PERSON] et al. (2017) first reported the preparation of \"DNA-mimetic\" homopolymers and block copolymers through controlled ring-opening metathesis polymerization of non-covalently protected monomers as early as 2002. Notably, an adenine-containing monomer was \"protected\" by succinimide, based on the hydrogen bonding between succinimide and the adenine moiety. Additionally, this simple introduction of succinimide increased the solubility of the resulting polymer, which prevents resultant polymers from precipitating before the reaction is complete. These nucleobase-containing block copolymers were produced by following the same ROMP procedure. Finally, the self-assembly of copolymers (\(m=50\), \(n=5\); shown in red in Figure 7) was investigated in THF solution and the results revealed the ability of these hydrogen-bonding-enabled self-complementary block copolymers to self-assemble into novel nanoscale rod morphologies. After that, a similar ROMP strategy was utilized by [PERSON] et al. [PERSON] et al. (2016) in the construction of a thymine-containing block copolymer with narrow molecular weight distribution. As shown in Scheme 6, adenine-containing monomers were aligned with hydrogen-bonding interactions to the thymine template, and subsequently underwent Sonogashira polymerization, leading to the templated synthesis of a conjugated polymer. The daughter strand is found to possess a narrow molecular weight distribution and a chain length nearly equivalent to that of the parent template, indicating that the daughter strand could copy the length and distribution information from the template strand. However, non-templated polymerization or polymerization with the incorrect template generated a short conjugated oligomer with a significantly broader molecular weight distribution. Peptide nucleic acids (PNAs) are synthetic DNA mimetics whose backbones are electrical neutral amide bonds, which contribute high stability towards nuclease degradation and high affinity towards complementary DNA/RNA strands [PERSON] et al. (2012); [PERSON] et al. (2012); [PERSON] et al. (2012). [PERSON] et al. (2012) reported a preparation of polymeric nucleic acids wherein single-stranded sequences of PNAs are incorporated as polymer brushes via graft-through polymerization using the ROMP initiator \"Ru\". This work clearly demonstrated the power of ROMP in preparing amphiphilic brush copolymers by graft-through polymerization of a relatively large side chain: oligonucleotides (Figure 8). This approach of combining PNAs and ROMP could provide an efficient strategy for the incorporation of nucleic acids into particles and polymer-based materials, which might be useful for various applications such as affinity purification  
Figure 7: **(A)** ROMP with noncovalently protected monomer; **(B)** preparation of block copolymers via ROMP [PERSON] et al. (2017). Reproduced with permission. Copyright American Chemical Society, 2006.  
of DNA, gene delivery, and programmable materials.