New insights into poly(lactic-co-glycolic acid) microstructure: using repeating sequence copolymers to decipher complex NMR and thermal behavior

Consequences of isolated critical monomer sequence errors for the hydrolysis behaviors of sequenced degradable polyesters

Despite the known sensitivity to sequence mutations of biological polymers, little is known about the effects of errors in sequenced synthetic copolymers.

Stereoselective Synthesis of Homofarnesenes: Establishment of an Efficient Method for the Stereoselective Preparation of Acyclic Tetrasubstituted Olefins

A new method for the stereoselective synthesis of tetrasubstituted olefins is described. β-Ketophosphonates are alkylated via conventional methods, and a Grignard reagent is used to diastereoselectively add to the carbonyl group of the resulting intermediates. The elimination of hydroxyl phosphonates yielded the desired tetrasubstituted olefins in a stereoselective manner. Thus, homofarnesenes of fire ant trail pheromones have been synthesized efficiently using this strategy.

Precision Aliphatic Polyesters via Segmer Assembly Polymerization

Precise structure-property relation of a biodegradable polymer (e.g., aliphatic polyester) is anticipated only if monomer units and chiral centers are arranged in a defined primary sequence as a biomacromolecule. An emerging synthetic methodology, namely segmer assembly polymerization (SAP), is introduced in this paper to reveal the latest progress in polyester synthesis. Almost any periodic polyester envisioned can be synthesized via SAP using a programed linear or cyclic monomer. In this context, the macroscopic properties of a biodegradable polymer are fundamentally determined by microstructural information through a bottom-up approach. It can be highlighted that SAP ideally combines the precision of organic synthesis and the high efficiency of a polymerization reaction. Previously reported strategies including nucleophilic displacement, polyesterification, cross-metathesis polymerization (CMP), ring-opening polymerization (ROP), ring-opening metathesis polymerization (ROMP) and entropy-driven ring-opening metathesis polymerization (ED-ROMP) are critically reviewed in this paper to shed light on precision synthesis of aliphatic polyesters via SAP. Emerging yet challenging, SAP is a paradigm which reflects the convergence of organic and polymer chemistries and is also an efficient pathway to microstructural control. The current status, future challenges and promising trends in this realm are analyzed and discussed in this overview of the state-of-the-art.

Synthesis, properties, and crystallization of the alternating stereocopolymer poly(l-lactic acid-alt-d-lactic acid) [syndiotactic poly(lactic acid)] and its blend with isotactic poly(lactic acid)

The alternating stereocopolymer poly(l-lactic acid-alt-d-lactic acid) or syndiotactic poly(lactic acid) was successfully synthesized by the condensation of the l-lactic acid-d-lactic acid dimer.

Monomer sequence in PLGA microparticles: Effects on acidic microclimates and in vivo inflammatory response

Controlling the backbone architecture of poly(lactic-co-glycolic acid)s (PLGAs) is demonstrated to have a strong influence on the production and release of acidic degradation by-products in microparticle matrices. Previous efforts for controlling the internal and external accumulation of acidity for PLGA microparticles have focused on the addition of excipients including neutralization and anti-inflammatory agents. In this report, we utilize a sequence-control strategy to tailor the microstructure of PLGA. The internal acidic microclimate distributions within sequence-defined and random PLGA microparticles were monitored in vitro using a non-invasive ratiometric two-photon microscopy (TPM) methodology. Sequence-defined PLGAs were found to have minimal changes in pH distribution and lower amounts of percolating acidic by-products. A parallel scanning electron microscopy study further linked external morphological events to internal degradation-induced structural changes. The properties of the sequenced and random copolymers characterized in vitro translated to differences in in vivo behavior. The sequence alternating copolymer, poly LG, had lower granulomatous foreign-body reactions compared to random racemic PLGA with a 50:50 ratio of lactic to glycolic acid.

