Hydroxyl capping

Macroinitiators for ATRP can be produced from polymers obtained from non-ATRP polymerization by appropriate conversion of their end groups. For example, hydroxyl-capped polymers obtained by cationic or anionic chain polymerization or even step polymerization can be esterihed with 2-bromopropanoyl bromide to yield Br-capped polymers that are ATRP macroinitiators. 

Here we report the results of using a series of hydroxyl-capped poly(ethylene glycols), PEGs. These PEG can be considered to be of the form below. 

The American Chemical Society) (b) Reaction sequence for the transformation of RAFT-made polymer into hydroperoxy terminal species (which can be readily reduced to hydroxyl-capped polymers) via the addition of molecular oxygen to the freed macroradicals. (Reproduced with permission from Ref [45aj 2009, CSIRO Publishing)... 

Imuta, J. Kashiwa, N. Toda, Y. Catalytic regioselective introduction of aUyl alcohol into nonpolar polyolefins Development of one-pot synthesis of hydroxyl-capped polyolefins mediated by a new metallocene IF catalyst. J. Am. Chem. Soc. 2002,124, 1176-1177. 

Gas chromatography (gc) has been used extensively to analyze phenoHc resins for unreacted phenol monomer as weU as certain two- and three-ring constituents in both novolak and resole resins (61). It is also used in monitoring the production processes of the monomers, eg, when phenol is alkylated with isobutylene to produce butylphenol. Usually, the phenoHc hydroxyl must be derivatized before analysis to provide a more volatile compound. The gc analysis of complex systems, such as resoles, provides distinct resolution of over 20 one- and two-ring compounds having various degrees of methylolation. In some cases, hemiformals may be detected if they have been properly capped (53). 

The preceding reactions do not occur if the terminal hydroxyl group in the polymer is no longer present, eg, if it has been end-capped by acetylation. 

Tirrell et al. [42,43] studied the role of interfacial chains in a more detailed fashion. Tirrell et al. [42,43] used a crosslinked PDMS cap in contact with a silicon wafer on to which a,o)-hydroxyl terminated PDMS chains are tethered by adsorption from a solution. The molecular weight of the narrow disperse PDMS samples was in the range of 20,000-700,000. The surface chain density was given by27 yj g e 0 is the volume fraction of PDMS in solution. 

A. Preparation of TMS Derivatives Add 0.25 ml of TRI-SIL TBT reagent (the only reagent we found to react with the hydroxyl group in position 17) and 0.25 ml of pyridine to the dried extract. Cap tightly and heat at 60° for 1-12 hours or overnight. 

The brush phases are made from the mono-chlorsilanes or the mono-alkoxysilanes which attach a single organic moiety to each reacted hydroxyl group. For example using octyldimethylchlorsilane, dimethyloctyl chains would be attached to the surface like bristles of a brush, hence the term brush phase. In all bonding processes, some hydroxyl groups remain unreacted which would allow the polar character of the silica play a part in retention and also render the surface liable to dissolution in water and dilute acids. For this reason the material is capped by treatment with either trimethylchlorsilane or hexamethyldisilazane to eliminate any unreacted or stearically... 

Hydrogenolysis. Hydrogenation of polycyclopropanone was carried out using supported catalysts. Before hydrogenation was attempted the polymers were end-capped in a refluxing mixture of acetic anhydride and pyridine. This was done to convert thermally unstable hydroxyl end groups, which may have been present in the polymer, to more thermally stable acetyl end groups as shown in Equation 8. This was... 

Di-hydroxyl end-capped PPG (18) is an intermediate in the formation of a common polyurethane prepolymer (20). End group functionality of this intermediate is important, as this hydroxyl functionality is modified to form the prepolymer. Any different end group structures could lead to the presence of prepolymer that will not form polyurethane of the desired structure. The desired reaction of the intermediate (18) to form the prepolymer (20) is described in Figure 21. Reaction of one unit of the intermediate (18) and two units of methylene diphenyl 4,4 -diisocyanate (MDI) results in the formation (nominally) of the prepolymer (20). 

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