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Catégorie :Category: mViewer GX Creator Lua TI-Nspire
Auteur Author: Emissione
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≈ 20 Kcal/mol, a value that allows to quantify approximately the rate increase
with temperature. Also note, as is typical of chain growth processes, that the
greater activation energy is relative to the first phase of the polyreaction
(initiation reaction).
As to the effect of concentration, being Xn ∝ [M] / [I]0.5, if the monomer
concentration is increased both the reaction rate and molecular weight
increase. On the other hand, if the concentration of the initiator is increased,
the reaction rate increases but the molecular weight decreases.
In practice, to moderate the molecular weight (radical processes often
generate high molecular weights that are then difficult to process
technologically) a small amount of initiator is generally used (because of its
high costs, significantly higher than for monomers) and transfer agents are
preferably added.


It is finally useful to summarize and compare the characteristic features of
polycondensations and free-radical polyadditions.
Polycondensations follow an ionic-type chemical mechanism with step-growth
kinetics (i.e. slow); the molecular weight increases with the extent of reaction
p (which in turn depends on time and on the stoichiometric ratio between the
reagents r. At each stage of the process the polymer is characterized by a
negligible level of residual monomer (as its weight fraction). Therefore,
purification processes are generally not necessary or easy.
Free-radical polyadditions follow a chain-growth-type polymerization kinetics
(i.e. fast). The molecular weight obtained is generally high already at the
beginning of the reaction with minor changes throughout the polymerization
time (unless extensive chain transfer reactions to products occur). Since the
propagation of a single macroradical is almost instantaneous (reaction times of
the order of ms), over time only the yield grows, i.e. the conversion of
monomer into polymer. The level of residual monomer at the end of the
reaction is often high and requires purification procedures from the produced
polymer.



Chapter 4: Ionic chain polyadditions


Many types of monomers with electronically interdependent or implicit
functionalities (thus unsaturated) can undergo polyaddition reactions with an
ionic-type chemical mechanism. The kinetic mechanism is usually chain-growth
but under conditions of controlled reactivity it can be step-growth.

42
As for the case of free-radical polymerizations, the ionic polyreaction consists
of several phases such as initiation, propagation and termination reactions,
while the chain transfer reaction is less typical. In some particular cases the
termination reaction can be avoided. This allows to obtain highly controlled
architectures, as will be described hereinafter (see living polymers).
The overall polymerization kinetics - and therefore also the degree of
regiochemical and stereochemical control of the process - largely depend on
the reactivity of the ionic pair, which is the chain end that propagates. This
reactivity is essentially correlated to the degree of dissociation of the ionic pair
which is in turn a function of the polarity of the reaction environment. Apolar
solvents (like hydrocarbons) are ineffective in the solvation of ionic pairs that
will thus remain highly associated and therefore relatively poorly reactive.
Conversely, polar solvents such as ethers allow for an effective solvation of the
ionic pair, which will thus be more available for the monomer addition reaction.




The degree of dissociation of the ionic pair will therefore depend on the
dielectric constant of the solvent. In general it will be possible to have both
cationic and anionic active centers, the latter being more important. The ionic
species that are formed are essentially carbocations, carbanions, oxanions and
less frequently oxocations (Figure 15).




Figure 15. Active species in ionic polyadditions


It is possible to distinguish two types of attack onto the unsaturated monomer
of C=C type.

43
In cationic polymerizations with ionic attack, the π electron pair of the
monomer is added to the carbocation, generally leading to a regular head-to-
tail linkage (in the - frequent - case of polymerization of vinyl monomers).
Note that monomers that can undergo cationic polymerization possess R
substituents with electron-donating effect, thus the formation of the most
substituted carbocation is always favored.
In anionic polymerizations with ionic attack the carbanion electronic pair will be
added to the less substituted carbon of the unsaturated monomer, with the
subsequent formation - also in this case - of the most substituted carboanion.
Usually monomers that can undergo anionic polymerization differ from the
previous ones because they have R substituent groups with electron-attracting
effect.
In polymerization processes with counter-ionic attack an essential role is
played by the M+ catalyst, which is typically a salt of a transition metal (high
atomic number). In these particular class of anionic polymerizations, the
unsaturated monomer first undergoes a polarization of the double bond π by
the transition metal M+, and then an insertion on the polymeric carbanion, with
a very good control of both regiochemistry (head-to-tail configuration) and in
many cases stereochemistry (pseudoasymmetrical carbon configuration).
Figure 16 shows the reaction chemical mechanisms in the various cases
mentioned above.