STATEMENT OF SIGNIFICANCE

This paper demonstrates that changing the monomer sequence in poly(lactic-co-glycolic acid)s (PLGAs) leads to dramatic differences in the rate of degradation and the internal acidic microclimate of microparticles degrading in vitro. We note that the acidic microclimates within these particles were imaged for the first time with two-photon microscopy, which gives an extremely clear and detailed picture of the degradation process. Importantly, we also document that the observed sequence-controlled in vitro processes translate into differences in the in vivo behavior of polymers which have the same L to G composition but differing microstructures. These data, placed in the context of our prior studies on swelling, erosion, and MW loss (Biomaterials2017, 117, 66 and other references cited within the manuscript), provide significant insight not only about sequence effects in PLGAs but into the underlying mechanisms of PLGA degradation in general.

Defining the Field of Sequence-Controlled Polymers

Over the last ten years, the development of synthetic polymers containing controlled monomer sequences has become a prominent topic in fundamental and applied polymer science. This emerging area is particularly broad and combines classical polymer chemistry tools with techniques imported from other domains such as biology, biochemistry, organic synthesis, engineering, and bioanalytics. Consequently, it also generates new structures, terminologies, and applications that are not within the traditional scope of polymer science. The term "sequence-controlled polymers" (SCPs) was recently proposed as a generic name to describe all these recent trends. However, since the field of SCPs has been growing very rapidly in recent literature, it is urgent to accurately define its scientific frontiers. In this important context, this review is an attempt to define, rationalize, and classify the field of SCPs. In particular, all synthetic approaches that have been reported for the synthesis of SCPs are discussed and categorized. In addition, the characterization tools, properties, and potential applications of these new polymers are described herein. Overall, this review serves as a reference guide for understanding the burgeoning field of SCPs.

The impact of monomer sequence and stereochemistry on the swelling and erosion of biodegradable poly(lactic-co-glycolic acid) matrices

Monomer sequence is demonstrated to be a primary factor in determining the hydrolytic degradation profile of poly(lactic-co-glycolic acid)s (PLGAs). Although many approaches have been used to tune the degradation of PLGAs, little effort has been expended in exploring the sequence-control strategy exploited by nature in biopolymers. Cylindrical matrices and films prepared from a series of sequenced and random PLGAs were subjected to hydrolysis in a pH 7.4 buffer at 37 °C. Swelling ranged from 107% for the random racemic PLGA with a 50:50 ratio of lactic (L) to glycolic (G) units to 6% for the sequenced alternating copolymer poly LG. Erosion followed an inverse trend with the random 50:50 PLGA showing an erosion half-life of 3-4 weeks while poly LG required ca. >10 weeks. Stereosequence was found to play a large role in determining swelling and erosion; stereopure analogs swelled less and were slower to lose mass. Molecular weight loss followed similar trends and increases in dispersity correlated with the onset of significant swelling. The relative proportion of rapidly cleavable G-G linkages relative to G-L/L-G (moderate) and L-L (slow) correlates strongly with the degree of swelling observed and the rate of erosion. The dramatic sequence-dependent variation in swelling, in the absence of a parallel hydrophilicity trend, suggest that osmotic pressure, driven by the differential accumulation of degradation products, plays an important role.

The Next Generation of Platinum Drugs: Targeted Pt(II) Agents, Nanoparticle Delivery, and Pt(IV) Prodrugs

The platinum drugs, cisplatin, carboplatin, and oxaliplatin, prevail in the treatment of cancer, but new platinum agents have been very slow to enter the clinic. Recently, however, there has been a surge of activity, based on a great deal of mechanistic information, aimed at developing nonclassical platinum complexes that operate via mechanisms of action distinct from those of the approved drugs. The use of nanodelivery devices has also grown, and many different strategies have been explored to incorporate platinum warheads into nanomedicine constructs. In this Review, we discuss these efforts to create the next generation of platinum anticancer drugs. The introduction provides the reader with a brief overview of the use, development, and mechanism of action of the approved platinum drugs to provide the context in which more recent research has flourished. We then describe approaches that explore nonclassical platinum(II) complexes with trans geometry or with a monofunctional coordination mode, polynuclear platinum(II) compounds, platinum(IV) prodrugs, dual-threat agents, and photoactivatable platinum(IV) complexes. Nanoparticles designed to deliver platinum(IV) complexes will also be discussed, including carbon nanotubes, carbon nanoparticles, gold nanoparticles, quantum dots, upconversion nanoparticles, and polymeric micelles. Additional nanoformulations, including supramolecular self-assembled structures, proteins, peptides, metal-organic frameworks, and coordination polymers, will then be described. Finally, the significant clinical progress made by nanoparticle formulations of platinum(II) agents will be reviewed. We anticipate that such a synthesis of disparate research efforts will not only help to generate new drug development ideas and strategies, but also will reflect our optimism that the next generation of approved platinum cancer drugs is about to arrive.

