Synthesizing New and Versatile Self-Immolative Polymers

One of the biggest environmental challenges that we face, is how to deal with the enormous accumulation of plastics (or synthetic polymers) in nature. Self-immolative polymers (SIPs) are an emerging class of degradable polymers that, upon removal of carefully designed end-caps, degrade from end-to-end to in a domino-like fashion. The main benefit of SIPs lies in the product obtained upon depolymerization, which consists of either monomer, or small molecules that can be converted to monomers, enabling recycling to virgin materials. This thesis is mainly concerned with SIPs and the synthesis of new versatile SIP backbones. In Chapter I the fundamental theory and the key concepts related to SIPs are introduced. The chapter gives an overview of degradable polymers in general, and then narrows down to describe the fundamental differences between degradable polymers and SIPs. A literature review reveals that the family of SIP backbones consists of five different scaffolds, which each possesses specific strengths and weaknesses related to cost, stability and toxicity. These issues are improved upon in this thesis, which presents the research conducted through 4 years of PhD work that has led to the development of two new SIP backbones, namely poly(DL-dithiothreitol) (pDTT) and poly(isocyanate) (pIC), presented in Chapter II and III respectively. pDTT is a novel poly(disulfide) polymer, produced from a monomer, DL-dithiothreitol (DTT), which under conventional polymerization protocols fails to produce anything but small molecules. Remarkably, the synthesis of pDTT was found to proceed in a simple fashion consisting of mechanical mixing of two solids, i.e. the monomer, DTT, and an activating agent, 2,2’-dithiodipyridine, for a few minutes. pDTT is thermally and chemically robust, however, once the terminal end-caps are removed, it depolymerizes smoothly within minutes, forming cyclic DTT through intramolecular thiol-disulfide exchange reactions from one end to the other. All constituents in pDTT were shown to be recyclable to starting materials using simple redox chemistry highlighting the strength of SIPs for recycling. Compared to other SIPs, pDTT stands out because of the simplicity and speed of all processing steps, and the high tolerance towards air, moisture, and impurities. Polymers from isocyanate monomers have been known for 60 years. However, their potential use as self-immolative polymers have never been studied. Proof-of-concept monomer, hexyl isocyanate, is polymerizable through an anionic addition polymerization reaction to poly(hexyl isocyanate) (pHIC), in a scalable synthesis with control of molecular weight, in up to hundred thousands of Daltons. pHIC is a durable plastic that, upon end-cap removal and exposure to a basic media, depolymerizes, even in its solid state. The strength of the pIC, compared to other SIPs, is the amount of available isocyanate monomers, either from commercial sources, or synthesized through a single organic chemical reaction step, from even simpler starting materials. This way the properties of pICs are interchangeable while its depolymerization path is conserved. A small library of 9 different commercially available isocyanates were investigated as homo- or co-polymers. Once triggered, they depolymerize selectively, in both solution and solid state. A comparison between the five existing SIP backbones and the two new backbones introduced in this thesis is shown in an extensive table in Chapter IV, which compiles synthesis parameters, molecular weights, stabilities, reaction times, depolymerization times and conditions, and a short overview of the application that each scaffold has been utilized in. In Chapter V, research conducted in an industrial collaboration aims towards solutions for dismantling fiber reinforced epoxy composites. The collaboration demands cheaper alternatives than a SIP can provide, thus fundamental research into surface chemistry was conducted with the aim of reaching smooth on-demand separation between a carbonaceous surface and a cured epoxy component. This is attempted, first, with conventional surface attached polymers (polymer brushes) and afterwards with SIP brushes.