Figure 16. Ionic (i) and counter-ionic (ii) attack addition mechanisms


A feature of ionic polyadditions is that the characteristics of the process are
strongly influenced by the experimental conditions adopted for the polyreaction

44
(type of initiator, type of solvent etc.). It is therefore less easy, compared to
the case of free-radical polyadditions, to obtain a true generalization of the
kinetic model.
Furthermore it is always to be remembered that the reactivity of monomers
(typically ethylenic, vinylic or cyclic) are influenced by the nature of the R
group: if this group is an electron-donating one such as an alkyl or ether group,
the corresponding olefin may be defined as electron-rich and cationic
polymerization is favored. Conversely, if the R group is an electron-attracting
one such as an acrylonitrile -CN or an ester -COOR, the olefin is defined
electron-poor and it will preferentially undergo anionic polymerization. In some
cases such as with styrene (R = phenyl) there will be a stabilizing effect both
of the cationic and anion charge, and both polymerization mechanisms can
take place.
Steric effects are also important since ionic polyadditions compared to radical
polyadditions are able to impart to the formed polymer a higher constitutive
control and often also configurational control.

Cationic polyadditions are essentially based on the growth of carbocations,
which are high energy - and therefore very reactive - species. The chemical
mechanism of these polyreactions is ionic, the kinetic mechanism is chain-
growth and the polymerization must be typically conducted in an inert
environment (anhydrous hydrocarbons) at low temperature (-40°C to -100°C)
to ensure an adequate lifetime of the carbocation. As initiators it is possible to
use protic acids such as sulfuric acid H2SO4, perchloric acid HClO4, phosphoric
acid H3PO4, and Lewis acids such as BF3 and AlCl3. Polymerizable monomers
are mostly alpha-substituted olefins and vinyl ethers of the type RO-CH=CH2.
Also epoxides can undergo cationic polymerization with a mechanism of
protonation of the oxygen and ring opening (Fig. 17). Such polyreaction can
also be conducted at room temperature and is quite used in the
photocrosslinkable ink and paint technology.




Figure 17. Mechanism of cationic polymerization of epoxides




45
The cationic polyaddition of unsaturated monomers of type C=C includes an
initiation phase, a propagation phase and a termination phase, as shown in
Figure 18. The termination can take place in different ways, for example it may
be a spontaneous dissociation of the ionic pair usually following a rise in
temperature, or the carbocation can be easily quenched by traces of moisture.
Processes must therefore be conducted under strictly anhydrous conditions.


1. initiation

H H
C C + HX CH3 − CH+ X −
H R R
2. propagation

CH+ X − + CH2 CH R CH2 CH CH2 CH + X −

R R R




3. termination

CH+ X − CH2 CH + HX (T effect)

R R



+ acqua
CH+ X − CH2 CH OH + HX (moisture)
R
R



Figure 18. Phases of cationic polymerization


There are relatively few cases of industrial production processes based on
cationic polymerizations. One of these is the process of polymerization of
isobutylene CH2=C(CH3)2, which leads to the synthesis of polyisobutylene, the
so-called "butyl rubber". Such polymerization is very fast and is carried out at -
100°C in the presence of AlCl3 acting as catalyst. The industrial product is
actually a copolymer which contains small amounts (1-2%) of isoprene, a
monomer that provides the allylic hydrogens –CH2-CH= suitable for
vulcanization with sulfur.



46
Anionic polyad...

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