Chemistry Reference
In-Depth Information
In discussions, the author was confronted with the following objection. When
the linear chains and their end groups vanish, the rate of equilibration slows down
and finally vanishes, the resulting 100 % cycles do not equilibrate anymore, and
thus, are outside the JS theory. For three reasons this argumentation is not helpful.
First, when a polycondensation in bulk with 99.999999 conversions is considered,
end groups and equilibration still exist. Now the question is in principle the same
as for 100 % conversion. Does the reaction mixture mainly (in terms of weight
fraction) consist of rings, or is their weight fraction in the range of 2.4-2.5 % as
calculated by J ? S and Flory. Second, this objection also concerns polyconden-
sations below the critical conversion, where J ? S have admitted as part of their
theory that 100 % cycles will be formed at p = 1. Unfortunately, neither J ? S
nor Flory explained if the rings formed at p = 1 are in equilibrium with each other
or not. Third, not all ring-ring equilibria require end groups as catalyst. It is well-
known from NMR experiments [
100
] that tributyltin alkoxides rapidly exchange
their alkoxide groups via strong DA-interactions (Formula
5.1
). This exchange
also occurs (for electronic and steric reasons even more rapidly) in the case of
dibutyltin alkoxides. Therefore, polycondensations of dibutyltin derivatives with
1,2-ethanadiol or 1,3-propanediol yield the cyclic dimers (see Formula
5.1
), but
the cyclic monomer with 1,4-butanediol, regardless which derivatives and which
reaction conditions are used [
101
-
103
]. The thermodynamically most stable
products are formed under all circumstances. Strong DA-interactions are also
known from other metal alkoxides, so that the tin alkoxides are not exceptions.
Finally, it should be mentioned that J ? S garnished their statement about the BE
phenomenon with a footnote (labeled 16) saying: ''A trivial case, the condensation
of the entire system into a single giant ring is possible at all concentrations''. This
statement is pretty strange, because the formation of a single giant ring is absolutely
unlikely from a kinetic point of view and impossible when equilibration occurs.
In summary, it remains to say that TC polycondensations are not fully understood
as long as the problem of the crIMC phenomenon is not definitely solved. This
problem is particularly important because technical polycondensates such as Nylon-
6, Nylon-6,6, PET, and PBT are produced at an IMC clearly above the crIMC.
References
1. Jacobson H, Stockmayer WH (1950) J Chem Phys 18:1600
2. Jacobson H, Beckman CO, Stockmayer WH (1950) J Chem Phys 18:1607
5. Flory PJ (1953) Principles of polymer chemistry. Cornell University Press, Ithaca
6. Flory PJ, Semlyen JA (1966) J Am Chem Soc 88:3209
7. Flory PJ, Suter UW, Mutter M (1976) J Am Chem Soc 98:5733
8. Suter UW, Mutter M, Flory PJ (1976) J Am Chem Soc 98:5740
9. Mutter M, Suter UW, Floy PJ (1976) J Am Chem Soc 98:5745
10. Semlyen JA (1976) Adv Polym Sci 21:43
