The case for poly(ylides) as a class of charge-neutral, hydrophilic polymers with applications in biomaterials science.

Many applications of biomaterials require hydrophilic polymers as building blocks, including hydrogels and nanomedicinal devices. Besides enabling sufficient swelling properties in aqueous environments, hydrophilic polymers provide hydration layers, which are considered a major requirement when designing non-fouling surfaces and materials. For the last few decades, polyethylene glycol has been seen as the gold standard for such applications. However, reports on its stability and immunogenicity have urged chemists to identify alternatives with comparable or superior properties. In addition to biopolymers, zwitterionic polymers have gained increasing attention by effectively offering an overall charge-neutral scaffold capable of forming strong hydration layers. Driven by an enhanced understanding of the structure-property relationship of zwitterionic materials, poly(ylides) have emerged as a new class of hydrophilic and charge-neutral polymers. By having the negative charge adjacent to the positive charge, ylides offer not only a minimal dipole moment but also maintain their overall charge-neutral nature. Despite some early reports on their synthesis during the 1980s, polymeric ylides were largely overlooked as a class of polymers, and their utility as unique hydrophilic building blocks for the design of biomaterials and nanomedicinal tools remained elusive. In recent years, several groups have reported -oxide and carbon-centered ylide-based polymers as highly effective building blocks for the design of antifouling materials and nanomedicines. Here, by reviewing recent progress and understanding of structure-property relationships, arguments are provided explaining why polymeric ylides should be classified as a standalone class of hydrophilic polymers. Consequently, the author concludes that the term 'poly(ylide)' or 'polymeric ylides' should be routinely used to adequately describe this emerging class of polymers.