Page 127 - Biodegradable Polyesters

4.5 Conclusions 105

k and k , which means faster transesterification and esterification rates and, as a
2
1
result, lower carboxyl concentrations at all temperatures and higher amounts of
hydroxyl consumption. As a result, the polyesters produced exhibit higher intrin-
sic viscosities and average molecular weights. Accordingly, use of a larger glycol
(i.e., PG and consequently BG) leads to lower esterification kinetic rate constant,
following the number of methylene units. However, the transesterfication reaction
rate constant of PPSu is much lower compared to that of PBSu, leading to higher
number of carboxyl and hydroxyl end groups and therefore eventually lower IV
values, meaning lower average molecular weights.
In addition, the values of the esterification rate constant, k , estimated are
2
always much larger compared to the corresponding value of k in all three
1
poly(alkylene succinate)s. This signifies esterification proceeding in a much
faster rate compared to transesterification. Since carboxyl end groups are only
consumed through the esterification reaction, this explains the very low values
of COOH end groups measured and as a result, the cessation of their reaction
after a point and the continuation of the reaction only through transesterification
(consumption of OH end groups).
Finally, the activation energies for the transesterification and esterification reac-
tions were determined. Results showed that the esterification reaction decreases
with increasing methylene units in glycols (i.e., PESu > PPSu > PBSu), although
the activation energy of the transesterification reaction was larger in PESu fol-
lowed by PBSu and significantly lower in PPSu [45]. Results on the estimated val-
ues of the inactive end group concentration, [OH] and [COOH] of PESu, PPSu,
i
i
and PBSu can be found in Ref. [45].

4.5
Conclusions

Production of 1,3-PD in high quality and quantity during the last 15–20 years
has led to the synthesis of several aliphatic polyesters with biodegradable prop-
erties. Poly(propylene alkylanedicarboxylate) polyesters can be prepared by the
two-stage melt polycondensation method (esterification and polycondensation)
using proper amounts of aliphatic acids and 1,3-PD and a procedure similar to
that used for aromatic polyesters.
PPSu presents lower crystallinity, crystallization rates, and melting point com-
pared to its homologs PESu and PBSu This in turn results in a polymer with higher
enzymatic hydrolysis rates and hence greater biodegradability. On the other hand,
retardation in PPSu crystallization is due to its reduced symmetry caused by the
propylene units.
Esterification and polycondensation kinetics of PPSu synthesis can be well
described on the basis of rather simple simulation models, taking into account
the reaction kinetics and the functional group modeling approach. The latter is
a very beneficial technique which includes aspects of the reaction mechanism
although with the minimum computational effort.