PNIPAM polymers

Scheme 3.15 Au NPs synthesized on template core-shell particles based on GMA and PNIPAM polymers and functionalized with thiol groups.

Fig. 4. Phase separation temperature ( ) before and (O) after UV irradiation of the aqueous solutions of PNIPAM polymers containing leuconitrile (LN) groups...

To complement this work, 92, a rhodium complex attached to a monophosphine PNIPAM polymer support, has also been prepared and screened " ... 

Hydroalkoxylation of alkynes, or the addition of alcohol to alkynes, is a fundamental reaction in organic chemistry that allows the preparation of enol ethers and a variety of oxygen-containing heterocycles such as furan, pyran, and benzofuran derivatives. Bergbreiter et al. found that a Mnear poly-(A-isopropylacrylamide) (PNIPAM) polymer exhibited inverse temperature solubility in water (i.e., soluble in cold water but insoluble in hot water). A recoverable homogeneous palladium catalyst was prepared based on the polymer. The PNIPAM-bound Pd(0) catalyst was effective for the reaction of 2-iodophenol with phenylacetylene in aqueous THE media to give the target product... 

Plasmid analysis is often used in recombinant DNA technology. Anew separation matrix, consisting of poly(A-isopropylacrylamide) (PNIPAM) polymer and mannitol as small molecule additive, was used for CE-based plasmid DNA separation. Supercoiled, linear, and nicked conformers of lambda plasmid were separated in 1% PNIPAM + 6% mannitol. The effect of the applied mannitol concentration on the separation quality is shown in Figure 6.6 [117]. 

Thiol-ene coupling was also used for the end-functionaHzation of PNIPAM in an analogous study conducted by the groups of Lowe and Hoyle [55]. In this case, PNIPAM was prepared by RAFT polymerization, followed by the aminolysis of the thiocarbonylthio end group to produce a terminal thiol simultaneous Michael addition with aUyl methacrylate yielded the alkene mono end-functional PNIPAM. The polymer terminus was then modified via a radical thiol-ene dick reaction with three different mercaptans to afford a series of PNIPAM polymers with varied LCSTs (Scheme 30.5). 

Certain Af-substituted acrylamide polymers exhibit phase separation characteristics with associated changes in their properties upon heating above a certain lower critical solution temperature (LCST) [38 0]. Polymers based on Al-isopropyl acrylamide (NIPAM) are the best-known examples. The homopolymer has an LCST of 32°C in aqueous solution [41]. NIPAM can also be polymerized with a wide variety of comonomers, and with the appropriate choice of comonomer, the LCST can be controlled to near physiological temperatures [42,43]. Poly(Af-isopropylacrylamide) (PNIPAM) polymers have therefore recently been investigated for use in drug delivery [44,45], biomolecule separation [46], and tissue engineering [47] applications. 

Similar to the fluorous biphasic concept, a system was developed that formed a homogenous phase at elevated temperature, but phase-separated at room temperature [49c]. It was found that a mixture of functionalized PNIPAM polymer, ethanol, heptane and water exhibited these properties. The hydrogenation of 1-octadecene and 1-dodecene using a phosphine functionalized PNIPAM with a rhodium precursor were taken as test reactions and the high activity was foimd was similar to that of RhCl(PPh3)3. At room temperature the mixture phase separated and the catalysis stopped since the catalyst is completely insoluble in heptane. The substrate is dissolved in the heptane allowing a facile catalyst/product separation without the loss of activity. The concept is obviously limited to substrates that show... 

Tagit and coworkers developed surfaces which can act as nano-thermometers using quantum dots (QDs) attached to PNIPAM polymer brushes grafted onto a gold substrate. The luminescence of these QDs could be quenched by increasing the temperature above the LCST of PNIPAM, and recovered by decreasing it (Tagit, 2009). 

