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

760 Polymers, Synthesis

requirements and have not been utilized in commercial
products.Achallengeremainstobroadenthescopeofsuit-
able polymerization chemistries and processes leading to
new families of high-performance polymeric products.

IV. LINEAR CHAIN-GROWTH
POLYMERIZATIONS

SCHEME 9 General polyurethane synthesis. The transformation of a vinyl or alkene monomer into a
long-chain macromolecule via a chain reaction process is
On the other hand, poly(amic diesters) can be stored for depicted in Scheme 10. This process basically involves
indefinite periods of time without degradation due to the the addition of a monomer to an activated or initiated
inability to form an intermediate carboxylate anion and form of the monomer. The reaction as indicated involves
3
2
have been utilized reproducibly in microelectronics ap- a change in the bonding from an sp bond to an sp bond.
plications. Earlier studies have shown that the formation This process of chain polymerization is usually not spon-
of tri- and tetramethylesters of dianhydrides increased the taneous, but rather must be catalyzed or initiated. For most
likelihoodofN-alkylationsidereactionsandacorrespond- monomer systems of this type, there are at least three basic
ingdecreaseinthemolecularweightandmechanicalprop- individual steps in the overall process of polymerization:
erties of the final product. A similar side reaction has been the initiation step, the propagating or growth step, and
observed in attempts to prepare polyamide esters using the termination step. The details of these three individ-
dimethyl esters of terephthalic acid. ual events will be highly dependent on the exact mech-
Melt-processible thermoplastic polyimides are pre- anism of polymerization. The active intermediates may
pared by the addition of flexible units, bulky side groups, be categorized as radicals, anions, cations, or coordinated
or, as mentioned earlier, flexible difunctional oligomers. species. For some of these mechanistic processes, notably
Examples of these modifications include GE’s Ultem anionic and controlled radical polymerization, it is often
(ether units), Amoco’s Torlon (amide units), Hoechst- possible to avoid or significantly reduce the termination
Celanese’s P150 (sulfone units), and GE’s Siltem (silo- step. There are many monomers that can be transformed
xane segments). Ultem polyimide is manufactured by into long-chain linear macromolecules via an addition or
General Electric and is an injection-moldable thermoplas- chain reaction process. A number of these are illustrated
tic poly(ether imide). This commercial product exhibits in Table II. The common feature is that one is convert-
high mechanical properties including modulus and ing the unsaturated carbon–carbon bond into a saturated
strength, excellent ductility, and high thermal stability. moiety. All samples here have a functionality of 2; divinyl-
Although polyimides have been prepared using a myriad benzene, in contrast, would have a functionality of 4 and
of other synthetic methodologies including aryl cou- would form networks.
pling with palladium catalysts, poly(amic silylesters), The nature of the pendent group will help discern which
silylethers with activated halides, and transimidization, reaction mechanism must be followed in order to effect
this section has focused on only two of the commercial this transformation. We have indicated that, in all cases,
polyimides in the marketplace today. Many specialty we are converting a carbon–carbon double bond to a long-
polyimides for composite applications have also evolved chain macromolecule. However, it should be pointed out
including LARC-TPI and CPI (Mitsui Toatsu) and that chain reaction polymerization is not limited simply
Hoechst-Celanese’s fluorinated polyimides, Due to the to carbon–carbon bond polymerization but includes many
tremendous scope of polyimide research and applications, other reactions. Nevertheless, it is most useful to illus-
several excellent comprehensive texts have been devoted trate chain reaction polymerization via a discussion of
to this family of high-temperature polymers.
It is easy to understand, based on the above discussions,
the serious restrictions that severely limit the number of
suitable organic reactions that have been used for the suc-
cessful preparation of high-molecular weight products via
step-growth polymerization. Although many synthetic or-
ganic reactions appear to be suitable for the preparation
of macromolecules via a step-growth polymerization pro-
cess, most organic reactions do not meet all the necessary SCHEME 10 Polymerization of an alkene monomer into polymer.