Effect of polymer characteristics on structure of polymer-liposome complexes

In the current study, we examined the effect of polymer characteristics on the structure of complexes formed between poly(methacrylic acid-co-n-alkyl methacrylate) and with phosphatidylcholine/cholesterol liposomes. We varied the polymer concentration in the vesicles, the preparation concentration of lipid and polymer components during preparation, the molecular weight of the polymer chain, the molecular weight of the polymer's hydrophobic side groups and their mole fraction. The vesicle behavior indicated polymer-free bilayers and bilayers complexed with polymer coexisted at low polymer concentrations. As the polymer concentration exceeds a critical level, however, the system became homogeneous, indicating bilayer uniformity of the bilayer. As the polymer content was raised, the vesicle size and fluidity increased, and the transition temperature decreased. We found that the vesicle size mostly affects the membrane fluidity. We also found that the thermal properties (transition temperature and the magnitude of heat capacity of the peak, DeltaCp) are governed by the effects of the polymer on the structure of bilayer. The length of the alkyl chain of the polymer is shown to significantly affect the structure of polymer-liposome complexes, as did the chain molecular weight and mole concentration of hydrophobic group in the polymer.     

Effect of Topology on the Properties of Poly(N-isopropylacrylamide) in Water and in Bulk

Three macrocyclic poly(N-isopropylacrylamide)s (PNIPAM) with molecular weight (MW) ranging from 6 to 19 kg/mol were synthesized by ‘click’ ring closure of the corresponding α-azido ω-propargyl telechelic linear PNIPAMs, themselves prepared by reversible addition fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide. Differential scanning calorimetry (DSC) studies revealed that both the thermal phase separation in water and the glass transition in bulk of PNIPAM were affected by polymer topology. In aqueous solution, the cyclic polymers exhibit a higher phase separation temperature and broader phase transition range than the corresponding linear counterparts. In bulk, the cyclic polymers display a higher glass transition temperature of lesser molecular weight dependence, as compared to their linear precursors.     

Specific ion effects on the water solubility of macromolecules: PNIPAM and the Hofmeister series

Aqueous processes ranging from protein folding and enzyme turnover to colloidal ordering and macromolecular precipitation are sensitive to the nature and concentration of the ions present in solution. Herein, the effect of a series of sodium salts on the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide), PNIPAM, was investigated with a temperature gradient microfluidic device under a dark-field microscope. While the ability of a particular anion to lower the LCST generally followed the Hofmeister series, analysis of solvent isotope effects and of the changes in LCST with ion concentration and identity showed multiple mechanisms were at work. In solutions containing sufficient concentrations of strongly hydrated anions, the phase transition of PNIPAM was directly correlated with the hydration entropy of the anion. On the other hand, weakly hydrated anions were salted-out through surface tension effects and displayed improved hydration by direct ion binding.     

PNIPAM chain collapse depends on the molecular weight and grafting density

This study demonstrates that the thermally induced collapse of end-grafted poly(N-isopropylacrylamide) (PNIPAM) above the lower critical solution temperature (LCST) of 32 degrees C depends on the chain grafting density and molecular weight. The polymer was grafted from the surface of a self-assembled monolayer containing the initiator (BrC(CH3)2COO(CH2)11S)2, using surface-initiated atom transfer radical polymerization. Varying the reaction time and monomer concentration controlled the molecular weight, and diluting the initiator in the monolayer altered the grafting density. Surface force measurements of the polymer films showed that the chain collapse above the LCST decreases with decreasing grafting density and molecular weight. At T > LCST, the advancing water contact angle increases sharply on PNIPAM films of high molecular weight and grafting density, but the change is less pronounced with films of low-molecular-weight chains at lower densities. Below the LCST, the force-distance profiles exhibit nonideal polymer behavior and suggest that the brush architecture comprises dilute outer chains and much denser chains adjacent to the surface.     

