Transamidation, polyamides

Transamidation, polyamide, 158 Transesterification, 529-530 Transesterification polymerizations, 69-74 Transimidization, 302-303 Transition metal coupling, 10, 467-523 applications for, 472-476 chemistry and analytic techniques for, 483-490... 

Transamidation and Transesteramidation. Transamidation is the mutual exchange of chain fragments in a polyamide, shown as follows where R, R" and R, R" represent polymer chain fragments of any length. 

Transamidation is an important process in the melt phase for polyamides because it is usually the process by which an equiUbrium molecular weight distribution is reestabUshed and, in the case of the melt blending of two or more polyamides to form a copolymer, it is the process by which randomi2ation of the individual monomers along the chain is effected. In the soHd phase, chain mobiUty is restricted and equiUbrium in either case often is not achieved. 

Transesteramidation is a process similar to transamidation, except that a polyamide is mixed with a polyester rather than another polyamide (67). 

The molecular weight increases with increasing conversion. Regulation of the molecular weight can be achieved by adding small amounts of substances (e.g., benzoic acid) that can react with the polyamide chains by transamidation. Because of the transamidation reaction and hydrolysis of amide bonds, an equilibrium molecular-weight distribution is finally attained (see Sect. 4.1). 

Many combinations of diacids—diamines and amino acids are recognized as isomorphic pairs (184), for example, adipic acid and terephthalic acid or 6-aminohexanoic acid and 4-aminocyclohexylacetic acid. In the type AABB copolymers the effect is dependent on the structure of the other comonomer forming the polyamide that is, adipic and terephthalic acids form an isomorphic pair with any of the linear, aliphatic C-6—C-12 diamines but not with -xylylenediamine (185). It is also possible to form nonrandom combinations of two polymers, eg, physical mixtures or blends (Fig. 10), block copolymers, and strictly alternating (187—188) or sequentially ordered copolymers (189), which show a variation in properties with composition differing from those of the random copolymer. Such combinations require care in their preparation and processing to maintain their nonrandom structure, because transamidation introduces significant randomization in a short time above the melting point. 

CycUc polyamides were reported to be isolated from Nylon 6 polymers in 1956 [18,19]. Thermal polycondensation of co-amino acid (carbon number > 6) gave a cycUc and linear polymer [82]. Moreover, upon heating polyamide in the presence of a transamidation catalyst, the cyclization equilibrium is eventually reached, and both Unear and cyclic constituents are present [83]. The proportion of the latter depends on the concentration, and cycUc compounds predominate in high dilute solutions. 

Examples of copolymer formation by redistribution reactions in PA/PA blends are given in Table 5.5. In related work Liu and Donovan [1995] failed to find evidence for transamidation in PA-6 blends with an aromatic polyamide during molding and aimealing. 

Generally, the initiators activate the inactive amide groups, causing them to react with other lactams through successive transamidations that result in formations of polyamides. Both acids and bases catalyze the transamidation reactions. The additions of electrophiles affect increases in the electrophilicity of the carbonyl caibon of the acylating lactam. The nucleophiles, on the other hand, increase the nucleophilic character of the lactam substrate (if they are bases). 

Examples of copolymer formation by redistribution reactions (sometimes referred to as transreactions) in PA/PA blends are given in Table 5.8. In related work, Liu and Donovan (1995) failed to find evidence for transamidation in PA-6 blends with an aromatic polyamide during molding and annealing. Aspects of transreactions in PA/PA blends have been described in Eersels et al. (1999) and in portions of other chapters included in Fakirov (1999) (transreactions in condensation polymers). 

The reaction of an initiator with an amide group of the polymer chain, instead of that of the monomer, may result in the breaking of the macromolecule. Intermolecular transamidations can also modify the length of the polyamide chains and may generate cyclic stmctures. These exchange reactions result in a modification of the molar mass distribution. 

P-lactams. Monomers having a low rate constant of propagation are preferable for the synthesis of uniform polyamides. Drawbacks in the preparation of uniform polyamides from P-lactams are the very frequent occurrence of depolymerization and transamidation (at the acyl lactam chain end and on the polyamide chain), both causing broadening of molar mass distribution of the resultant polyamide. The latter reaction occurs more frequentiy at high conversion. Therefore, it is advisable to stop the polymerization at low to intermediate... 

Note 2 Transamidations commonly occur during polycondensations to form polyamides. 

For instance, the synthesis of segmented poly(ether ester amide) copolymers with an aromatic polyamide block was attempted in N,N-dimethylacetamide solution, using a dihydroxy-terminated poly(oxyethylene) oligomer, >phenyl-enediamine, and isophthaloyl chloride [30]. Unfortunately, only low viscosity materials were obtained, probably due to side reactions involving the amide solvent (transamidation and acylation). 

H2N< C00H Polyamides 1. Amine (PA) + anhydride or acid (modifier) 2. Amine/carboxyl (PA) + epoxy (modifier) 3. Amine/carboxyl (PA) + isocyanate (modifier) 4. Carboxyl (PA) + oxazoline (modifier) 5. Carboxyl (PA) + nitrile (modifier) 6. Transamidation reactions (between PA and usually diamine type modifier)... 

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