Controlled organocatalyzed d,l-lactide ring-opening polymerizations: synthesis of low molecular weight oligomers

A systematic approach to the synthesis of organocatalyzed oligo(d,l-lactide) demonstrates that choice of initiator, catalytic ratio, and reaction time yields well-controlled oligomers. Ring-opening polymerization of d,l-lactide with the initiator α-methyl propargyl alcohol, a secondary alcohol, used in excess of 4-dimethylaminopyridine catalyst mitigates cyclicization, transesterification, and catalyst-initiated side reactions. This approach enables the design of uniform lactide oligomers for controlled release applications, such as delivery systems for drugs, prodrugs, and molecular sensors.

A systematic approach to the synthesis of organocatalyzed oligo(d,l-lactide) demonstrates that choice of initiator, catalytic ratio, and reaction time yields well-controlled oligomers.

Poly(lactide) (PLA) is a versatile polymer with properties suitable for a range of controlled release applications. Molecular weight and copolymer composition control degradation and drug release, with degradation yielding biocompatible lactic acid products. PLA and copolymers of lactide and glycolide (PLGA) have been used pre-clinically and clinically for release of drugs,^1–3 prodrugs,^4–6 and molecular sensors.^7–9 Highly reproducible PLA chemistries are particularly important to ensure control over the release of drugs with narrow therapeutic windows and to enhance efficacy by reducing dependence on patient compliance.^10–13 Stannous octoate is commonly employed as an organometallic catalyst of lactide ring-opening polymerization (ROP).^14,15 However, tin catalysts, which are challenging to fully remove during purification, can result in toxicity.¹⁶ Alternatives such as strongly basic amine organocatalysts are favored, particularly 4-dimethylaminopyridine (DMAP), which was pioneered by Nederberg et al.¹⁷ DMAP-mediated ROP is used for one-pot PLA polymerizations and conjugations,¹⁸ diblock or triblock copolymerizations with lactide as a first or second block,^19–21 grafting lactide to cellulose polymer fibers,²² and synthesizing star-shaped/cross-linked PLA networks.²³ PLA has also been exploited to tether and release drugs from linear polymers and hydrogel depots.²⁴ Drug tethers are typically low molecular weight oligo(lactide) composed of fewer than seven lactide repeat units to avoid crystallinity and associated challenges with solubility and control over degradation rates.²⁵ Self-catalysis, or direct polycondensation, of lactic acid at increased temperature and reduced pressure for an extended time yields low-molecular weight (800–3200 Da) oligo(lactide), in contrast to organocatalysts that are commonly employed for high molecular weight PLA synthesis. However, extensive setup and environmental control decrease the accessibility of these reactions.^26,27 Furthermore, with neither an initiator nor a catalyst used in polycondensation, mixtures of α-hydroxy and ω-carboxy PLA are formed, limiting the versatility of post-polymerization drug functionalization.²⁸ Alternatively, click chemistry²⁹ may be exploited to control subsequent modification of oligo(lactide) by employing a ‘clickable’ alcohol to initiate oligo(lactide) synthesis.³⁰Here, we explored bulk ROP of d,l-lactide by propargyl alcohol initiator and DMAP catalyst to synthesize low molecular weight oligo(lactide) linkers. To enable subsequent click reactions and to mitigate crystallinity, propargyl alcohol (PA) and d,l-lactide (L) were used.²⁷ Although not studied here, similar approaches have shown products do not epimerize, and we expect oligomers to be atactic and amorphous.¹⁷ To investigate the molecular weight and polydispersity of oligo(lactide), an initial polymerization was designed similar to that of Nederberg et al.¹⁷ and Coulembier and Dubois¹⁸ using a PA : L : DMAP ratio of 1 : 20 : 4 (Scheme 1; see Table S1, ESI^†). The neat polymerization was stirred at 130 °C under a nitrogen environment. After 5, 10, 15, 30, and 60 minutes, reaction vials were opened to atmosphere and cooled before dissolution in dichloromethane (DCM) and precipitation in hexanes. ¹H-NMR spectroscopy identified successful synthesis of PA-functional oligo(d,l-lactide) (PA-ODLA) with ∼99% conversion of d,l-lactide after only 5 minutes of polymerization (Fig. 1a; see Fig. S3, ESI^†). Integration of peaks C, E, and F indicated linkers were ∼19 lactide units, or an average M_n of 2825 Da. However, matrix assisted laser desorption ionization time of flight mass