Real-time 31P NMR reveals different gradient strengths in polyphosphoester copolymers as potential MRI-traceable nanomaterials

Polyphosphoesters (PPEs) are a class of polymers with diverse applications spanning biomedicine (drug delivery, tissue engineering) and materials science (flame retardants, electrolytes). While primarily studied as homopolymers, the copolymerization of different PPE subclasses remains largely unexplored, particularly regarding reactivity ratios and resulting copolymer structures. This gap is significant as copolymerization offers a route to tailor polymer properties such as solubility, degradation, and thermal/mechanical characteristics by combining the features of different PPE subclasses. Previous work has primarily focused on phosphates, with limited exploration of other subclasses like phosphonates and thiophosphates. Ring-opening polymerization (ROP) offers a route to well-defined PPEs, and organocatalysis offers advantages over metal-catalyzed ROP by avoiding potentially harmful metallic residues. This research aims to systematically investigate the organocatalyzed copolymerization of four PPE subclasses, determine their reactivity ratios, characterize the resulting copolymer microstructures (gradient, random, block), and assess their potential as MRI-traceable nanomaterials for biomedical applications. This comprehensive study addresses the need for a deeper understanding of PPE copolymerization to facilitate the design of tailored materials for advanced applications.

The literature on PPEs highlights their versatility and potential. Various synthetic routes, including polycondensation, olefin metathesis, and ROP, have been employed. ROP, particularly with organocatalysts like DBU, is preferred for its ability to produce well-defined polymers without metallic residues. Previous studies have reported on the organocatalyzed ROP of individual PPE subclasses, but the copolymerization of different subclasses has received scant attention. The reactivity ratios of various cyclic phospholanes have not been thoroughly characterized, which is crucial for understanding and controlling the copolymer microstructure. While some work exists on copolymerization with other monomers (lactides, lactones, carbonates), a systematic study of the copolymerization behavior of diverse PPE subclasses is lacking. This study builds upon existing knowledge by systematically investigating the copolymerization of multiple PPE subclasses to generate a library of copolymers with controllable microstructures and properties.

The researchers synthesized binary and ternary PPE copolymers using four different cyclic comonomers representing distinct PPE subclasses: side-chain phosphonates, phosphates, thiophosphates, and in-chain phosphonates. Organocatalyzed ring-opening copolymerization was employed, utilizing DBU (for side-chain phosphonates) and a combination of DBU/TU or DBU/TrisUrea (for other subclasses) to optimize reaction kinetics and prevent transesterification. Copolymerizations were performed in dichloromethane at -10 °C directly within an NMR tube inside the spectrometer to enable real-time 1H/31P NMR monitoring. This real-time monitoring provided kinetic data for determining reactivity ratios. Different nonterminal models (Jaacks, BSL, Frey) were used to analyze the kinetic data and calculate reactivity ratios for both binary and ternary copolymers. The obtained reactivity ratios, along with Monte Carlo simulations, allowed for the determination of copolymer microstructure (gradient strength, blockiness). The amphiphilic properties of the copolymers were investigated using interfacial tension measurements (spinning drop technique). Self-assembly in aqueous solutions was characterized by dynamic light scattering (DLS). Finally, the potential of these copolymers as MRI-traceable nanomaterials was evaluated by measuring NMR relaxation times (T1 and T2) and performing 31P MRI imaging.

The study revealed a wide range of reactivity ratios (0.02 to 44) depending on the comonomer combination and organocatalyst. Copolymerization of phosphates and phosphonates yielded gradient copolymers with varying gradient strengths, classified as soft, medium, or hard gradients based on the difference in reactivity ratios. In-chain polyphosphonates and polythiophosphates, known for slower homopolymerization, resulted in block-like structures. The electron density at the phosphorus atom, along with ring strain, influenced the reactivity and resulting microstructure. Ternary copolymerization with different catalysts allowed for control over the block order and microstructure, producing amphiphilic structures resembling triblock terpolymers. Interfacial tension measurements demonstrated the amphiphilicity of the copolymers, with lower surface tensions observed for copolymers containing more hydrophobic segments. Dynamic light scattering showed self-assembly in aqueous solutions, forming nanostructures with diameters ranging from 5 nm to 320 nm. NMR relaxation time measurements indicated that T1 and T2 relaxation times could be tuned by adjusting the gradient strength, leading to favorable MRI properties. 31P MRI phantom images confirmed the potential of these copolymers as MRI-traceable nanomaterials with signal-to-noise ratios as high as 171.4 for some formulations.

The findings demonstrate the successful synthesis of a library of amphiphilic PPE copolymers with controllable microstructures via organocatalyzed ROP. The ability to tune the gradient strength by selecting comonomers and catalysts offers a powerful tool for tailoring polymer properties. The self-assembly behavior and the correlation between microstructure and NMR relaxation times highlight the potential of these materials as MRI-traceable nanocarriers. The high signal-to-noise ratios observed in the MRI phantom images suggest their suitability for biomedical imaging applications. This work expands the scope of PPEs beyond their existing applications and provides a foundation for developing advanced theranostic materials.

This study successfully demonstrated the synthesis of a diverse library of amphiphilic and potentially biodegradable PPE copolymers via organocatalyzed ring-opening copolymerization. Real-time 31P NMR kinetics were used to determine reactivity ratios and copolymer microstructures. The resulting gradient copolymers self-assembled into nanostructures suitable for MRI tracing, expanding the potential applications of PPEs in drug delivery and theranostics. Future research will focus on investigating the use of these copolymers for implant coatings and drug encapsulation, further exploring their potential in biomedical applications.

The study primarily focuses on in vitro characterization. Further in vivo studies are needed to fully assess the biocompatibility and efficacy of these copolymers. The NMR measurements were conducted in a controlled environment; therefore, additional investigation is needed to see if the results translate to more complex biological environments. Also, the copolymerization was conducted without stirring, potentially affecting molar mass dispersity and microstructure.