Tert-butyl End Group Research Articles

Influence of Cholesterol and Bilayer Curvature on the Interaction of PPO-PEO Block Copolymers with Liposomes.

Interactions of nonionic poly(ethylene oxide)- b-poly(propylene oxide) (PEO-PPO) block copolymers, known as Pluronics or poloxamers, with cell membranes have been widely studied for a host of biomedical applications. Herein, we report how cholesterol within phosphatidylcholine (POPC) lipid bilayer liposomes and bilayer curvature affects the binding of several PPO-PEO-PPO triblocks with varying PPO content and a tPPO-PEO diblock, where t refers to a tert-butyl end group. Pulsed-field-gradient NMR was employed to quantify the extent of copolymer associated with liposomes prepared with cholesterol concentrations ranging from 0 to 30 mol % relative to the total content of POPC and cholesterol and vesicle extrusion radii of 25, 50, or 100 nm. The fraction of polymer bound to the liposomes was extracted from NMR data on the basis of the very different mobilities of the bound and free polymers in aqueous solution. Cholesterol concentration was manipulated by varying the molar percentage of this sterol in the POPC bilayer preparation. The membrane curvature was varied by adjusting the liposome size through a conventional pore extrusion technique. Although the PPO content significantly influences the overall amount of block copolymer adsorbed to the liposome, we found that polymer binding decreases with increasing cholesterol concentration in a universal fashion, with the fraction of bound polymer dropping 10-fold between 0 and 30 mol % cholesterol relative to the total content of POPC and cholesterol. Increasing the bilayer curvature (decreasing the radius of the liposome) in the absence of cholesterol increases polymer binding between 2- and 4-fold over the range of liposome sizes studied. These results demonstrate that cholesterol plays a dominant role, and bilayer curvature has a less significant impact as the curvature decreases, on polymer-membrane association.

Myoblast Protection by Polyethylene Oxide-Polypropylene Oxide Block Copolymers against Hypo-Osmotic Stress

Amphiphilic poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers, known as poloxamers, have shown the ability to rescue cells from various types of membrane injury. However, the mechanism of cell protection by block copolymers is still unclear. To explore the role of block copolymer architecture on membrane protection, we synthesized diblock copolymers with control over the composition, molecular weight, and endgroup hydrophilicity/hydrophobicity. Membrane protection efficacy was studied in cultured murine myoblasts by the lactate dehydrogenase (LDH) release assay, in which cells are exposed to the stresses of hypo-osmotic shock and isotonic recovery, and membrane integrity is quantitatively evaluated with or without the presence of block copolymers. The cell protection required sufficient hydrophobicity of the block copolymers, controlled by either the hydrophobic PPO block or endgroup modification. At least 9 PPO units were needed for protection at 150 μM. Addition of hydrophobic tert-butyl endgroup enhanced the protection ability at reduced concentration. With fixed hydrophobicity, increasing hydrophilic PEO block length improved protection. In addition to the cellular integrity assay, we synthesized fluorophore-labeled block copolymers and performed confocal microscopy to image subcellular localization to further understand the membrane protection mechanism.

PEO-PPO Diblock Copolymers Protect Myoblasts from Hypo-Osmotic Stress In Vitro Dependent on Copolymer Size, Composition, and Architecture.

Poloxamer 188, a triblock copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), protects cellular membranes from various stresses. Though numerous block copolymer variants exist, evaluation of alternative architecture, composition, and size has been minimal. Herein, cultured murine myoblasts are exposed to the stresses of hypotonic shock and isotonic recovery, and membrane integrity was evaluated by quantifying release of lactate dehydrogenase. Comparative evaluation of a systematic set of PEO-PPO diblock and PEO-PPO-PEO triblock copolymers demonstrates that the diblock architecture can be protective in vitro. Short PPO blocks hinder protection with >9 PPO units needed for protection at 150 μM and >16 units needed at 14 μM. Addition of a tert-butyl end group enhances protection at reduced concentration. When the end group and PPO length are fixed, increasing the PEO length improves protection. This systematic evaluation establishes a new in vitro screening tool for evaluating membrane-sealing amphiphiles and provides mechanistic insight to guide future copolymer design for membrane stabilization in vivo.

