Alexandru D. Asandei

Mild-temperature Mn2(CO)10-photomediated controlled radical polymerization of vinylidene fluoride and synthesis of well-defined poly(vinylidene fluoride) block copolymers.

By contrast to typical high-temperature (100-250 °C) telo-/polymerizations of gaseous fluorinated monomers, carried out in high-pressure metal reactors, the visible light, Mn(2)(CO)(10)-photomediated initiation of vinylidene fluoride (bp = -83 °C) polymerization occurs readily from a variety of alkyl, semifluorinated, and perfluorinated halides at 40 °C, in low-pressure glass tubes and in a variety of solvents, including water and alkyl carbonates. Perfluorinated alkyl iodide initiators also induce a controlled radical polymerization via iodine degenerative transfer (IDT). While IDT proceeds with accumulation of the less reactive P(m)-CF(2)-CH(2)-I vs the P(n)-CH(2)-CF(2)-I chain ends, Mn(2)(CO)(10) enables their subsequent quantitative activation toward the synthesis of well-defined poly(vinylidene fluoride) block copolymers with a variety of other monomers.

Stabilization of graphene sheets by a structured benzene/hexafluorobenzene mixed solvent.

Applications requiring pristine graphene derived from graphite demand a solution stabilization method that utilizes an easily removable media. Using a combination of molecular dynamics simulations and experimental techniques, we investigate the solublization/suspension of pristine graphene sheets by an equimolar mixture of benzene and hexafluorobenzene (C(6)H(6)/C(6)F(6)) that is known to form an ordered structure solidifying at 23.7 °C. Our simulations show that the graphene surface templates the self-assembly of the mixture into periodic layers extending up to 30 Å from both sides of the graphene sheet. The solvent structuring is driven by quadrupolar interactions and consists of stacks of alternating C(6)H(6)/C(6)F(6) molecules rising from the surface of the graphene. These stacks result in density oscillations with a period of about 3.4 Å. The high affinity of the 1:1 C(6)H(6)/C(6)F(6) mixture with graphene is consistent with observed hysteresis in Wilhelmy plate measurements using highly ordered pyrolytic graphite (HOPG). AFM, SEM, and TEM techniques verify the state of the suspended material after sonication. As an example of the utility of this mixture, graphene suspensions are freeze-dried at room temperature to produce a sponge-like morphology that reflects the structure of the graphene sheets in solution.

Visible‐Light Hypervalent Iodide Carboxylate Photo(trifluoro)methylations and Controlled Radical Polymerization of Fluorinated Alkenes

IFAB-ulous trifluoromethylation: (CX3COO)2I(III) h (X=F, H) and (CH3COO)3I(V)(C6H4COO) are introduced as CX3·/CX3I precursors for metal-free, visible-light, radical (trifluoro)(iodo)methylations of alkenes, illustrated by their use as photoinitiators for the controlled radical polymerization of vinylidene fluoride with external (I(CF2)6I) and in situ generated (CF3I) iodine chain transfer agents, and for block copolymer synthesis.

Photomediated Controlled Radical Polymerization and Block Copolymerization of Vinylidene Fluoride

This review summarizes recent research on novel photochemical methods for the initiation and control of the polymerization of main chain fluorinated monomers as exemplified by vinylidene fluoride (VDF) and for the synthesis of their block copolymers. Such reactions can be carried out at ambient temperature in glass tubes using visible light. Novel, original protocols include the use of hypervalent iodide carboxylates alone or in conjunction with molecular iodine, as well as the use of photoactive transition metal carbonyls in the presence of alkyl, fluoroalkyl, and perfluoroalkyl halides. An in-depth study of the reaction parameters highlights the use of dimethyl carbonate as a preferred polymerization solvent and outlines the structure-property relationship for hypervalent iodide carboxylates and halide initiators in both the free radical and iodine degenerative transfer controlled radical polymerization (IDT-CRP) of VDF. Finally, the rational selection of metal carbonyls that are successful not only as IDT mediators but, more importantly, in the quantitative activation of both PVDF-CH2-CF2-I and PVDF-CF2-CH2-I chain ends toward the synthesis of well-defined PVDF block copolymers is presented.

