Books
in black and white
Main menu
Share a book About us Home
Books
Biology Business Chemistry Computers Culture Economics Fiction Games Guide History Management Mathematical Medicine Mental Fitnes Physics Psychology Scince Sport Technics
Ads

Polymer Chemistry. The Basic Concepts - Himenz P.C.

Himenz P.C. Polymer Chemistry. The Basic Concepts - Copyright, 1984. - 736 p.
Download (direct link): polymerchemistry1984.djvu
Previous << 1 .. 211 212 213 214 215 216 < 217 > 218 219 220 221 222 223 .. 312 >> Next

Polypropylene polymerized with triethyl aluminum and titanium trichloride
has been found to contain various kinds of chain ends. Both terminal
vinylidene unsaturation and aluminum-bound chain ends have been
identified. Propose two termination reactions which can account for these
observations. Do the termination reactions allow any discrimination
between the monometallic and bimetallic propagation mechanisms?
Solution
A reaction analogous to the alkylation step of reaction (7.Q) can account
for the association of an aluminum species with chain ends;
(b)
Figure 7.14 (a) The insertion of a propylene molecule into a site
vacancy in the Ziegler-Natta catalyst, (b) The
cross section shows the origin of stereoregulation. [From P. Cosee,
Tetrahedron Lett. 17:17 (I960).]
494 Polymers with Microstructure
Preview of Things to Come
495
The transfer of a tertiary hydrogen between the polymer chain and a
monomer can account for the vinylidene group in the polymer:
/X
CH.- ------H
! j
I I 1 '
Ti-CH.-C - + CH =C - Ti -^CH,-C~J
2 I I I
XX x
These reactions appear equally feasible for titanium in either the
monometallic or bimetallic intermediate. Thus they account for the
different types of end groups in the polymer, but do not differentiate
between propagation intermediates.

In the commercial process for the production of polypropylene by
Ziegler-Natta catalysts, hydrogen is added to terminate the reaction, so
neither of these reactions is pertinent to this process.
7.13 Preview of Things to Come
In part 2 of this book we have focused attention on some classes of
reactions which produce polymers and on some properties of the resulting
products. In the final three chapters we shall consider some of the
methods that are used to characterize the polymeric products of these
syntheses.
In the concluding chapters we again consider assemblies of molecules-
this time, polymers surrounded by solvent molecules which are comparable
in size to the repeat units of the polymer. Generally speaking, our
efforts are directed toward solutions which are relatively dilute with
respect to the polymeric solute. The reason for this is the same reason
that dilute solutions are widely considered in discussions of ionic or
low molecular weight solutes, namely, solute-solute interactions are
either negligible or at least minimal under these conditions.
We shall discuss three types of phenomena for polymer solutions:
thermodynamic properties in Chap. 8, frictional properties in Chap. 9,
and light-scattering properties in Chap. 10. A common feature of
virtually all phenomena in these areas is that they all depend on the
molecular weight of the solute. Thus observations of these properties can
be interpreted to yield values for M; we shall use this capability as a
unifying theme throughout these chapters.
In Chap. 8 we discuss the thermodynamics of polymer solutions,
specifically with respect to phase separation and osmotic pressure. We
shall devote considerable attention to statistical models to describe
both the entropy and the enthalpy of mixtures. Of particular interest is
the idea that the thermodynamic
X
I
- Ti-CH2-CH2X +^C=CH2
496
Polymers with Microstructure
nonideality of polymer solutions can be used as a quantitative measure of
either the volume of polymer molecules or the strength of solute-solvent
interactions. We shall also briefly consider charged polymers in Chap. 8;
this is the only place where polymeric electrolytes are discussed in this
text.
At first glance, the contents of Chap. 9 read like a catchall for
unrelated topics. In it we examine the intrinsic viscosity of polymer
solutions, the diffusion coefficient, the sedimentation coefficient,
sedimentation equilibrium, and gel permeation chromatography. While all
of these techniques can be related in one way or another to the molecular
weight of the polymer, the more fundamental unifying principle which
connects these topics is their common dependence on the spatial extension
of the molecules. The radius of gyration is the parameter of interest in
this context, and the intrinsic viscosity in particular can be
interpreted to give a value for this important quantity. The experimental
techniques discussed in Chap. 9 have been used extensively in the study
of biopolymers.
Chemistry students are generally more familiar with the absorption of
light by molecules than with more classical optical phenomena such as
refraction and scattering. In Chap. 10 we attempt to remedy this
situation by detailed development of those aspects of light scattering
Previous << 1 .. 211 212 213 214 215 216 < 217 > 218 219 220 221 222 223 .. 312 >> Next