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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
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ease of formation of lactones, cyclic esters, and cyclic anhydrides is
greatest in the neighborhood of five- or six-atom rings, drops to a
minimum in the range of 8- to 12-membered rings, and then increases and
levels off at a low plateau value for still larger rings. The broad
minimum in the rate of formation of rings with 8 < 1 < 12 is attributed
to the crowding between chain hydrogens in the interior of the rings in
this size range.
On the basis of both thermodynamic and kinetic evidence-both of which
are interpretable in terms of the strain associated with rings of certain
sizes or similar structural factors-we see that only rings with five or
six atoms have any significant stability. Accordingly, we conclude the
following:
1. AA and BB monomers and also AB monomers invariably react to form
predominantly linear structures in all but the rather special case where
the ring structure in reaction (5.CC) has a value of 1 = 5 or 6. This
explains why so many of the monomers in step-growth polymerizations are
tetra-, hexa-, and decamethylene compounds.
2. The cyclization of a polymer that has already grown through the
dimer or trimer stage is also insignificant.
3. The formation of polymers by the ring-opening reaction (5 .FF)
requires an initiator to get the reaction started, and perhaps a catalyst
to assure a suitable rate, but otherwise is quite feasible for 1 < 4 and
1 > 7.
We conclude this section by citing some examples of ring-opening
polymerizations. Table 5.9 lists several examples of ring-opening
polymerizations. In addition to the reactions listed, we recall the
polymerizations of lactones and lactams exemplified by equations in Table
5.3 and 5.4, respectively.
Ring-opening polymerizations are catalyzed by a wide variety of
substances, including the bases OH' and RO' and the acids H+ and BF3;
water is also used as a catalyst. The reactions proceed by the opening of
the ring by the catalyst to form an active species,
followed by the subsequent attachment of additional monomers to the
active site,

(5.GG)
Cab* + Qb -> Cabab* -> -> polymer
(5.HH)
Rings 'n Things
Table 5.9 Some Typical Ring-Opening Polymerization Reactions
1. Epoxides:

/\
" 2-2 ----fCH2 CHrO+n
2. Cyclic formals (formaldehyde-cyclic ether dimer):
(CH2>5
n o' X0 ------>" +0-CH2-0-(CH2)5in
3. Cyclic sulfides (including S8):
CH,
I 3
n CH2-CHCH3 -----" fCH2-C-S4n
s H
4. Alkylenimines:
n 2-2 H
\ / I
N -----^ ^CH2CH2-N+r
H
5. Cyclic acetals (trioxane: cyclic trimer of formaldehyde):
-CH, n / \
3 / ------- tCHj-O+n
&-CH2
333
6. Cyclic siloxanes:
334
Condensation or Step-Growth Polymerization
5.11 More Rings
The search for substances which qualify for proposed applications has
always been a driving force for the synthesis and characterization of new
compounds. This is especially true in polymer chemistry, where it is the
potential of polymers as engineering materials that often stimulates
research. Polymeric materials frequently fail to be serviceable in
engineering applications for one of the following reasons:
1. They lack resistance to solvents or other chemicals.
2. They lack thermal stability at high temperatures.
3. They are not sufficiently rigid.
To some extent each of these objections is met by the presence of either
chemical or crystallite crosslinking in the polymer. Another approach
which complements the former is to incorporate rings into the backbone of
the chemical chain. As an example, contrast the polyesters formed between
ethylene glycol and either suberic or terephthalic acid. Structures [V]
and [VI], respectively, indicate the repeat units in these polymers:


II II II
yCZA II
to-(CH2)2-o-c-(CH2)6-c4 *2)2--C-
^JV-C*
[V]
[VI]
In both instances the two carboxyl groups are separated by six carbons,
but these occur as six methylene groups in the suberate and as a benzene
ring in the terephthalate. The suberate melts at about 62C and the
terephthalate at about 260C. At least in part, this difference is
traceable to the greater chain stiffness of the backbone containing the
ring structure, which decreases the AS for the melting process and
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