Imaging and Thermal Studies of Wheat Gluten/Poly(vinyl alcohol) and Wheat Gluten/Thiolated Poly(vinyl alcohol) Blends

The morphology of wheat protein (WG) blends with polyvinyl alcohol (PVA) and respectively with thiolated polyvinyl alcohol (TPVA) was investigated by atomic force (AFM) and transmission electron microscopy (TEM) as well as by modulated dynamic scanning calorimetry (MDSC). Thiolated additives based on PVA and other substrates were previously presented as effective means of improving the strength and toughness of compression molded native WG bars via disulfide-sulfhydryl exchange reactions. Consistent with our earlier results, AFM and TEM imaging clearly indicate that the addition of just a few mole percent of thiol to PVA was sufficient to dramatically change its compatibility with wheat protein. Thus, TPVA is much more compatible with WG and phase separates into much smaller domains than in the case of PVA, although there are still two phases in the blend: one WG-rich phase and another TPVA-rich phase. The WG/TPVA blend has phase domains ranging in size from 0.01 to 0.1 microm, which are roughly 10 times smaller than those of the WG/PVA blend. MDSC further illustrates the compatibilization of the protein with TPVA via the dependence of the transition temperatures on composition.

Water soluble poly(styrene sulfonate)-b-poly(vinylidene fluoride)-b-poly(styrene sulfonate) triblock copolymer nanoparticles

A visible light, Mn2(CO)2-photomediated process was used to enable the iodine degenerative transfer controlled radical polymerization of vinylidene fluoride (VDF) initiated from I–(CF2)6–I, and the successive quantitative activation of the ∼CH2–CF2–I and ∼CF2–CH2–I chain ends of I–PVDF–I, to produce a series of PNpSS-b-PVDF-b-PNpSS triblock copolymers (NpSS/VDF/NpSS = 4/60/4, 15/60/15, 34/60/34) with neopentyl styrene sulfonate (NpSS), which upon NaN3 deprotection, afforded the corresponding PNaSS-b-PVDF-b-PNaSS. All blocks, as well as PVDF, PNpSS and PNaSS formed water stable dispersion/solutions following either nanoprecipitation from acetone (PVDF, PNpSS, and PNpSS-b-PVDF-b-PNpSS) or direct dissolution in water (PNaSS, PNaSS-b-PVDF-b-PNaSS). Remarkably, all PNaSS-b-PVDF-b-PNaSS triblocks also provided indefinitely stable systems even under the high ionic strength conditions of phosphate buffered saline (PBS) solutions, corresponding to cell-isotonic, pH = 7.4 conditions. These trends were found to be consistent with the block composition dependence of the apparent hydrodynamic radius (Rh), conductivity, and zeta potential (ζ), where Rh increases upon deprotection from about 35–25 nm to about 168–136 nm in water, and decreases to 133–86 nm in PBS solutions, ζ decreases from ∼−40 mV to ∼−70 mV in water and increases to ∼−18 mV in PBS, and where the conductivity is negligible for PNpSS-b-PVDF-b-PNpSS but then increases linearly to ∼0.2 mS cm−1 for PNaSS-b-PVDF-b-PNaSS in water to reach ∼17 mS cm−1 in PBS. Finally, the blocks were evaluated as promising 19F-MRI contrast agents.

Normal, ICAR and photomediated butadiene-ATRP with iron complexes

The ligand (L) and halide effects of a series of iron complexes (FeX2 or FeX3, X = Cl, Br)/L supported by carbon (Cp2Fe2(I)(CO)4 > Cp2Fe > Fe(CO)5 > (Ph2PCp)2Fe), nitrogen (phthalocyanine ≫ bpy ≥ MeO-bpy ≫ PMDETA > phen), halide (FeXmY4−m/Bu4N, X, Y = Cl ≫ Br > I), oxygen (12-crown-4 ≫ 15-crown-5 ≥ dibenzo-18-crown-6) and phosphorous (P[Ph(2,4,6-OMe)3]3 > P(t-Bu)3 ≫ P(n-Bu)3, PPh3, P[Ph(4-CF3)]3, P(C6F5)3) ligands, as well as ligand-free FeX3, were evaluated in the normal, ICAR, and photo-ATRP of butadiene (BD) initiated from bromoesters, α,α-dichloro-p-xylene, or FeX3 in toluene at 110 °C. Good polymerization control was observed in many cases, and two clear trends i.e. P[Ph(OMe)3]3 ≫ Bu4NX > crown ethers > amines > C-ligands and FeCl2, FeCl3 ≫ FeBr2, FeBr3 occur consistently across all polymerizations. These effects correlate with the higher stability of the allyl PBD-Cl vs. PBD-Br chain ends and with FeCl3 likely being a better deactivator than FeBr3. Conversely, while basic enough to reduce FeX3, P[Ph(2,4,6-OMe)3]3 is not nucleophilic enough to quaternize PBD-X in the apolar toluene and successfully enables a faster activation/deactivation equilibrium than all other ligands. As such, e.g. N-ATRP with [BD]/[R–Br]/[FeCl3]/P[Ph(2,4,6-OMe)3]3 = 100/1/2/3 affords a linear Mnvs. conversion profile with PDI as low as 1.15–1.2 and a halide chain end functionality (CEF) = 0.65 at up to 50% conversion. While controlled polymerizations occur in photo-ATRP even without ligand and initiator, photoirradiation of catalytic N-ATRP with BD/R–Br/FeCl3/P[Ph(2,4,6-OMe)3]3 = 100/1/0.05/0.15 significantly improves the rate (×10 vs. dark), conversion (up to 70%) and X-CEF (0.9) via the additional initiation afforded by FeX3 photolysis, albeit with a slight PDI increase to ∼1.4. Thus, Fe-mediated BD-ATRP is achievable, and the rational selection of the polymerization variables enables minimization of side reactions and the successful synthesis of well-defined PBD with a wide range of molecular weights, narrow PDI and reasonably high X-CEF, suitable for the preparation of e.g. block copolymers.

