Relative Reactivity of C4 Olefins toward the Polyisobutylene Cation

The capping reaction of living polyisobutylene cation (PIB+) with 1-butene, cis-2-butene, trans-2-butene, or 1,3-butadiene in hexanes (Hex)/methyl chloride (MeCl) 60/40 (v/v) solvent mixtures at −80 °C was studied. The reaction products were characterized by elemental analysis, gel permeation chromatography, and NMR spectroscopy. Monoaddition (capping) was observed with 1-butene and cis-2-butene but not with trans-2-butene, which did not react with PIB+. Monoaddition of 1,3-butadiene followed by instantaneous halide transfer from the counteranion and selective formation of the 1,4-adduct were observed in Hex/MeCl 60/40 (v/v) solvent mixtures at −80 °C at [1,3-butadiene] ≤ 0.05 mol L-1. The polymerization of isobutylene (IB) was studied in the presence of 1-butene, cis-2-butene, trans-2-butene, or 1,3-butadiene as capping agent in Hex/MeCl 80/20 to 40/60 (v/v) in the temperature range of −40 to −80 °C. From the limiting conversion and limiting number-average degrees of polymerization the reactivity ratio, kp±/kc± (where kp± is the absolute rate constant of propagation for IB and kc± is the cross-propagation (capping) rate constant from PIB+ cation to the C4 olefins), was calculated. Since the propagation of IB (addition of IB to PIB+) does not have an enthalpic barrier, the activation energy for cross-propagation was calculated to be 5.1−6.0 kJ mol-1. By extrapolating the plot of kp±/kc± vs hexanes (vol %), the reactivity ratios were calculated for 100% hexanes at −80 °C. From the cross-propagation activation energy and the calculated reactivity ratios at −80 °C in 100% hexanes, the reactivity ratios for 0 °C have also been calculated. According to these values at 0 °C, isobutylene is ∼17 times more reactive than 1,3-butadiene, ∼543 times more reactive than 1-butene, and ∼294 times more reactive than cis-2-butene

Living Carbocationic Polymerization of <i>p</i>-Methoxystyrene Using <i>p-</i>Methoxystyrene Hydrochloride/SnBr₄ Initiating System: Determination of the Absolute Rate Constant of Propagation for Ion Pairs

The carbocationic polymerization of p-methoxystyrene (p-MeOSt) was studied in CH2Cl2 at −30 °C using 1-chloro-1-(p-methoxyphenyl)ethane (p-MeOStCl) as initiator in conjunction with different Lewis acids, BF3·O(C2H5)2, BCl3, ZnCl2, TiCl4, SnCl4, and SnBr4. The best results, moderate rates, theoretical Mns, and low polydispersities, were obtained in conjunction of SnBr4. The living nature of the polymerization was verified by linear first-order ln([M]0/[M]) vs time and linear Mn vs conversion plots in the temperature range of −60 to −20 °C. The number-average molecular weight of the polymers increased in direct proportion to monomer conversion up to Mn = 120 000 and agreed with the calculated molecular weight, assuming that one polymer chain forms per molecule of p-MeOStCl. Kinetic studies suggest that the polymerization is first order in [SnBr4], and the rate, Mn, and polydispersity index are not affected by excess proton trap, 2,6-di-tert-butylpyridine. The living nature of the polymerization was further demonstrated by chain extension experiment. The stability of the propagating chain end of poly(p-MeOSt) and monomeric chain end of p-MeOSt+ was studied, and a slow decomposition of the active chain end was observed under monomer starved conditions. Employing the model compound p-MeOStCl in conjunction with SnBr4 in CH2Cl2 UV−vis spectroscopy was used to determine the equilibrium constant of ionization (Ki) at −30 and −60 °C. From Ki values and the apparent rate constant of propagation (kapp), the absolute rate constant of propagation for ion pairs, kp± = 1.07 × 105 L mol-1 s-1 for −30 °C and 3.83 × 104 L mol-1 s-1 for −60 °C, was calculated. To determine kp± separately, competition experiments were carried out in the presence of a nucleophile, phenylsilane. NMR spectroscopy, gel permeation chromatography, and MALDI−TOF MS analysis suggested complete capping of the polymeric cation and the absence of side reactions. From the limiting conversion and limiting number-average degree of polymerization kp± was calculated using the known rate constant of capping. The kp± values obtained from the competition experiments agreed well with those determined from the UV−vis spectroscopy

Crosslinked polymethacrylate absorbent with phenylalanine and stearate pendants

Herein, we report a new class of oil absorbents via conventional free radical polymerization method to prepare polymethacrylate gels with a range of crosslinking density having both phenylalanine and stearate moieties in the gel matrix. Analogue crosslinked gel was also synthesized with only stearate pendants without phenylalanine. Solvent absorbing capacity of gels was studied in various solvents with different dielectric constants. The mechanical property and morphology of the absorbents were studied by rheological method and field emission scanning electron microscopy, respectively. Modulation of porosity, absorption capacity and mechanical property of the absorbent were achieved with the variation of crosslinking density. The side chain pendants in the absorbent played an important role toward the solvent uptake capabilities. The absorbent readily can uptake 66 times chloroform and 14 times petrol of its dry weight at room temperature. Overall, the solvent uptake capacity of a new class of absorbents was well studied in this report.</p

