Page 247 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN012B-596 July 27, 2001 18:18

756 Polymers, Synthesis

BB system, which requires two distinct molecules to be mic reactions facilitates the use of bulk polymerization
reacted together in the step-growth or polycondensation processes in step-growth polymerization. For example,
process. In the second stage of the ester interchange route polyesters and polyamides are manufactured commer-
to polyesters, application of heat and a gradually increas- cially in the absence of solvents and the final products are
ing vacuum down to a fraction of a torr eliminates any directly usable without further isolation and purification.
excess ethylene glycol that may be present. Then (usually Although molecular weights gradually increase through-
in the presence of a catalyst), ester interchange processes out most of a step-growth polymerization process, the final
begin where a hydroxyl end group can attach an in-chain stages of the polymerization process involve the rapid in-
ester carbonyl. On removal of additional ethylene glycol, crease in molecular weight. It is well known that the melt
the molecular weight and degree of polymerization begin viscosity for thermoplastics increase with the 3.4 power
to decrease. of the weight-average molecular weight. It is not surpris-
Finally, when the n value in Scheme 4 approaches a ing, therefore, that significant attention has been devoted
few hundred, which corresponds to molecular weights in recent years to the development of novel agitation and
of >20,000 g/mole, one may stop the process. Clearly, reactor designs to facilitate the transport of the viscous
for different applications somewhat different molecular melt and the transport of polymerization by-products in
weights may be required, but typically 20,000–30,000 the latter stages of step-growth polymerization processes.
would be a representative value. As the interchange Although high molecular weight is often required
process takes place, the reaction proceeds to higher and for the maximization of thermal and mechanical prop-
◦
higher molecular weight. Typically, this is done 25 C erties, the synthesis of difunctional oligomers (less
above the crystalline melting point in order to achieve ad- than 10,000 g/mole) is accomplished in step-growth
equate reactivity. Although one could intuitively propose polymerization using monofunctional reagents or a
the use of longer polymerization times to achieve higher stoichiometric excess of a difunctional monomer. The
molecular weight products, the importance of relatively difunctional oligomers are suitable starting materials for
slow side reactions during the polymerization process the preparation of crosslinked coatings, adhesives, or
becomes more important when using either longer poly- segmented block copolymers. Due to the reactive nature
merization times or higher polymerization temperatures. of many polymers prepared using step-growth poly-
For example, the manufacture of PET is accompanied by merization processes, reactive end groups and internal
the formation of diethylene glycol (DEG), vinyl esters, reactive functionalities offer the potential for subsequent
and acetaldehyde during the polymerization process. deriviatization, chain extension, or depolymerization.
DEG is incorporated into the chain during the polymer- For example, the depolymerization of polyesters is easily
ization and its presence in the backbone has been shown to accomplished with the addition of a suitable difunctional
negatively affect the chemical resistance, crystallization acid or glycol. In addition, polyester and polycarbonate
rate, thermal stability, and ability for strain-induced crys- blends are easily compatibilized in a twin-screw extruder
tallization in many PET product applications. In addition, to prepare optically clear coatings.
acetaldehyde has been observed to affect the packaging of Polyesters derived in part from unique rigid aro-
beverage and other food products. Typically, the process matic monomers including hydroquinone and biphenyl
is completed at slightly below the crystalline melting derivatives offer the potential for new families of
point in what is often termed a solid-phase finishing step. high-temperature, high-performance polyester resins.
In addition to reducing the propensity of undesirable side Although liquid crystalline polyesters (LCPs) were
reactions, the lower temperature solid-phase or solid-state discovered in the late 1960s, this family of engineering
process serves to efficiently remove any small molecular thermoplastics continues to receive intense academic and
impurities that were generated in the previous higher industrial attention. In addition to inherent flammability
temperature melt-phase process. resistance, LCPs offer exceptional moldability due to the
As discussed earlier, a salient feature of step-growth shear-induced alignment of the rigid polyester back-
polymerization processes is the gradual increase in molec- bones. Therefore LCPs have become very important in
ular weight throughout the polymerization process. This the manufacture of small parts for the electronics and
observation is a direct result of the stepwise addition of re- other industries. Ticona (formerly Hoechst-Celanese) has
active species (monomers, dimers, trimers, tetramers, etc.) pioneered the commercialization of all-aromatic liquid
to give high molecular weight. In contrast, high-molecular crystalline polyesters based on 2-hydroxy-6-naphthaoic
weight products are obtained very quickly in chain poly- acid (HNA) and p-hydroxybenzoic acid (PHB). Ester
merization processes and are typically accompanied by formation based on the reaction between an aromatic
highlyexothermicreactions.Consequently,thegradualin- carboxylic acid and an aromatic phenol is not suitable
crease in melt viscosity and the absence of highly exother- for polyester manufacture, and all-aromatic polyesters