It turns out that in solutions of c < 0.1 gL 1 thermosensitive homopolymers, such as PNIPAM, PVCL, and PVME, themselves, form stable colloids in water at elevated temperature in the absence of additives or chemical modification [141-147]. The colloids remain stable upon prolonged heat treatment, without detectable aggregation or precipitation. Also, core-shell particles consisting of PNIPAM and a hydrophobic block are stable not only below but also above the LCST up to 50 °C, when the PNIPAM block is expected to be insoluble [185]. Factors that determine the colloidal stability as defined in Sect. 1.1 do not explain, it seems, their stability. In this review we have compiled a fist of all the reported instances where the formation of stable particles was detected in aqueous solutions of neutral thermosensitive neutral polymers at elevated temperature. We present studies of homopolymers, as well as their copolymers consisting of thermosensitive fragments and ei-... 

Turbidimetry is ideally suited to detect the temperature at which a transparent polymer solution turns opaque. The temperature corresponding to the onset of the increase of the scattered light intensity is usually taken as the cloud-point temperature, TcP, although some authors define the cloud point as the temperature for which the transmittance is 80% (or 90%) of the initial value. This technique is commonly known as the cloud-point method [199]. Turbidimetry was employed, for instance, to show that the cloud-point temperature of aqueous PNIPAM solutions does not depend significantly on the molar mass of the polymer [150]. 

Kujawa and Winnik [209] reported recently that other volumetric properties of dilute PNIPAM solutions can be derived easily from pressure perturbation calorimetry (PPC), a technique that measures the heat absorbed or released by a solution owing to a sudden pressure change at constant temperature. This heat can be used to calculate the coefficient of thermal expansion of the solute and its temperature dependence. These data can be exploited to obtain the changes in the volume of the solvation layer around a polymer chain before and after a phase transition [210], as discussed in more detail in the case of PVCL in Sect. 3.2.2. 

In all cases, the cloud-point temperature was slightly dependent on polymer concentration for a given copolymer it increased with decreasing concentration. This effect is enhanced with increasing number of PEO grafts per chain. Also, the PNIPAM collapse seemed to be less abrupt with decreasing concentration. Upon dilution of the solution the distance between polymer chains increases, which favours intrapolymeric interactions over in-terpolymeric attractions. Dilution also enhances the surface stabilisation of the polymer particles by PEO. 

Recent theoretical studies suggest that copolymers consisting of structural units with different solubilities may remember the conformations they adopted during their synthesis [257-260]. Such polymers, of which PNIPAM-... 

Fig. 6 Formation of an aggregate and the dependence of its average hydrodynamic radius ((Rh ) on temperature. The polymer is PNIPAM-g-PEO-51, c= 1.0gI.. Top model describing the steps for the formation of an aggregate and its shrinking upon slow heating from a 20 °C to b 45 °C and to c 60 °C. (Adapted from Refs. [165,170])...

PNIPAM-co-GMA (Table 1). Thus, different distributions of substituents are possible, in principle. It was of interest to see (1) whether the distribution of the PEO grafts on the PNIPAM main chain influences the thermal properties of the polymer and (2) whether the polymer grafted at elevated temperature adopted the collapsed conformation in which it was synthesised when its aqueous solution was heated. 

Fig. 9 Size distribution of the aggregates at 45 °C. Polymer concentration 1.0 gL l a PNIPAM-g-PEO-6, b PNIPAM-g-PEO-7, c PNIPAM-g-PEO-10 grafted in an organic solvent is used as a reference. (Adapted from Refs. [166,170])...

Fig. 10a Average hydrodynamic radii ((Rh)agg) and b weight-average molar mass ((Mw)agg) of the aggregates in aqueous block copolymer solutions at different polymer concentrations (c given in moles of PNIPAM blocks) at 45 °C a NE-A, b NE-B, c NE-C, d NE-1, e NE-2, and/ NE-3. (Reprinted with permission from Ref. [169] copyright 2002 American Chemical Society)... 

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