Determination of the Absolute Rate Constant of Propagation for Ion Pairs in the Cationic Polymerization of <i>p</i>-Methylstyrene

The cationic polymerization of p-methylstyrene (p-MeSt) was studied in dichloromethane, chloroform, and methylcyclohexane/methyl chloride 60/40 (v/v) at different temperatures with various initiators in conjunction with different Lewis acids, TiCl4, BCl3, SnBr4, and SnCl4. Well-controlled living cationic polymerization was obtained in dichloromethane, in conjunction with SnCl4 as Lewis acid, and the living nature of the polymerization was verified by linear first-order ln([M]0/[M]) vs time and linear number-average molecular weight (Mn) vs conversion plots in the temperature range of −15 to −70 °C. The number-average molecular weight of the polymers increased in direct proportion to monomer conversion up to Mn ≈ 90 000 and agreed reasonably well with the calculated molecular weight, assuming that one polymer chain forms per molecule of initiator. The kinetics of p-MeSt polymerization suggests that the polymerization is first order with respect to SnCl4 concentration. Employing the model compound 1-chloro-1-(4-methylphenyl)ethane in conjunction with SnCl4 in CH2Cl2, UV−vis spectroscopy was used to follow the capping reaction with 2-phenylfuran to determine the equilibrium constant of ionization (Ki) at −30 °C. From the Ki value and the apparent rate constant of propagation (kapp) the absolute rate constant of propagation for ion pairs, kp± = 6.8 × 108 L mol-1 s-1 at −30 °C, was calculated as a lower limit. The absolute rate constant of propagation for ion pairs, kp±, was also determined at different temperatures from competition experiments, where polymerizations were carried out in the presence of 2-phenylfuran as capping agent. Gel permeation chromatography and NMR spectroscopy suggested complete capping of the polymeric cation and the absence of side reactions. From the limiting conversion and limiting number-average degrees of polymerization kp± = 1 × 109 L mol-1 s-1 was calculated using the known rate constant of capping. The kp± value remained unaffected in the temperature range of −15 to −70 °C, indicating that propagation does not have an enthalpic barrier

Carbocationic Polymerization of Isobutylene Using Methylaluminum Bromide Coinitiators: Synthesis of Bromoallyl Functional Polyisobutylene

The carbocationic polymerization of isobutylene (IB) was studied in conjunction with AlBr3, MeAlBr2, Me1.5AlBr1.5, and Me2AlBr coinitiators in hexanes(Hex)/methyl chloride (MeCl) 60/40 (v/v) solvent mixtures at −80 °C in the presence of a proton trap, 2,6-di-tert-butylpyridine. The observed Mns were directly proportional to monomer-to-initiator ratio with 2-chloro-2,4,4-trimethylpentane (TMPCl) as initiator and MeAlBr2, Me1.5AlBr1.5, and Me2AlBr coinitiators; however, with AlBr3 the Mns are much lower than the theoretical values. Chain extension “incremental monomer addition” (IMA) experiments resulting in bimodal distributions demonstrate that termination is operational with Me2AlBr. With MeAlBr2 the chain-extended PIBs exhibited close to theoretical Mns, but the molecular weigh distribution was broad. Using Me1.5AlBr1.5, Mn of the polymers increased in direct proportion and the molecular weight distributions remained narrow. 1H and 13C NMR spectroscopy of the polyisobutylene (PIB) obtained with Me1.5AlBr1.5 suggested virtually quantitative bromo end functionality. With MeAlBr2 and Me2AlBr the bromo functionality was lower (0.8−0), decreasing with the increase of Lewis acid concentration and polymerization time. The capping reaction of living polyisobutylene cation (PIB+) with 1,3-butadiene (BD) in Hex/MeCl 60/40 (v/v) solvent mixtures at −80 °C was also studied in conjunction with methylaluminum bromide coinitiators. Quantitative crossover reaction from living PIB chain end to BD followed by instantaneous termination and selective formation of 1,4-addition product bromoallyl functional PIB (PIB-AllylBr) was obtained only with Me1.5AlBr1.5 coinitiator

Folate-Conjugated Thermoresponsive Block Copolymers: Highly Efficient Conjugation and Solution Self-Assembly

