Carboxyl-ended polymers

The N-to-C assembly of the peptide chain is unfavorable for the chemical synthesis of peptides on solid supports. This strategy can be dismissed already for the single reason that repeated activation of the carboxyl ends on the growing peptide chain would lead to a much higher percentage of racemization. Several other more practical disadvantages also tend to disfavor this approach, and acid activation on the polymer support is usually only used in one-step fragment condensations (p. 241). 

Huffman, K. R., and Casey, D. J., Effect of Carboxylic end groups on hydrolysis of polyglycolic acid, J. Polym. Sci. 

The carboxyl end can also thermally degrade to form vinyl end groups [22-24], which occur along with the formation of acetaldehyde (Figure 11). This is a side reaction that needs to be avoided, as acetaldehyde can taint mineral water and carbonated soft drinks if present at high levels (>20ppm) in the bottle polymer. Hence lower temperatures (around 160°C) are used in the SSP phase to minimise the formation of acetaldehyde. 

Esterification is the first step in PET synthesis but also occurs during melt-phase polycondensation, SSP, and extrusion processes due to the significant formation of carboxyl end groups by polymer degradation. As an equilibrium reaction, esterification is always accompanied by the reverse reaction being hydrolysis. In industrial esterification reactors, esterification and transesterification proceed simultaneously, and thus a complex reaction scheme with parallel and serial equilibrium reactions has to be considered. In addition, the esterification process involves three phases, i.e. solid TPA, a homogeneous liquid phase and the gas phase. The respective phase equilibria will be discussed below in Section 3.1. 

The monomers TPA and EG are mixed upstream to the esterification reactor in a jacketed slurry preparation unit equipped with a stirrer for highly viscous fluids (e.g. Intermig ). The typical molar ratio of EG to TPA lies between 1.1 and 1.3. The esterification temperature and the molar ratio of monomers are the main controlling factors for the average degree of polycondensation of the esterification product (prepolymer), as well as for its content of carboxyl end groups and DEG. The latter mainly occurs as randomly distributed units of the polymer molecules. 

Figure 5.23 Variation in the concentration of carboxylic end groups and intrinsic viscosity during the postcondensation of PET powder produced from DMT (1) and TPA (2) prepolymers (7, 240 °C) [49]. From Gerking, L., Modifications of fiber properties by polymer and within spinning line, presentation (Paper 52b) given at the 32nd International Man-Made Fibre Congress, 22-24 September, 1993, Dornbirn, Austria, and reproduced with permission of EMS Inventa-Fischer, GmbH Co. KG...

A very important factor of bottle polymer is its thermal stability, which depends on the conditions of its manufacture and the thermal history of the polymer. The amount of carboxylic end groups (CEGs) is a good indicator of the qualification of the chips. Continuously produced polymer should contain no more than 25 eq/kg CEG. Little differences between the TPA and DMT routes towards bottle polymers are observed. Chips from batch processes show higher CEG values (30meq/kg and more). The thermal stability depends on the use and efficiency, i.e. mainly the concentration, of stabilizers. 

Note 2 The term halatopolymer is used for a linear polymer formed by the coupling of halato-telechelic polymer molecules, for example, for the linear polymer formed by the coupling of carboxylate end-groups with divalent metal cations [2]. 

Termination may also occur by chain transfer with the initiator (e.g., water or alcohol) or a deliberately added chain-transfer agent. Deliberate termination of growth is carried out to produce polymers with specific molecular weights or, more often, telechelic polymers with specific end groups. Hydroxyl and amine end groups are obtained by using water and ammonia as chain-transfer agents. Carboxyl-ended telechelics can be obtained by termination with ketene silyl acetal followed by hydrolysis with base [Kobayashi et al., 1989]. 

Amidines are formed in hydrolytic polymerizations of lactams but do not limit the polymer molecular weight. Molecular weight buildup is not impeded since the carboxyl end groups of growing polymer are quite reactive toward amidine groups [Bertalan et al., 1984]. 

The polydispersity of polymers prepared in this way is usually very low for example, a value MJM of 1.05 was found for a sample of poly(a-methylsty-rene). Living polymers can also be used for the preparation of block copolymers after the consumption of the first monomer, a second anionically polymerizable monomer is added which then grows onto both ends of the initially formed block. By termination of the living polymer with electrophilic compounds the polymer chains can be provided with specific end groups for example, living polystyrene reacts with carbon dioxide to give polystyrene with carboxylic end groups. 

Living polymers usually require special reagents to achieve termination. Any electrophilic reagent that reacts with the carbanion active center and also allows preparation of polymers with desired terminal functionalities can be used for this purpose.168,174,181 Hydrogen-terminated polymers can be produced by proton donors, whereas carbon dioxide results in a carboxylate end group. Terminal alcohol functionalities can be formed through reaction with ethylene oxide and carbonyl compounds. 

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