基于星型杂臂环糊精聚合物的纳米胶束: 构筑及包合特性

通过胺化反应和原子转移自由基聚合(ATRP),合成了以β-环糊精为“核”,以1条聚乙二醇和2~4条聚N-异丙基丙烯酰胺为“臂”的双亲水性星型杂臂聚合物(MPEG-CD-PNIPAMx)。通过1H NMR,13C NMR和凝胶渗透色谱/多角度激光光散射联用(SEC/MALLS)对其结构进行了表征。对1H NMR峰面积积分计算得聚N-异丙基丙烯酰胺“臂”数为2~4。通过紫外-可见分光光度计测得该星型大分子的较低溶液临界温度(LCST)为37℃。MPEG-CD-PNIPAMx在其水溶液温度达到LCST以上时呈现两亲性,并通过疏水相互作用自组装成以聚N-异丙基丙烯酰胺为“核”,以β-环糊精及聚乙二醇为“壳”的纳米级胶束粒子。通过MPEG-CD-PNIPAMx及其胶束粒子在芘溶液中的荧光光谱,发现胶束粒子对疏水性客体小分子的包合可发生在处于壳层的β-环糊精的疏水性空腔和胶束粒子的疏水性内核。     

Effects of Hofmeister anions on the phase transition temperature of elastin-like polypeptides

The modulation of the lower critical solution temperature (LCST) of two elastin-like polypeptides (ELPs) was investigated in the presence of 11 sodium salts that span the Hofmeister series for anions. It was found that the hydrophobic collapse/aggregation of these ELPs generally followed the series. Specifically, kosmotropic anions decreased the LCST by polarizing interfacial water molecules involved in hydrating amide groups on the ELPs. On the other hand, chaotropic anions lowered the LCST through a surface tension effect. Additionally, chaotropic anions showed salting-in properties at low salt concentrations that were related to the saturation binding of anions with the biopolymers. These overall mechanistic effects were similar to those previously found for the hydrophobic collapse and aggregation of poly(N-isopropylacrylamide), PNIPAM. There is, however, a crucial difference between PNIPAM and ELPs. Namely, PNIPAM undergoes a two-step collapse process as a function of temperature in the presence of sufficient concentrations of kosmotropic salts. By contrast, ELPs undergo collapse in a single step in all cases studied herein. This suggests that the removal of water molecules from around the amide moieties triggers the removal of hydrophobic hydration waters in a highly coupled process. There are also some key differences between the LCST behavior of the two ELPs. Specifically, the more hydrophilic ELP V5A2G(3)-120 construct displays collapse/aggregation behavior that is consistent with a higher concentration of anions partitioning to polymer/aqueous interface as compared to the more hydrophobic ELP V(5)-120. It was also found that larger anions could bind with ELP V5A2G(3)-120 more readily in comparison with ELP V(5)-120. These latter results were interpreted in terms of relative binding site accessibility of the anion for the ELP.     

Fabrication of novel thermo-responsive electrospun nanofibrous mats and their application in bioseparation

To fulfill the development of biotechnology and biomedicine, environmental-responsive polymer materials are wanted for isolation and purification of biomolecules. Herein, a novel thermo-responsive poly(methyl methacrylate) (PMMA)/poly(N-isopropylacrylamide) (PNIPAM) blend nanofibrous mat was developed, which can adsorb and release a model solute, bovine serum albumin (BSA), through the way of hydrophilicity–hydrophobicity transition behavior of PNIPAM. The uniform bead-free electrospun nanofibers were obtained from the homogeneous PMMA solution in the presence of different amount of PNIPAM. Scanning electron microscopy (SEM) analysis showed that the electrospinnability of PMMA was improved by the addition of PNIPAM, and the diameter of resultant nanofibers could be modulated by controlling the amount of PNIPAM. The thermo-responsive swelling behavior of the blend nanofibrous mats was reversible and reproducible by changing environmental temperature across the lower critical solution temperature (LCST) of PNIPAM. Moreover, the separation property of the blend nanofibrous mats was found to be related to the amount of PNIPAM as well as the concentration of BSA. As for a better separation effect, the nanofibers with higher content of PNIPAM were favorable.     