Group interval-controlled polymers: an example of epoxy functional polymers via step-growth thiol–yne polymerization

Here, we successfully synthesized a series of epoxy GICPs via one-step UV-triggered thiol–yne polymerization of commercial glycidyl propargyl ether and dithiols at 0 °C..

A coacervate-forming biodegradable polyester with elevated LCST based on bis-(2-methoxyethyl)amine

This work details a thermoresponsive, biodegradable coacervate-forming polyester, which exhibits tunable T_cp under various conditions.

Sequence-Controlled Copolymers Prepared via Entropy-Driven Ring-Opening Metathesis Polymerization

A new general synthetic approach to sequenced macromolecules was developed and applied to the synthesis of polymers comprising lactic acid (L), glycolic acid (G), and ε-caprolactone (C)-derived monomer units. The new method employs entropy-driven ring-opening metathesis polymerization (ED-ROMP) to prepare copolymers with embedded sequences and controlled molecular weights. Cyclic macromonomer precursors were prepared by ring-closing metathesis of ethylene glycol (Eg)-linked sequenced oligomers bearing terminal olefins. ED-ROMP of the resulting macrocycles using Grubbs' second generation catalyst yielded poly(CL-Eg-LC-Oed), poly(CLL-Eg-LLC-Oed), poly(LGL-Eg-LGL-Oed), and poly(LGL-Eg-LGL-Hed) (Oed = octenedioc acid; Hed = hexenedioc acid). Hydrogenation produced the saturated sequenced copolymers. Molecular weight was well-controlled and could be adjusted by varying the monomer-to-catalyst ratio. M_ns of 26-60 kDa were obtained (dispersities = 1.1-1.3). The methodology proved general for three different sequences and two olefinic metathesis groups.

Monomer Sequence Regulation in Main and Side Chains of Vinyl Copolymers: Synthesis of Vinyl Oligomonomers via Sequential Atom Transfer Radical Addition and Their Alternating Radical Copolymerization

We propose a novel strategy of monomer sequence regulation in main and side chains of vinyl copolymers using sequential atom transfer radical addition (ATRA) for maleimide-ended sequence-regulated vinyl "oligomonomers" and their alternating radical copolymerization with styrene. To establish this strategy, a series of sequence-regulated vinyl oligomers were prepared by the ATRA of styrene (S) or methyl acrylate (A) to a halide-possessing A or S unit (methyl α-bromopropionate or 1-phenylethylhalide). The obtained halide-ended sequence-regulated vinyl oligomers were converted into maleimide-ended oligomonomers by S_N2 reaction with the potassium salt of furan-protected maleimide (M) followed by deprotection. The maleimide-ended oligomonomers were then radically copolymerized with styrene in an alternating fashion to result in sequence-regulated vinyl copolymers consisting of a controlled monomer sequence (SA, AS, AA, and SS) in the side chain and an alternating sequence (MS) in the main chain. The solubility of the copolymers depended on not only the monomer composition but also the sequence of the side chains, whereas the thermal properties were negligibly affected by the side-chain monomer sequences.