A combination of controlled radical polymerization and azide−alkyne click chemistry was employed to prepare temperature-responsive block copolymer micelles conjugated with biological ligands with potential for active targeting of cancer tissues. Block copolymers of N-isopropylacrylamide (NIPAM) and N,N-dimethylacrylamide (DMA) were synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization with an azido chain transfer agent (CTA). Pseudo-first-order kinetics and linear molecular weight dependence on conversion were observed for the RAFT polymerizations. Cu(I)-catalyzed coupling with propargyl folate resulted in folic acid residues being efficiently conjugated to the α-azido chain ends of the homo and block copolymers. Temperature-induced self-assembly resulted in aggregates capable of controlled release of a model hydrophobic drug. Cu(I)-catalyzed azide−alkyne cycloaddition has proven superior to conventional methods for conjugation of biological ligands to macromolecules, and the general strategy presented herein can potentially be extended to the preparation of folate-functionalized assemblies with other stimuli susceptibility (e.g., pH) for therapeutic and imaging applications

Remarkable Swelling Capability of Amino Acid Based Cross-Linked Polymer Networks in Organic and Aqueous Medium

This work reports design and synthesis of side chain amino acid based cross-linked polymeric gels, able to switch over from organogel to hydrogel by a simple deprotection reaction and showing superabsorbancy in water. Amino acid based methacrylate monomers, <i>tert</i>-butoxycarbonyl (Boc)-l/d-alanine methacryloyloxyethyl ester (Boc-l/d-Ala-HEMA), have been polymerized in the presence of a cross-linker via conventional free radical polymerization (FRP) and the reversible addition–fragmentation chain transfer (RAFT) technique for the synthesis of cross-linked polymer gels. The swelling behaviors of these organogels are investigated in organic solvents, and they behave as superabsorbent materials for organic solvents such as dichloromethane, acetone, tetrahydrofuran, etc. Swollen cross-linked polymer gels release the absorbed organic solvent rapidly. After Boc group deprotection from the pendant alanine moiety, the organogels transform to the hydrogels due to the formation of side chain ammonium (−NH₃⁺) groups, and these hydrogels showed a significantly high swelling ratio (∼560 times than their dry volumes) in water. The morphology of organogels and hydrogels is studied by field emission scanning electron microscopy (FE-SEM). Amino acid based cross-linked gels could find applications as absorbents for oil spilled on water as well as superabsorbent hydrogels

Carbohydrate-Conjugated Amino Acid-Based Fluorescent Block Copolymers: Their Self-Assembly, pH Responsiveness, and/or Lectin Recognition

An effective strategy has been documented to combine both carbohydrate and amino acid biomolecules in a single synthetic polymeric system via a reversible addition–fragmentation chain transfer (RAFT) polymerization technique. The resultant unique block copolymer was engineered to form uniform micelles with the desired projection of either selective or both amino acid/sugar residues on the outer surface with multivalency, providing pH-based stimuli-responsiveness and/or lectin recognition. The self-assembly process was studied in detail by field emission scanning electron microscopy (FE-SEM), dynamic light scattering (DLS), and UV–visible spectroscopy. The enhanced lectin binding behavior was observed for glyconanoparticles with both amino acid/sugar entities on the shell as compared to the only glycopolymer nanoparticle because of the higher steric hindrance factor in the case of only the glycopolymer nanoparticle. Fluorophore conjugation by postpolymerization functionalization was further exploited by fluorescence spectroscopy for evidencing the lectin recognition process

Amino Acid-Based Polymer-Coated Silver Nanoparticles as Insulin Fibril Inhibitors

To explore the impact of polymer-coated silver nanoparticles (PC-AgNPs) on the extent of the insulin aggregation process, herein, we have synthesized three copolymers comprising poly(ethylene glycol) methyl ether methacrylate (PEGMA) and tert-butoxycarbonyl (Boc)-protected amino acid (alanine, leucine, and phenylalanine) containing methacrylate monomers, via reversible addition-fragmentation chain transfer (RAFT) polymerization. After deprotection of the Boc groups, the as-prepared water-soluble copolymers were coated on silver nanoparticles (Ag NPs), and the role of these NPs on insulin aggregation pathways was examined by multifarious spectroscopic and microscopic techniques. The extent of the inhibitory effect against the insulin fibrillation process was found to be related to the surface properties of the NPs, with the highest inhibitory effect detected for phenylalanine-based polymer-coated Ag NPs (PPhe-AgNPs). Using circular dichroism (CD) spectroscopy and Nile red (NR) fluorescence spectroscopy, we investigated the conformational changes and examined the role of hydrophobic interaction in inhibiting the aggregation properties of insulin upon treatment with PC-AgNPs. Furthermore, PC-AgNPs were also able to disintegrate the matured insulin fibrils and efficiently decreased the fibril-induced cytotoxicity, as confirmed by transmission electron microscopy (TEM) and the hemolysis study, respectively. Together, our findings established the novel amino acid-based PC-AgNPs as potent nanomaterials with 77–96% insulin fibril inhibition and marked disaggregation of matured insulin fibrils