spectrometry suggested M_n was 752 Da. As M_n determined by NMR was based on average end-group analysis and assumed PA-initiated oligo(lactide), and M_n determined by MALDI analysis represented all species present, these data indicated that not all oligo(lactide) chains were initiated by PA. Rather, distinct series of peaks periodically separated by 144 Da, the M_n of lactide, were formed during polymerization (α, β, γ, δ, ε, and ζ; Fig. 1b; see Table S2, ESI^†). Peaks 72 Da less than these peaks were also identified (α′–ζ′). Additional reactions were conducted to identify the formed products: one with the initiator α-methyl propargyl alcohol (αMPA; see Fig. S4, ESI^†), and another with only d,l-lactide and DMAP but no initiator (see Fig. S9, ESI^†).Open in a separate windowScheme 1Ring-opening polymerization of lactide by alcohol initiator. X = H, propargyl alcohol; CH₃, α-methyl propargyl alcohol.Open in a separate windowFig. 1Five minute polymerizations of 1 : 20 : 4 propargyl alcohol : lactide : 4-dimethylaminopyridine form varied products. (a) ¹H-NMR, with hydrogen peaks assignments shown. (b) MALDI-TOF, where colors represent peak sets that repeat every 72 Da and the inset shows a full 144 Da peak set.The desired product, PA-ODLA “α” (Fig. 1a), was formed via DMAP base activation of PA, which initiated ROP (see Fig. S5, ESI^†). Base activation of the alcohol was confirmed similar to previous reports^20,31 using ¹H-NMR of PA mixed with DMAP in CDCl₃ to identify ppm shifts for hydroxyl groups (see Fig. S6, ESI^†). Peak α′, 72 Da less than α, was the result of transesterification of PA-ODLA (see Fig. S7, ESI^†), which is a common side-reaction during ROP of lactide.^18,20 Two undesired products, peaks ζ and ζ′, were cyclic PA-ODLA with and without transesterification, respectively. Cyclicization increases over time during ROP of lactide^32–34 and is undesired because hydroxyl end groups are not available for subsequent conjugation. These peaks were due to radical-mediated dehydration with hydroxyl end group participation,³⁵ as the addition of hydroquinone (HQ), a radical scavenger, eliminated peaks ζ and ζ′ (see Fig. S8, ESI^†). However, a peak appeared that corresponded to HQ-DMAP-ODLA and lacked alkyne functionalities for subsequent click reactions.Another undesired product, DMAP-ODLA, or peak γ, formed via DMAP nucleophilic attack of lactide (see Fig. S9, ESI^†). Peak γ′, separated by 72 Da from peak γ, resulted from transesterification of DMAP-ODLA. By conducting reactions of only DMAP and lactide, only PA and lactide, and only αMPA and lactide, it was confirmed that DMAP can both initiate and catalyze lactide ROP, but PA and αMPA cannot (see Fig. S10, ESI^†). DMAP-ODLA formed due to PA : DMAP ratios less than 1, as previously described,¹⁸ and is undesired because ‘clickable’ propargyl end groups are not present. Finally, peaks β, δ, and ε are likely ion fragments, as these peaks were only present when using MALDI in linear, but not reflector, mode (see Fig. S11, ESI^†).To increase the amount of oligo(lactide) with alkyne and hydroxyl functionalities, side reactions were systematically addressed. It was noted that transesterification increased with time of polymerization and was greater for PA than for αMPA, as less nucleophilic secondary alcohols are unable to participate in transesterification reactions.³⁰ Thus, reactions were conducted at 130 °C for 5 minutes using αMPA as the initiator to optimize desired product (αMPA-ODLA, peak α).Ratios of αMPA : L : DMAP were investigated to mitigate undesired DMAP-initiated and cyclic ODLA (Fig. 2a; see Fig. S12, ESI^†). Increasing αMPA : L at a constant αMPA : DMAP increased the intensity of α relative to both ζ and γ (Fig. 2b; see Fig. S13, ESI^†). Similar behavior was identified when holding L : DMAP constant and increasing αMPA : L (Fig. 2c; see Fig. S14, ESI^†). These results suggest that αMPA : L controls cyclicization, as the distribution of ζ is similar among reactions in Fig. 2a, and the ratio of ζ : α decreases between ratios of 1 : 10 and 1 : 2 in Fig. 2b. αMPA : DMAP controls DMAP-ODLA generation, as the distribution of γ is similar within Fig. 2b and smallest at a ratio of 2 : 1. Finally, lower ratios of L : DMAP appear to increase α, with 2 : 1 yielding the greatest amount of desired product α. Altogether, higher αMPA : L, higher αMPA : DMAP, and lower L : DMAP ratios yield higher levels of α relative to γ and ζ.Characterization of 5 minute αMPA polymerizations. X = % conversion, M_n = molecular weight, PDI = polydispersity