Writing on polymer chains

Synthetic polymer materials are currently limited by their inability to store information in their chains, unlike some well-characterized biopolymers. Nucleic acids store and transmit genetic information, and amino acids encode the complex tridimensional structures and functions within proteins. To confer similar properties on synthetic materials, researchers must develop"writing" mechanisms, facile chemical pathways that allow control over the primary structure of synthetic polymer chains. The most obvious way to control the primary structure is to connect monomer units one-by-one in a given order using iterative chemistry. Although such synthesis strategies are commonly used to produce peptides and nucleic acids, they produce limited yields and are much slower than natural polymerization mechanisms. An alternative strategy would be to use multiblock copolymers with blocks that have specified sequences. In this case, however, the basic storage element is not a single molecular unit, but a longer block composed of several repeating units. However, the synthesis of multiblock copolymers is long and tedious. Therefore, researchers will need to develop other strategies for writing information onto polymer chains. In this Account, I describe our recent progress in the development of sequence controlled polymerization methods. Although our research focuses on different strategies, we have emphasized sequence-regulation in chain-growth polymerization processes. Chain-growth polymerizations, particularly radical polymerization, are very convenient methods for synthesizing polymers. However, in most cases, such approaches do not lead to controlled monomer sequences. During the last five years, we have shown that controlled/living chain-growth polymerization mechanisms offer interesting advantages for sequence regulation. In such mechanisms, the chains form gradually over time, and therefore the primary structure can be tuned by using time-controlled monomer additions. For example, the addition of small amounts of acceptor comonomers, such as N-substituted maleimides, during the controlled radical polymerization of a large excess of donor monomer, such as styrene, allows the writing of information onto polymer chains in a robust manner. Even with these advances, this strategy is not perfect and presents some of the drawbacks of chain-growth polymerizations, such as the formation of chain-to-chain sequence defects. On the other hand, this approach is experimentally easy, rapid, scalable, and very versatile.

A new generation of poly(lactide/ε-caprolactone) polymeric biomaterials for application in the medical field

Thermoplastic biodegradable polymers displaying an elastomeric behavior are greatly valued for the regeneration of soft tissues and for various medical devices. In this work, terpolymers composed of ε-caprolactone (CL), D-lactide (D-LA), and L-lactide (L-LA) were synthesized. These poly(lactide-ε-caprolactone) (PLCLs) presented an elevated randomness character (R∼1), glass transition temperatures (Tg ) higher than 20°C and adjusted L-LA content. In this way, the L-LA average sequence length (/L-LA ) was reduced to below 3.62 and showed little or no crystallization capability during in vitro degradation. As a result, the obtained materials underwent homogenous degradation exhibiting KMw ranging from 0.030 to 0.066 d(-1) and without generation of crystalline remnants in advanced stages of degradation. Mechanical performance was maintained over a period of 21 days for a rac-lactide-ε-caprolactone copolymer composed of ∼85% D,L-LA and ∼15% CL and also for a terpolymer composed of ∼72% L-LA, ∼12% D-LA and ∼16% CL. Terpolymers having L-LA content from ∼60 to 70% and CL content from ∼10 to 27% were also studied. In view of the results, those materials having CL and D-LA units disrupting the microstructural arrangement of the L-LA crystallizable chains, an L-LA content <72% and a random distribution of sequences, may display proper and tunable mechanical behavior and degradation performance for a large number of medical applications. Those with a CL content from 15 to 30% will fulfill the demand of elastomeric materials of Tg higher than 20°C whereas those with a CL content from 5 to 15% might be applied as ductile stiff materials.

Physicochemical properties and applications of poly(lactic-co-glycolic acid) for use in bone regeneration

Poly(lactic-co-glycolic acid) (PLGA) is the most often used synthetic polymer within the field of bone regeneration owing to its biocompatibility and biodegradability. As a consequence, a large number of medical devices comprising PLGA have been approved for clinical use in humans by the American Food and Drug Administration. As compared with the homopolymers of lactic acid poly(lactic acid) and poly(glycolic acid), the co-polymer PLGA is much more versatile with regard to the control over degradation rate. As a material for bone regeneration, the use of PLGA has been extensively studied for application and is included as either scaffolds, coatings, fibers, or micro- and nanospheres to meet various clinical requirements.