Dually responsive multiblock copolymers via RAFT polymerization: Synthesis of temperature- and redox-responsive copolymers of PNIPAM and PDMAEMA

We report synthesis of temperature- and redox-responsive multiblock copolymers by reversible addition-fragmentation chain transfer (RAFT) polymerization. Well-defined α,ω-bis(dithioester)-functionalized poly(N-isopropylacrylamide) (PNIPAM) and poly(2-(dimethylamino) ethyl methacrylate) (PDMAEMA) were prepared using 1,4-bis(thiobenzoylthiomethyl)benzene and 1,4-bis(2-(thiobenzoylthio)prop-2-yl)benzene as RAFT agents, respectively. Dually responsive multiblock copolymers were synthesized in a single aminolysis/oxidation step from the α,ω-bis(dithioester)-terminated PNIPAM and PDMAEMA. The copolymers and their stimulus-responsive behavior were characterized by size exclusion chromatography, NMR, light scattering and atomic force microscopy. Due to the presence of redox-sensitive disulfide bonds between the blocks, the copolymers were readily reduced to the starting polymer blocks. The presence of temperature-responsive PNIPAM blocks provided the copolymers with the ability to assemble into core-shell nanostructures with hydrophobic PNIPAM as a core and cationic PDMAEMA as stabilizing shell when above the phase transition temperatures of PNIPAM. The temperature-induced assembly of the copolymers also showed substantial pH sensitivity. The phase transition temperature increased with decreasing pH, while molecular weight of the assemblies decreased.     

Direct observation of the phase transition for a poly(N-isopropylacryamide) layer grafted onto a solid surface by AFM and QCM-D

The temperature-induced structural changes of a thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) layer grafted onto a silica substrate were investigated in aqueous solution using an atomic force microscope (AFM) and a quartz crystal microbalance with dissipation (QCM-D). A PNIPAM layer was grafted onto the silicon wafer surface by free radical polymerization of NIPAM to obtain a high molecular weight polymer layer with low-grafting density overall. By AFM imaging, the transition of the grafted PNIPAM chains from a brush-like to a mushroom-like state was clearly visualized: The surface images of the plate were featureless at temperatures below the LCST commensurate with a brush-like layer, whereas above the LCST, a large number of domain structures with a characteristic size of approximately 100 nm were seen on the surface. Both frequency and dissipation data obtained using QCM-D showed a significant change at the LCST. Analysis of these data confirmed that the observed PNIPAM structural transition was caused by a collapse of the brush-like structure as a result of dehydration of the polymer chains.     

Activity of enzymes immobilized on microspheres with thermosensitive hairs

Poly(N-isopropylacrylamide)s (PNIPAMs) carboxylated at one chain end or both ends were prepared by polymerization using 4,4-azobis(N,N,-cyanopentanoic acid) (V-501) as an initiator and β-mercaptopropionic acid (MPA) as a chain transfer reagent. One end group of PNIPAM carboxylated at both ends was conjugated with latex particles, and another with trypsin using carbodiimide. Differential scanning calorimetry (DSC) revealed that PNIPAM on the particles exhibited a drastic phase transition, and that the transition temperature was largely elevated when the enzyme was immobilized at the chain end. Therefore, PNIPAM on the particles showed two phase transitions because of the coexistence of the enzyme-conjugated and non-conjugated PNIPAMs. The activity of trypsin immobilized on the particles with the PNIPAM spacer showed significant temperature dependence. The apparent relative activity increased above the transition temperature of non enzyme-conjugated PNIPAM on the particles. One of the reasons for this is that the diffusion of the substrate changed discontinuously around the transition temperature. Therefore, the temperature dependence of the enzymatic activity was significantly affected by the molecular size of the substrates. The enzymatic activity was also influenced by the surface density of trypsin and PNIPAM on the particle, and the molecular weight of the PNIPAM spacer.     

Synthesis and "schizophrenic" micellization of double hydrophilic AB4 miktoarm star and AB diblock copolymers: structure and kinetics of micellization