Quantification of polystyrene blend surfaces based on end group ToF-SIMS analysis

In this work, two sets of low molecular weight polystyrene blends were studied. The first one is composed of two polystyrenes with, at one end, tertiary or secondary butyl end group ( sec- and tert-) and hydrogen end group (–H) at the other end. ToF-SIMS spectra show a linear dependency with bulk concentration. Due to the slight difference of molecular structures between sec- and tert-butyl end group, no difference of surface free energy and surface state is expected. Then, the observed linear dependency implies that the matrix effect is insignificant. The second set of polymer blends are composed of polystyrenes with, at one end, tertiary or secondary butyl end group ( sec- or tert-) and at the other end, hydrogen (–H) or trimethyl silyl (–Si(CH 3) 3, TMS) end group, respectively. With the results from the first set of blends, we were able to quantify the sec-PS–Si concentration at the surface as compared to the bulk, using univariate and multivariate techniques. The different methods establish that all these polystyrene blends present a bulk-like surface. The principal component analysis (PCA) seems to be interesting for the study of ternary polymer blends.

Molecular Weight Effects on Polystyrene Fingerprint Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) Spectra

Monodisperse polystyrenes (PS) of different molecular weights (M-n) synthesized by living anionic polymerization with three types of butyllithium initiator (linear, n; secondary, sec; and tertiary, tert) were analyzed by ToF-SIMS (time-of-flight secondary ion mass spectrometry). The influence of the molecular weight on the secondary ion intensities was studied in detail for the fingerprint part of the mass spectra (with m/z < 200). A drastic effect was observed for M-n values below 10(4), related to the presence of the saturated butyl end group. An extra hydrogen transfer originating from this end group during the secondary ion formation must be invoked to explain the data. Only the first neighbor monomer repeat units seem to be affected. This H exchange increases the intensity of ions containing more hydrogen or needing H transfer for their formation as the tropylium ion (C7H7+ at m/z = 91). The molecular structure of the butyl end group is found to influence greatly not only the intensity of their parent ion but also the PS characteristic ion intensities. Indeed, the tert-butyl end group is seen unable to produce the H transfer observed for the n- and sec-butyl ones. A model is proposed to take the influence of the end group on the PS SIMS fragmentation pattern into account. The parameters of this model allow the quantification of the end group interaction.

Effect of molecular weight on formation of non-volatile oligomers by thermal degradation of polyisobutylene and its kinetic analysis

The formation of functional groups of non-volatile oligomers by the thermal degradation of polyisobutylene is characterized by a kinetic approach including intermolecular hydrogen abstraction of primary (p) and tertiary (t) terminal macroradicals (R p • and R t •) and volatile small radicals (S •), followed by β-scission. By assuming in the kinetic analysis that the reaction occurs competitively under a steady state regarding the on-chain macroradicals, various composition ratios for the functional groups can be represented in terms of the rates of respective hydrogen abstraction. The ratio between the tert-butyl endgroup (t-Bu) and the isopropyl endgroup (i-Pr), which corresponds to that between the abstraction rates of R p • and R t • , is expressed by the product of the rate constant ratio and the integrated macroradical concentration ratio (R p •]/[R t •]). The observed value of the ratio [ t-Bu] [ i-Pr] decreases with reaction time. This is induced by a decrease in the molecular weight ( M) of the reaction medium. The molecular weight dependence ( M a ) of [ t-Bu] [ i-Pr] is expressed by that of R p bull;/[R t •]. The value of exponent a was determined to be about 1.2 and 0.7 at 300 and 320°C, respectively, from the analysis of data at various reaction times. These values are roughly consistent with average values (1.0 and 0.9) of the same power-law exponent for the volatile oligomers. These results support the hypothesis that the concentrations of the respective radicals decrease in different ways.

Preparation and characterization of end-reactive oligomers by thermal degradation of polyisobutylene

The chemical structure of the non-volatile oligomers ( M n = 2600–9000) isolated from the polymer residues obtained by the thermal degradation of polyisobutylene at 300 and 320°C was determined by 400 MHz 1H n.m.r. spectroscopy with regard to the reactive end groups. The functional groups formed in the degradation process are as follows: tert-butyl end group (t-Bu), isopropyl end group (i-Pr), terminal trisubstituted double bond (TTD), terminal vinylidene double bond (TVD), and non-terminal trisubstituted double bond (NTTD). The average number of TTD and TVD per molecule ( f t) is in a range from 1.46 to 1.64. This value suggests that 53–67 mol% of the non-volatile oligomers are telechelic oligomers having both TTD and/or TVD, and 39-29 mol% of the oligomers are macromonomer-like oligomers having a double bond at one chain end. Both t-Bu and i-Pr are produced by the intermolecular hydrogen abstraction of primary and tertiary terminal macroradicals, and the subsequent β scission of the resulting on-chain macroradicals at the skeletal CC bond of the main chain yields TTD and TVD. The formation of telechelic oligomer is interpreted by a major contribution of the hydrogen abstraction of volatile radicals, in addition to a minor contribution of the hydrogen abstraction of terminal macroradicals.