29 июня, 2015 
Malyar The mechanical, physical and chemical properties conferred on the polymer by the monomers utilised are discussed in detail under the relevant headings in the sections of this book dealing with applications and uses. However, it is pertinent to discuss here, in more general terms, the effect of molecular structure on the chemical and physical behaviour of polymers.
1. Orientation of the Repeating Unit
During the propagation stage, vinyl monomer can add to the growing chain, in one of two distinct ways, to form a “head to head” (and “tail to tail”) linkage or a “head to tail” linkage as illustrated below:
|
X |
X |
X X і і |
|
|
—*o— I X о |
+ |
т -o- II CN X 0 1 |
I I. ► CH2-C-CH2-C 1 1 |
|
У |
Y |
1 1 Y Y |
|
|
(head to tail linkage) |
or
|
X I |
X 1 |
X I |
X 1 |
|
|
-o- 1 X о |
+ |
1 — CH2= C ————- ► 1 |
1 CH2—c— |
1 C— CH2 |
|
Y |
Y |
1 Y |
1 Y |
|
(head to head linkage) |
Figure 1 -54
The type of linkage formed depends on the relative stability of the monomer radical formed. For vinyl compounds with the structure CH2=CYX in general the — CYX radical is more stable than the — Сн2 radical since the electron excess can be resonance stabilised over the substituent groups X and Y.
In general, monomers which form free radicals easily, tend to polymerise more slowly than monomers which do not form free radicals readily. This can be illustrated by reference to styrene which readily forms a monomer radical. However, the radical is stabilised by delocalisation in the К electron cloud of the aromatic ring, which results in a relatively slow rate of propagation when forming a homopolymer.
Some of the more important factors influencing the orientation of the repeating units are:
Опубликовано в рубрике 