Poly(N-isopropylacrylamide) (PNIPAM)-based tetrafunctional atom transfer radical polymerization (ATRP) macroinitiator (1b) was synthesized via addition reaction of mono-amino-terminated PNIPAM (1a) with glycidol, followed by esterification with excess 2-bromoisobutyryl bromide. Well-defined double hydrophilic miktoarm AB4 star copolymer, PNIPAM-b-(PDEA)4, was then synthesized by polymerizing 2-(diethylamino)ethyl methacrylate (DEA) via ATRP in 2-propanol at 45 degrees C using 1b, where PDEA was poly(2-(diethylamino)ethyl methacrylate). For comparison, PNIPAM-b-PDEA linear diblock copolymer with comparable molecular weight and composition to that of PNIPAM-b-(PDEA)4 was prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. The pH- and thermoresponsive "schizophrenic" micellization behavior of the obtained PNIPAM65-b-(PDEA63)4 miktoarm star and PNIPAM70-b-PDEA260 linear diblock copolymers were investigated by 1H NMR and laser light scattering (LLS). In acidic solution and elevated temperatures, PNIPAM-core micelles were formed; whereas at slightly alkaline conditions and room temperature, structurally inverted PDEA-core micelles were formed. The size of the PDEA-core micelles of PNIPAM65-b-(PDEA63)4 is much smaller than that of PNIPAM70-b-PDEA260. Furthermore, the pH-induced micellization kinetics of the AB4 miktoarm star and AB block copolymers were investigated by the stopped-flow light scattering technique upon a pH jump from 4 to 10. Typical kinetic traces for the micellization of both types of copolymers can be well fitted with double-exponential functions, yielding a fast (tau1) and a slow (tau2) relaxation processes. tau1 for both copolymers decreased with increasing polymer concentration. tau2 was independent of polymer concentration for PNIPAM65-b-(PDEA63)4, whereas it decreased with increasing polymer concentration for PNIPAM70-b-PDEA260. The chain architectural effects on the micellization properties and the underlying mechanisms were discussed in detail.     

Coil-to-globule-type transition of poly(N-isopropylacrylamide) adsorbed on colloidal silica particles

 The temperature dependence of the dimensions of poly(N-isopropylacrylamide) (PNIPAM) adsorbed on two different colloidal silica particles was studied with dynamic light scattering. The hydrodynamic diameter was measured when the temperature was varied stepwise from 10 to 60 °C. PNIPAM molecules free in solution undergo a conformational transition at the θ temperature. We have found that PNIPAM adsorbed onto silica particles also undergoes a transition below the θ temperature. When a small amount of polymer was adsorbed the coil-to-globule transition at the θ temperature did not occur. Potentiometric titrations showed that the surface charge of the silica particles was not affected by the polymer adsorption. Sodium dodecyl sulfate (SDS) (100–1200 mg/l) was added to improve the stability. The particles with a higher zeta potential required a smaller addition of SDS to prevent coagulation compared to the particles with a smaller surface potential. For low additions of SDS the transition curves of adsorbed PNIPAM were unaffected. For larger additions of SDS the collapse of PNIPAM was shifted to higher temperatures. When as much as 1200 mg/l SDS was added, two regions with weak transitions were observed before the collapse. It was also observed that the presence of SDS results in a smaller adsorption of PNIPAM onto the particles. The addition of SDS strongly increased the magnitude of the electrophoretic mobility of the polymer–particle unit. From the electrophoretic measurements an electrokinetic layer thickness was calculated and it was found to be smaller than the corresponding hydrodynamic layer thickness, as obtained by dynamic light scattering. Received: 14 December 1999/In revised form: 22 February 2000/Accepted: 6 March 2000     

Molecular dynamics simulations of PNIPAM‐co‐PEGMA copolymer hydrophilic to hydrophobic transition in NaCl solution

Classical molecular dynamics simulations were carried out to investigate the hydrophilic to hydrophobic transition of PNIPAM‐co‐PEGMA close to its lower critical solution temperature (LCST) in 1 M NaCl solution. PNIPAM‐co‐PEGMA is a copolymer of poly(N‐isopropylacrylamide) (PNIPAM) and poly(ethylene glycol) methacrylate (PEGMA). The copolymer consists of 38 monomer units of NIPAM with two PEGMA chains attached to the PNIAPM backbone. The PNIPAM‐co‐PEGMA was observed to go through the hydrophilic?hydrophobic conformational change for simulations at temperature slightly above its LCST. Na⁺ ions were found to bind strongly and directly with amide O, even more strongly with the O atoms on PEGAMS chains, whereas Cl^? ions only exhibit weak interaction with the polymer. Significantly a novel caged stable metal‐organic complex involving a Na⁺ ion coordinated by six O atoms from the copolymer was observed after the PNIPAM‐co‐PEGMA copolymer went through conformational transition to form a hydrophobic folded structure. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

