Hydroxyl chain

Commercial end functional polymers have been converted to alkoxyamincs and used to prepare PKO-Worri-PS.040 The hydroxyl group of alkoxyamine 284 was used to initiate ring-opening polymerization of caprolactonc catalyzed by aluminum tris(isopropoxide) and the product subsequently was used to initiate S polymerization by NMP thus forming polycaprolactone-Wodr- P8.641 The alternate strategy of forming PS by NMP and using the hydroxyl chain end of the product to initiate polymerization of caprolactonc was also used. 

The described procedure has been widely used by Smith III and coworkers [250] in the efficient total synthesis of natural products containing extended 1,3-hydroxylated chains. This architecture is often found as a structural element in polyene macrolide antibiotics [251] such as mycotoxin A and B, dermostatin, and roxaticin. The Smith group used the above-mentioned approach (e. g., as five-component coupling) for the synthesis of the pseudo-C2-symmetric trisacetonide (+)-2-471 [252], which was employed by Schreiber and coworkers [253] within the synthesis of (+)-mycotoxin A (2-470a) (Scheme 2.108). Thus, lithiation of 2.5 equiv. dithiane 2-462b followed by treat-... 

Preparation of poly(7V-vinyl-2-pyrrolidone) containing a hydroxyl chain terminus... 

Hence, the most plausible explanation of our FTIR observations of the simultaneous appearance of hydroxyl, carbonyl and ether groups upon TGDDM epoxide consumption is epoxide isomerization and/or oxidation followed by epoxide-hydroxyl chain extension reactions. 

The samples in which the Si02 was introduced in situ have higher decomposition temperatures. A possible mechanism for this improvement would be increased capability of the in situ produced silica to tie up hydroxyl chain ends that participate in the degradation reaction. 

The relative number of immobilised adsorbed chain units in a single type of silica is proportional to Mn 0 5 [117]. The fraction of immobilised chain units is larger in hydroxyl-terminated PDMS [117]. This suggests that the majority of hydroxyl chain ends are linked to the silica surface due to the formation of double hydrogen bonds. 

A particularly interesting block copolymer made by the coupling approach was a multi-block copolymer of PMPS and poly (ethylene oxide) (PEO). This was prepared by reacting the Si— X chain ends of PMPS with the hydroxyl chain ends of well-defined commercial sample of poly(ethylene glycol) [47] (Fig. 3). Allhough... 

Esterification of Hydroxyl End Groups The reaction of hydroxyl chain ends with anhydride... 

IR spectra of ammonia adsorbed on oxovanadium species supported on porous sihca are shown in Figure 2.33. Two kinds of interactions with hydroxyl groups are evident (282). The broad band at about 1470 cm corresponds to the bending vibrations of ammonium ions formed by reaction with addic vanadia hydroxyls. The band at 1635 cm is ascribed to the bending mode of ammonia molecules H-bonded to silanols. Simultaneously, a shift of the OH stretching modes of the silanol groups (external silanols at 3745 cm and terminal and H-bonded silanols in hydroxyl chains at 3710 and 3547 cm, respectively) by about —800 cm is observed. The bands at 3390 and 3330 cm are assigned to N-H modes. 

The band at 3715 cm is attributed to hydroxyls acting as H-donors, while the band at 3510 cm to hydroxyls acting as H-acceptors. Some authors support assignment of the 3510 cm band to Si—OH pairs (646), whereas others have broadened the assignment to include H-bonded hydroxyl chains (614). 

Lactide is produced by degradation reactions, mainly via intramolecular chain scission of the prepolymer. Lactide synthesis from a prepolymer with a DP in the range of 10-15 in the presence of various catalysts at 4-5 mbar and 190-245°C is reported by Noda and Okuyama [9]. The best performances were reported using 0.05-0.2 wt% tin catalysts and tin octoate (stannous 2-ethylhexanoate) in particular, which is widely available. The catalyst increases the rate of backbiting reactions from hydroxyl chain ends of prepolymers to form lactide molecules [9, 15]. The melt viscosity of the prepolymer increases because of the esterification reactions during the process, which results in decreased rate of mass transfer. 

PEN blends, the hydroxyl chain ends of PET were capped with benzoyl chloride [66]. The end-capping of the hydroxyl chain ends seems to influence the mechanism and the kinetic of transesterification. The end capped copolymers showed a higher degradation stability as the unmodified PET and in blends with PEN, an efficient slowing of transesterification occurs. 

As evidenced by laboratory studies, this product may then relatively easily cyclise to form tetrahydrofuran (THF). Some workers have also suggested that THF could be formed directly from the hydroxyl chain end via a cyclic intermediate. 

This led to a suggestion by Maerov [20] that a key step in condensation may involve a chemical reaction that ties up a hydroxyl chain end with the catalyst molecule. Introduction of a second hydroxyethoxy chain end is followed by the right electronic bond shift ... 

A comparative study of thermal stabilities involving PDMS-silica composites showed that commercial fume sihca caused more severe degradation problem than did in situ precipitated silica [61], This improvement in the case of in situ precipitated silica could be due to the increased capability of sihca introduced in situ to tie up hydroxyl chain ends [3,61],... 

Cohen-Addad, J. P. Ebengou, R., Silica-Siloxane Mixtures. Investigations into Adsorption Properties of End-Methylated and End-Hydroxylated Chains. Polymer 1992, 33,379-383. 

RS)-BL with distannoxanes as catalyst in the presence of 1,4-butanediol as initiator gave optically active poly [(R)-BL] or atactic poly [(RS )-BL] with secondary hydroxyl chain ends and oxytetramethylene units in the backbone. These polymers may be used to initiate the copolymerization of LA at the chain ends and form block copolymers. The optically active P[(7 )-BL] is brittle whereas atactic P[(R5 )-BL] is elastomeric in nature and has been used to prepare elastomeric polymers with brittle PLLA. However, Sn(IV) compounds are active transesterification catalysts and may cause unit scrambling when LLA is used as a monomer. 

To clarify the effects of chain-end structures of PLA, Lee et al. [34] synthesized C1-, NH2-, and COOH-terminated PLLAs from OH-terminated PLLAs. The thermal stability of OH-terminated PLLAs was poor, whereas NH2- and Cl-terminated PLLAs were more resistant to thermal degradation. The main mechanisms of PLLA thermal degradation are transesterification and backbiting reactions that cause random degradation and unzipping depolymerization, respectively, starting from the carboxyl and/or hydroxyl chain ends [7, 8, 10, 29, 49]. 

Figure 4-5. (a) Schematic example of isolated and terminal hydroxyl groups, (b) H-bonded hydroxyl chain example ofsilanol condensation with elimination of water andformation of isolated hydroxyls on the silica film surfaces. (Reprinted from Journal of Non-Crystalline Solids, 316, P. Innocenzi, Infrared spectroscopy of solr-gel derived silica-based films a spectra-microstructure overview , p. 318, Copyright (2003), with permission from Elsevier.)... 

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