The ubiquity of polymeric materials in daily life comes with an increased fire risk, and sustained research into efficient flame retardants is key to ensuring the safety of the populace and material goods from accidental fires. Phosphorus, a versatile and effective element for use in flame retardants, has the potential to supersede the halogenated variants that are still widely used today: current formulations employ a variety of modes of action and methods of implementation, as additives or as reactants, to solve the task of developing flame‐retarding polymeric materials. Phosphorus‐based flame retardants can act in both the gas and condensed phase during a fire. This Review investigates how current phosphorus chemistry helps in reducing the flammability of polymers, and addresses the future of sustainable, efficient, and safe phosphorus‐based flame‐retardants from renewable sources.

Novel phosphorus-based hyperbranched polymers provide multifunctional flame retardancy to epoxy resins.

Flame retardants (FR) are inevitable additives to many plastics. Halogenated organics are effective FRs but are controversially discussed due to the release of toxic gases during a fire or their persistence if landfilled. Phosphorus-containing compounds are effective alternatives to halogenated FRs and have potential lower toxicity and degradability. In addition, nitrogen-containing additives were reported to induce synergistic effects with phosphorus-based FRs. However, no systematic study of the gradual variation on a single phosphorus FR containing both P–O and P–N moieties and their comparison to the respective blends of phosphates and phosphoramides was reported. This study developed general design principles for P–O- and P–N-based FRs and will help to design effective FRs for various polymers. We synthesized a library of phosphorus FRs that only differ in their P-binding pattern from each other and studied their decomposition mechanism in epoxy resins. Systematic control over the decomposition pathways of phosphate (PO­(OR)3), phosphoramidate (PO­(OR)2(NHR)), phosphorodiamidate (PO­(OR)­(NHR)2), phosphoramide (PO­(NHR)3), and their blends was identified, for example, by reducing cis-elimination and the formation of P–N-rich char with increasing nitrogen content in the P-binding sphere. Our FR epoxy resins can compete with commercial FRs in most cases, but we proved that the blending of esters and amides outperformed the single-molecule amidates/diamidates due to distinctively different decomposition mechanisms acting synergistically when blended.

Competitive copolymerization gives access to gradient copolymers with simple one-step and one-pot strategies. Due to the living nature of the sulfonyl-aziridine polymerization, gradient copolymers can be obtained with low dispersities and adjustable molar masses. The combination of different sulfonyl activating groups allowed to fine-tune the reactivity difference of the comonomers and thus an exact adjustment of the gradient strength. Sulfonyl-activated aziridines are to date the only monomer class providing access to gradient copolymers with reactivity ratios ranging from (1 ≤ r 1 ≤ 2; 1 ≥ r 2 ≥ 0.5) for statistical or soft gradient copolymers to block copolymers (r 1 ≥ 20, r 2 ≤ 0.02), only by adjusting the electron-withdrawing effect of the activation groups: the reactivity ratios were calculated by different models for a library of eight comonomers. This library was further used to classify between hard, medium, and soft gradients. From the data obtained from the monomer library, it was possible to predict polymerization rate coefficients (k p ) for aziridines, which were not prepared so far: correlation of the shifts in the 13 C NMR spectra, the Hammett parameters and secondary parameters such as calculated lowest unoccupied molecular orbital (LUMO) levels of the monomers and the natural charge at the electrophilic carbon, etc., were used to predict (co)monomer reactivity and the resulting gradient strength. We believe that our findings allow us to access tailored gradient copolymers with a controlled monomer sequence distribution depending on the chemical control of comonomer reactivity. With these systematic data on activated aziridines, also more complex copolymer structures can be predicted and prepared. Such materials might find application as linear polyethylenimine derivatives to act as functional polyelectrolytes, or predesigned compatibilizers and surface-active gradient copolymers by a predictable one-step copolymerization.

Reactive poly(phosphoester)s (PPEs) have been prepared via the acyclic diene metathesis polymerization of the monomers di(buten-3-yl) chlorophosphate and di(undecen-10-yl) chlorophosphate. Molecular weights can be adjusted from 3000 to ca. 50 000 g/mol and have been prepared and characterized in detail. This is the first report on olefin metathesis polymerization of highly electrophilic phosphochlorides, which were postmodified with different nucleophiles, i.e., alcohols, amines, and water, thus allowing the synthesis of side chain polyphosphoamidates, poly(phosphoester)s, and free acids from the same starting polymer. High side-chain functionality was found in all cases.

We report orthogonal ambipolar semiconductors that exhibit hole and electron transport in perpendicular directions based on aligned films of nanocrystalline "shish-kebabs" containing poly(3-hexylthiophene) (P3HT) and N,N'-di-n-octyl-3,4,9,10-perylenetetracarboxylic diimide (PDI) as p- and n-type components, respectively. Polarized optical microscopy, scanning electron microscopy, and X-ray diffraction measurements reveal a high degree of in-plane alignment. Relying on the orientation of interdigitated electrodes to enable efficient charge transport from either the respective p- or n-channel materials, we demonstrate semiconductor films with high anisotropy in the sign of charge carriers. Films of these aligned crystalline semiconductors were used to fabricate complementary inverter devices, which exhibited good switching behavior and a high noise margin of 80% of 1/2 Vdd. Moreover, complementary "NAND" and "NOR" logic gates were fabricated and found to exhibit excellent voltage transfer characteristics and low static power consumption. The ability to optimize the performance of these devices, simply by adjusting the solution concentrations of P3HT and PDI, makes this a simple and versatile method for preparing ambipolar organic semiconductor devices and high-performance logic gates. Further, we demonstrate that this method can also be applied to mixtures of PDI with another conjugated polymer, poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene]) (PBTTT), with better hole transport characteristics than P3HT, opening the door to orthogonal ambipolar semiconductors with higher performance.

Vitrimers are a promising alternative to conventional composite materials as they can be recycled and reshaped but still need additives. Herein, intrinsic flame-retardant phosphorus-containing vitrimers are presented, which were used in composites.

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