温度敏感性PNIPAM-b-PZLL-b-mPEG三嵌段共聚物的合成与自组装研究

通过大分子引发剂引发ε-苄氧羰基-L-赖氨酸-N-羧酸酐(Lys-NCA)开环聚合和大分子缩合的方法合成了聚(N-异丙基丙烯酰胺)-b-聚(ε-苄氧羰基-L-赖氨酸)-b-聚乙二醇单甲醚三嵌段共聚物(PNIPAM-b-PZLL-b-mPEG).用GPC和1H-NMR对其结构进行了表征.用芘荧光探针法证明了该三嵌段聚合物形成胶束的性质并测定了临界胶束浓度(CMC).动态光散射(DLS)研究表明,在固定PNIPAM-b-PZLL链段长度的情况下,mPEG分子量为2000时,胶束在温度高于临界溶解温度(LCST)时发生聚集,mPEG分子量为5000时,胶束在LCST以上没有发生聚集.     

Surface interaction forces mediated by poly(N-isopropylacrylamide) (PNIPAM) polymers: effects of concentration and temperature

Total internal reflection microscopy was used to directly measure the interaction potentials between a micron-sized silica sphere and a flat silica surface in the presence of a linear poly(N-isopropylacrylamide) (PNIPAM) aqueous solution. When the PNIPAM concentration was low, no discernible forces were detected. A further increase in PNIPAM concentration resulted in a long-range attraction which was likely due to a combined of the reduced electrostatic interaction between the silica particle and the flat surface after the polymer adsorption and polymer bridges formation. On the other hand, for a fixed PNIPAM concentration, the interaction potential profiles between the particle and flat surface were once again characterized by attraction as temperature was increased. This attractive force can be explained in terms of the conformational changes of PNIPAM chains at the surfaces, which subsequently affected the polymer adsorption and enhanced the segment–segment interaction among the adsorbed polymer chains.     

Temperature response of aqueous solutions of pyrene end‐labeled poly(N‐isopropylacrylamide)s probed by steady‐state and time‐resolved fluorescence

Aqueous solutions of a series of monodisperse poly(N‐isopropylacrylamide)s end‐labeled with n‐butyl‐1‐pyrene at one or both chain ends (Py_n‐PNIPAMs with n = 1 or 2) were studied by turbidimetry, light scattering, and fluorescence. For a given polymer concentration and heating rate, the cloud point (T_c) of an aqueous Py_n‐PNIPAM solution, determined by turbidimetry, was found to increase with the number‐average molecular weight (M_n) of the polymer. The steady‐state fluorescence spectra and time‐resolved fluorescence decays of Py_n‐PNIPAM aqueous solutions were analyzed and all parameters retrieved from these analyses were found to be affected as the solution temperature passed through T_c, the solution cloud point, and T_m, the temperature where dehydration of PNIPAM occurred. The trends obtained by fluorescence to characterize the aqueous Py_n‐PNIPAM solutions as a function of temperature were found to be consistent with the model proposed for telechelic PNIPAM by Koga et al. in 2006. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 308–318     

Synthesis of twin‐tail tadpole‐shaped hydrophilic copolymers and their thermo‐responsive behavior

Twin‐tail tadpole‐shaped hydrophillic copolymers composed of cyclic poly(ethylene gycol) (PEG) and two linear poly(N‐isopropylacrylamide) (PNIPAM) chains have been successfully synthesized by the combination of single‐electron‐transfer living radical polymerization and click chemistry under high concentration. Click cycloaddition reaction occurred between linear PNIPAM‐b‐PEG‐b‐PNIPAM with two azide groups at block junctions and dipropargyl oxalylate with high yield and efficiency. The resulting intermediates and the targeted polymers were characterized by proton nuclear magnetic resonance, fourier transform infrared spectroscopy, and gel permeation chromatography. The thermal phase transition behaviors of twin‐tail tadpole‐shaped polymers and their linear precursors were investigated by temperature‐dependent turbidity measurements, micro differential scanning calorimetry, and laser light scattering. The twin‐tail tadpole‐shaped polymers possess higher critical solution temperature (LCST) and smaller average aggregate size compared with their linear precursors with the same molecular weight. The above differences in the thermal phase transition behaviors should be due to the repulsive forces caused by the ring topology, which prohibited the intermolecular association. © 2009 Wiley Periodicals, © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009     

Effects of end group polarity and molecular weight on the lower critical solution temperature of poly(N‐isopropylacrylamide)