Recent advances in pre-clinical diagnosis of Alzheimer’s disease

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Lipoprotein(a) and Oxidized Phospholipids Promote Valve Calcification in Patients With Aortic Stenosis

Background

Lipoprotein(a) [Lp(a)], a major carrier of oxidized phospholipids (OxPL), is associated with an increased incidence of aortic stenosis (AS). However, it remains unclear whether elevated Lp(a) and OxPL drive disease progression and are therefore targets for therapeutic intervention.

Opening of ATP-sensitive potassium channels causes generation of free radicals in vascular smooth muscle cells

Recent evidence suggests that opening of mitochondrial K_ATP channels in cardiac muscle triggers the preconditioning phenomenon through free radical production. The present study tested the effects of K_ATP channel openers in a vascular smooth muscle cell model using the fluorescent probe MitoTracker (MTR) Red™ for detection of reactive oxygen species (ROS). Rat aortic smooth muscle cells (A7r5) were incubated with 1 μM reduced MTR (non-fluorescent) and the MTR oxidation product (fluorescent) was quantified. Thirty-minute pretreatment with either diazoxide (200 μM) or pinacidil (100 μM), both potent mitochondrial K_ATP channel openers, increased fluorescent intensity (FI) to 149 and 162 % of control (p < 0.05 for both), respectively, and the K_ATP channel inhibitor 5-hydroxydecanoate (5HD) blocked it. Valinomycin, a potassium-selective ionophore, raised FI to 156 % of control (p <: 0.05). However, 5HD did not affect the valinomycin-induced increase in FI. Inhibition of mitochondrial electron transport (myxothiazol) or uncoupling of oxidative phosphorylation (dinitrophenol) also blocked either valinomycin- or diazoxide-induced increase in FI, and free radical scavengers prevented any diazoxide-mediated increase in fluorescence. Finally the diazoxide-induced increase in fluorescence was not blocked by the PKC inhibitor chelerythrine, but was by HMR 1883, a putative surface K_ATP channel blocker. Thus opening of K_ATP channels increases generation of ROS via the mitochondrial electron transport chain in vascular smooth muscle cells. Furthermore, a potassium-selective ionophore can mimic the effect of putative mitochondrial KATP channel openers. We conclude that potassium movement through KATP directly leads to ROS production by the mitochondria. Received: 7 January 2002, Returned for revision: 31 January 2002, Revision received: 21 February 2002, Accepted: 14 March 2002     

Duodenal tumor risk in Lynch syndrome

Background and aims

Lynch syndrome (LS) is associated with an increased risk of small bowel tumors but routine screening is not recommended in international guidelines. The aim of our study was to determinate the prevalence of duodenal tumors in a French cohort of LS patients.

Stigma,sex work,and substance use: a comparative analysis

Stigma is a widely used concept in social science research and an extensive literature claims that stigmatisation contributes to numerous negative health outcomes. However, few studies compare groups that vary in the extent to which they are stigmatised and even fewer studies examine stigma's independent and mediating effects. This article addresses these gaps in a comparative study of perceived stigma and drug use among three low‐income feminised service occupations: sex work, food and alcoholic beverage serving, and barbering and hairstyling. An analysis of longitudinal data shows positive associations between sex work, perceived stigma, and socially less acceptable drug use (for example, heroin and cocaine), and that stigma mediates part of the link between sex work and the use of these drugs. Our overall findings suggest that perceived stigma is pronounced among those who work in the sex industry and negatively affects health independently of sex work involvement.     

Fluid resuscitation in pre-hospital management of septic shock

Background

Septic shock is associated with hypovolemia resulting in organs failure and poor prognosis. The first step in hemodynamic resuscitation relies on early fluid expansion. In this study, we describe qualitative and quantitative fluid resuscitation of septic shock initially managed in a pre-hospital setting by a mobile intensive care unit.