End groups of polyesters

There are many other methodologies that can be used to measure end groups of polyesters by NMR. The examples above represent methods that we are using in our own work. 

To study the influence of polymer structure, molar mass, and end groups on the performance of these polyesters, 2D LACCC-SEC separations were carried out. The contour map that is most useful for quantitative analysis and interpretation is reproduced in Plate 2. The ordinate is proportional to the LACCC retention of the polyester. Specific end groups of polyester model compounds are also shown as a guideline. These model compounds with various end group and polymer structure characteristics were run under identical conditions... 

Polyesters and polyamides have variable amounts of hydroxy, amine, or carboxylic end groups. Consequently, the chain extenders have to match with the type as well as with the concentrations of these end groups. In Scheme 1, an example is given of a chain extension reaction via carboxylic end groups of polyesters, which will not affect the concentration of the hydroxy groups. 

Ethoxylation of the carboxylic acid end groups of aliphatic polyesters significantly changes the biodegradation rate as well as the crystallinity of these materials (41). 

Unfunctionalized, i.e., 2-hydroxypropylamide functional, hyperbranched polyesteramides have been tested in powder coating formulations together with stoichiometric (OH/COOH) amounts of acidic polyesters (Uralac). It was anticipated that the reaction of the hydroxyl end groups of the polyesteramide with the carboxylic acid end groups of the polyester would provide a well-crosslinked film with good mechanical properties by polymer/polymer cure. 

It is known from the literature [12] that aliphatic acids are less reactive towards 2-hydroxyalkylamide groups than aromatic ones. Since the final network formation with the polyesteramides involves predominantly aliphatic acid groups, this could explain (in part) why the gel times found were longer than the commercial 2-hydroxyalkylamide crosslinkers, which react directly by esterification alone with the aromatic acid end groups of the polyester. 

The principle of dick chemistrf presented by Hawker et al. (see Section 2.3.5) offers an efficient and versatile method of functionalisation. For example, it utilises [3+2]-cycloaddition of azide-functionalised reagents with ethyne end groups of a dendrimer precursor to prepare dendrimers with triazole-functionalised end groups. The mild reaction conditions, almost quantitative reaction, and not least the tolerance towards numerous functional groups permit the use of widely differing molecular frameworks (e.g. poly(benzyl ether), POPAM dendrimer structures or hyperbranched polyesters) and different functionalised azides... 

Methods for determining carboxyl end-groups and hydroxyl end-groups in polyesters have been described in detail (77). Estimation of carboxyl end-groups by hydrazinolysis has been described by Zahn et al. (78). These authors also describe methods for analysis of comonomers in polyesters. Infrared spectroscopy has also been used for measuring end-group concentrations (108). DP determinations can be made from viscosity measurements in o-chlorophenol (109) or in phenoltetrachloro-ethane solution (50 50) (110). At times it may be sufficient to express results simply as relative viscosities (101). 

Table II. Quantitative Analysis of Various End Groups of Adipic Acid-Hexane Glycol Polyesters as Determined by Critical Chromatography...

It was noted that the efficiency is increased in the presence of amines (Scott, 1993b), so the effectiveness in conjunction with aromatic amine free-radical (CB-D) stabilizers or the amine end groups of nylon would be expected. It is also noted that cuprous salts are often used as the iodide, and the effective alkyl-scavenging ability of iodo compounds has been demonstrated (Henman, 1979). Copper salts are also effective in the melt stabilization of polyesters, but in this case the effectiveness is improved by using a hindered phenol antioxidant. This performance is in contrast to the strong prodegradant effect of copper and other transition metals on the polyolefins as discussed later. 

To bring the nanocontainer to a specific place where it should release its pay-load, targeting is a required approach. Hence, much work has been carried out to attach ligands or antibodies to the hydroxyl end-group of PEG-based assemblies [150,181,243], Biotinylated nondegradable block copolymer assemblies have been shown to attach to surfaces coated with the biotin receptor avidin [146,147, 150,244], Coupling chemistry has been applied to conjugate either an antihuman IgG, or antihuman serum to PEG-carbonate- or PEG-polyester-assembled polymer vesicles [149,245], HIV-derived Tat peptide attached to PEG-PBD polymersomes enhanced the cellular delivery of nanoparticles [246] and increased dendritic cell uptake in vitro [181]. 

The reaction between the epoxy and the end groups of the polyester was modeled by using l,2-epoxy-3-phenoxypropane and n-octanoic acid. An equimolar solution of the two was prepared in tetrahydrofuran (THF) with a catalytic amount of pyridine a few drops of this solution were placed on a NaCl window and the THF allowed to evaporate. The reaction was carried out at 100 °C, and the IR spectrum of the mixture was recorded every 15 min. Similar experiments were performed without a catalyst. The reaction between... 

IR data from the model compounds indicate that the amino end groups of the ATBN can react with the double bond in the polyester. In fact, the 1645 cm-1 absorption due to the double bond almost disappeared after 40 min at 100 °C, and the absorption due to the amino groups (N-H) also diminished considerably (Figure 1). A Michael-type addition reaction between the secondary amino hydrogen of piperazine derivatives and the activated double bonds of diethylfumarate in the polyester is known (5, 6). 

Similar to enol esters, oximes can also be used as irreversible acyl transfer agents for lipase catalysis. Thus, instead of a di-enol ester, Athavale et al. [67] polymerized diols with bis(2,3-butane dione monoxime) alkanedioate using Lipozyme IM-20. The results obtained by activation with enol-esters and their corresponding oximes were comparable. No attempts were made to analyze the end-group of the polyester. 

End-group an ysis has been applied mainly to condensation polymers, since these polymers by their very nature have reactive functional end groups. The end groups are often acidic or basic in nature, as exemplified by the carboxylic groups of polyesters or the amine groups of polyamides such groups are conveniently estimated by titration. From the experimental data M is derived according to... 

As shown in Table 5.35, blends of immiscible polyesters may be compatibilized through copolymer formation mediated by addition of a phosphite condensing agent. Block copolymer results when the phosphite-activated end-group of one PEST reacts with a nucleophilic end-group of another PEST. The reaction takes place at the phase interface. A secondary phosphite is a byproduct. The relative proportions of copolymer vs. simple chain-extended PEST may depend upon the relative solubihty of condensing agent in each of the immiscible polymer phases. 

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