The lower critical solution temperatures (LCSTs) for mass fractionated samples of poly(N‐isopropylacrylamide) (PNIPAM) were studied to determine the effect of polymer molecular weight on the LCST using a high throughput temperature gradient apparatus. PNIPAM fractions prepared by a conventional radical polymerization using azoisobutyronitrile (AIBN) as the initiator had LCSTs that were largely invariant with molecular weight or dispersity. Only slight deviations were noted with lower molecular weight samples. An 18‐kDa sample had a 0.6 °C higher LCST. A 56‐kDa sample had a 0.2 °C higher LCST. PNIPAM derivatives prepared with a triphenylmethyl (trityl) functionalized azo initiator were also prepared and mass fractionated. These samples' LCSTs were identical to those of PNIPAM samples prepared using AIBN initiation when higher molecular weight samples were compared. The trityl‐containing PNIPAM fractions' LCSTs varied when the molecular weight decreased below 100 kDa. Acidolysis of the trityl end groups provided a third set of PNIPAM derivatives whose LCST differed only with samples with M_w values < 60 kDa. These results show there is no effect of molecular weight on LCST until the degree of polymerization is such that end group structure becomes significant. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1492–1501, 2006     

Two phase transitions of poly(N-isopropylacrylamide) brushes bound to gold nanoparticles

The thermally induced phase transition of the poly(N-isopropylacrylamide) (PNIPAM) brush covalently bound to the surface of the gold nanoparticles was studied using high-sensitivity microcalorimetry. Two types of PNIPAM monolayer protected clusters (MPCs) of gold nanoparticles were employed, denoted as the cumyl- and the cpa-PNIPAM MPCs, bearing either a phenylpropyl end group or a carboxyl end group on each PNIPAM chain, respectively. The PNIPAM chains of both MPCs exhibit two separate transition endotherms; i.e., the first transition with a sharp and narrow endothermic peak occurs at lower temperature, while the second one with a broader peak occurs at higher temperature. With increase of the MPC concentration, the transition temperature corresponding to the first peak only slightly changes but the second transition temperature strongly shifts to lower temperature. The calorimetric enthalpy change in the first transition is much smaller than that in the second transition. The ratio of the calorimetric enthalpy change to the van't Hoff enthalpy change indicates that in the first transition PNIPAM segments show much higher cooperativity than in the second one. The investigation of pH dependence of two-phase transitions further indicates the PNIPAM brush reveals two separate transitions even with a change in interchain/interparticle association. The observations are tentatively rationalized by assuming that the PNIPAM brush can be subdivided into two zones, the inner zone and the outer zone. In the inner zone, the PNIPAM segments are close to the gold surface, densely packed, less hydrated, and undergo the first transition. In the outer zone, on the other hand, the PNIPAM segments are looser and more hydrated, adopt a restricted random coil conformation, and show a phase transition, which is dependent on both concentration of MPC and the chemical nature of the end groups of the PNIPAM chains. Aggregation of the particles, which may also affect the phase transition, is briefly discussed.     

New insights into the effects of molecular weight and end group on the temperature-induced phase transition of poly(N-isopropylacrylamide) in water

In an attempt to clarify issues related to the molecular weight dependence of the phase transition of poly(N-isopropylacrylamide) (PNIPAM) in water,we prepared a library of PNIPAM samples of well-controlled molecular weight (7000 to 45000 g/mol) bearing identical groups on each chain end.The polymers were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) with a bifunctional chain tranfer agent and further end group modification.The effects of the end group chemical structure,hydroxyethyl (HE),propargyl (Pr),chloroethyl (CE),n-butyl (nBu),n-hexyl (nHe),and isobutylsulfanylthiosulfanyl (IBS) on the phase transition temperature of aqueous PNIPAM solutions were investigated by high-sensitivity differential scanning calorimetry (HS-DSC),yielding the enthalpy ΔH and the endotherm maximum temperature (T M),and turbidimetry,providing the cloud point (T CP) of each solution.The T CP and T M of the PNIPAM sample of lowest molar mass (M n 7,000 g/mol,0.5 g/L) ranged,respectively,from 38.8 to 22.5 °C and 42.2 to 26.0 °C,depending on the structure of the end-group,whereas H showed no strong end-group dependence.The phase transition of all polymers,except,-di(n-butyl-PNIPAM),exhibited a marked dependence on the polymer molar mass.