Page 435 - Carrahers_Polymer_Chemistry,_Eighth_Edition

398 Carraher’s Polymer Chemistry

thermal analysis (DTA), and thermomechanical analysis (TMA). Examination by optical micros-
copy reveals that the crystalline structure is not entirely lost but persists throughout the extended
temperature range to a higher temperature transition, which appears to be T , the true melting tem-
m
perature. The nature of this transitional behavior resembles the transformation to a mesomorphic
state similar to that observed in nematic liquid crystals. It appears from the relationship between
the equilibrium melting temperature (heat and entropy of fusion; T = H /S ) and the low value of
m m m
H at T compared with the lower transition temperature that the upper transition, T , is character-
m m m
ized by a very small entropy change. This may be due to an onset of chain motion between the two
transitions leading to the small additional gain in conformational entropy at T . The lower transition
m
is believed to correspond to the T .
g
Allcock and coworkers have employed polyphosphazenes in a variety of uses, including the
broad areas of biomedical and electrical. From a practical point of view, polyphosphazenes are
usually soft just above the lower transition so that compression molding of films can be carried out.

This suggests that the lower transition temperature represents the upper temperature for most useful
engineering applications of polyphosphazenes in unmodifi ed forms.

11.5.3 BORON-CONTAINING POLYMERS
Organoboron has been incorporated into polymers employing a variety of techniques. Stock in
the 1920s first created a boron hydride polymer during his work on boron hydrides. Much of the

current interest in boron-containing polymers is a consequence of three factors. First, the presence
of a “low-lying” (meaning low energy) vacant “p” orbital allows its use in moving electrons in a
conjugated system. This is being taken advantage of through the synthesis of various pi-conjugated
systems and their use in optical and sensing applications. This includes use in light-emitting diodes
(LEDs), nonlinear optical systems, energy storage in batteries, and the construction of sensing
devices. One such polymer structure is given in structure 11.43.

R

(11.43)
B
R

These polymers are mainly synthesized employing the hydroboration reaction which is simply
the addition of a hydrogen from a boron hydride to a double or triple bond (Equation 11.44).

R
H C
2
BH
HC CH + B R
(11.44)
R R
This low-lying vacant p orbital also allows boron-containing polymers to be luminescent again
signaling potential optical applications. Many of these luminescent materials are NLO materials.
The second reason for interest in boron polymers involves their use as catalysts. While the
boron atom can be used as the site of catalytic activity, more effort has involved the use of boron-
containing materials as blocking and protecting agents and as cocatalysts. They are increasingly
being used in catalytic asymmetric syntheses.

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