Preparation of Drug Delivery Biodegradable PLGA Nanocomposites and Foams by Supercritical COsub2 Expanded Ring Opening Polymerization and by Rapid Expansion from CHCIF2 Supercritical Solutions

Abstract : The synthesis of poly(lactic-co-glycolic acid) (PLGA) by the ring opening copolymerization of D,L-lactide and glycolide was performed at 110 deg C to 130 deg C using Sn(Oct)2 as catalyst, 1,10-decanediol as initiator in a supercritical sc-CO2 expanded medium at pressures of up to 2,500 psi. Due to the limited monomer solubility in sc-CO2 at low temperatures (70 deg C), only Mn=2,500 is typically obtained. However, molecular weight increases with both temperature and sc-CO2 pressure. Thus, Mn = 13,000 (PDI = 1.28) was obtained at 110 deg C - 130 deg C even in the absence of fluorinated surfactants. Biodegradable drug delivery nanocomposites based on dexamthasone and poly(lactic acid) (PLA) and poly(lactide-co-glycolide) (PLGA) were prepared by the rapid expansion of the corresponding supercritical CHCIF2 solutions (110 deg C, 200-300 bar) in air (RESS) and in toluene (RESOLV). The RESS process leads to a broad particle size distribution (100-500 nm) while the RESOLV generates a narrower distribution centered around 100 nm and is accompanied by the formation of a few large particles, most likely due to aggregation.

TPB′: a constitutional isomeric mesogen based on conformational isomerism which generates pairs of completely isomorphic polyethers

Abstract The synthesis and characterization of 1,-(4-hydroxyphenyl)-2-(4-hydroxy-4′-biphenyl)butane (TPB′) which is the constitutional isomer of 1-(4-hydroxy-4′-biphenylyl)-2-(4-hydroxyphenyl)butane (TPB) and of the corresponding polyethers with α,ω-dibromoalkanes (TPB′-X with X = 4–19, where X = number of methylenic groups in the spacer) are described. The phase behaviour of TPB′-X is similar to that of TPB-X previously reported from our laboratory (Macromolecules 1991, 24, 6318), except that both the crystalline melting and the nematic-isotropic temperatures of TPB′-X are higher than those of TPB-X. Since the increase of the isotropization temperature is larger than that of the melting temperature, at long spacers virtual mesophases of TPB-X became monotropic for TPB′-X. This is explained by the difference between the axial ratio ( x = L d ) of these two constitutional isomers, i.e., x = 1.46 for TPB and x = 2.01 for TPB′. TPB-X and TPB′-X are isomorphic within their nematic and crystalline phases.

TiCp 2 Cl-Catalyzed Living Radical and Ring Opening Polymerizations Initiated from Epoxides and Aldehydes in the Synthesis of Linear, Graft and Branched Polymers

Ti(III)Cp2Cl-catalyzed radical ring opening (RRO) of epoxides or single electron transfer (SET) reduction of aldehydes generates Ti alkoxides and carbon centered radicals which add to styrene, initiating a radical polymerization. This polymerization is mediate in a living fashion by the reversible termination of growing chains with the TiCp2Cl metalloradical. In addition, polymers or monomers containing pendant epoxide groups (glycidyl methacrylate) can be used as substrates for radical grafting or branching reactions by self condensing vinyl polymerization. In addition, Ti alkoxides generated in situ by both epoxide RRO and aldehyde SET initiate the living ring opening polymerization of e-caprolactone. Thus, new initiators and catalysts are introduced for the synthesis of complex polymer architectures.