Hydroxyl group Capping

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. 

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... 

Figure 14.12 A core/shell polymeric particle made of a hydrophobic polystyrene core that is capped with a hydrophilic polyHEMA shell, which contains numerous hydroxyl groups.

When the ethylene-2-methylene-l,3-dioxepane copolymer was hydrolyzed, it gave oligomers which were capped with a hydroxyl group at one end and a carboxylic acid group at the other. For material with an ester-containing unit content of 2.1 mole-%, the value of n was approxinately 47 and for material with a content of 10.4 mole-%, the value of n was approximately 9. The copolymers with 6 or less mole-% of the ester-containing units had melting [joints in excess of 90°C. 

For example, XI will copolymerize with styrene to give a copolymer containing carbonate groups in the main polymer chain (20). Ifydrolysis gives the oligomeric polystyrene capped with reactive hydroxyl groups (2). 

Commercial polymers of formaldehyde are also produced using cationic polymerization. The polymer is produced by ring opening of trioxane. Since the polyacetal, POM, is not thermally stable, the hydroxyl groups are esterified (capped) by acetic anhydride (structure 5.22). These polymers are also called poly(methylene oxides). The commercial polymer is a... 

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