Polymerization with rapid initiation

Conventional radical polymerization usually produces polymers with a broad distribution in DP. The polymers are mixtures of the instantaneous polymers with DPw/DPn of at least 1.5 for the termination by recombination or 2.0 either for the termination by disproportionation or for the chain transfer to small molecules. In this respect, any living polymerization with rapid initiation will afford polymers with a narrow DP distribution of the Poisson type. Ring-opening met-hathesis polymerization of norbornenyl-terminated macromonomers, 8, 15, and 16, appears promising in this regard [22,23]. 

A complete separation of the elementary processes is possible only in very special and rarely occurring cases (e. g. in living polymerizations with rapid initiation). Even a mere change in the velocity of partial steps is of great importance. Such change can be induced in many ways, the most important of which are physical effects (temperature, pressure, viscosity, etc.), choice of initiator and purity of the medium. 

Fig. 6. Types of conversion curves. Conversion curve 1, 2,4 polymerization with rapid initiation rate decreases (1) only inconsequence of monomer consumption (living polymerization) (2) due to the consumption of monomer and of active centres (3), (5) polymerization with slow initiation Atind is the time interval of the concentration growth of active centres (4), (5) polymerization with an inhibition period tinh (A and A are points of inflection).

The molecular mass of the generated macromolecules is most easily predicted for living polymerizations with rapid initiation (see Chap. 5, Sect. 8.1). Somewhat more complicated is the case of systems that, although are not living, at least polymerize without transfer. The number average polymerization degree, P, of the product is a simple function of the mean kinetic chain length, v... 

Radical polymerizations are almost always considered as kinetically stationary. However, the stationarity conditions are not always fulfilled. Living polymerizations with rapid initiation are stationary, but the character of the medium should not significantly change during polymerization in order to prevent shifts in the equilibria between ion pairs and free ions. All other polymerizations are non-stationary even, to some extent, living polymerizations with slow initiation. It is usually very difficult to define initiation and termination rates so as to permit exact kinetic analysis. When the concentration of active centres cannot be directly determined, indirect methods must be applied, and sometimes even just a trial search for best agreement with experiment. 

The values of v are only slightly higher than 1 for products of living polymerizations with rapid initiation. They are around 2 for radical polymers, and in extreme cases values of several tens for sets of macromolecules generated by some coordination polymerizations. 

The rate of polymerization follows the standard equation obtained for an equilibrium polymerization without termination and with rapid initiation (20) ... 

Very rapid initiations are known, manifested by an instantaneous start to the polymerization after which the number of active centres is not further increased. Polymerizations with slow initiation are also quite frequent, starting only after some inhibition and/or induction period. In the course of these polymerizations, the concentration of active centres is not usually constant. A stationary state is not excluded, of course but it occurs much less frequently than with radical polymerizations. 

When a water-soluble monomer dispersed in a continuous oil phase is polymerized with an initiator soluble in the continuous phase, we speak of inverse emulsion polymerization [159]. This system has all the advantages of emulsion polymerizations (rapid polymerization, high degree of polymeriza-... 

In the simplest case, with rapid initiation and participation of a single type of active centre, the rate of propagation is equal to the polymerization rate, and kp is the overall polymerization rate constant. Rapid initiation can be established in ionic processes the presence of several kinds of centres means unequal numbers of monomer molecule additions to different centres. Long macromolecules will be formed on "rapid centres, shorter ones on "slow centres. A practical example of this situation is anionic living polymerization with the participation of contact and solvent-separated ion pairs, and of free ions. 

Active centres of ionic polymerizations do not usually decay by mutual collisions as the radical centres. The stationary state, when it exists at all, results from quite different causes, mostly specific to the given system. Therefore the kinetics of ionic polymerizations is more complicated and its analysis more difficult. The concentration of centres cannot usually be calculated. On the other hand, ionic systems with rapid initiation give rise to the kinetically very simple living polymerizations (see Chap. 5, Sect. 8.1). 

The mechanism of initiation is based on an examination of reaction products in an early stage of the polymerization. After short-stopping the polymerization by addition of sodium methoxide/methanol solution, the main product subsequently identified by gas chromatography was C2H5OCH2CH2OCH2OCH3. The rate of polymerization is presumed to follow the standard eqn. (6) for an equilibrium polymerization without termination and with rapid initiation. It is acknowledged that there is an induction period (presumably due to reaction of catalyst with adventitious water, since rigorous drying reduced the induction time to only... 

In addition to the initiation of gaseous formaldehyde with formic acid, HCl, boron trifluoride and stannic chloride were studied and found to be more active than formic acid (Fig. 17). The rate of gaseous formaldehyde polymerization with the initiator was measured under the same conditions as the formic acid initiated polymerization by determining the decrease in pressure of formaldehyde. HCl as initiator (at about 3—4% in the mixture) caused kinetic branching, i.e. a rapid increase in the rate of polymerization. 

The theoretical molecular weight distributions for cationic chain polymerizations (see Problem 8.30) are the same as those described in Chapter 6 for radical chain polymerizations terminating by disproportionation, i.e., where each propagating chain yields one dead polymer molecule. The poly-dispersity index (PDI = DP /DPn) has a limit of 2. Many cationic polymerizations proceed with rapid initiation, which narrows the molecular weight distribution (MDI). In the extreme case where termination and transfer reactions are very slow or nonexistent, this would yield a very narrow MDI with PDI close to one (p. 681). 

As already described, aluminum complexes of tetra-azaannulene (9) and phthalocyanine (10) have much lower reactivities than aluminum porphyrins for the polymerization of epoxides (11). However, in the presence of appropriate Lewis acids such as 44 or 45, the polymerizations with these initiators take place rapidly to give polyethers with fairly narrow MWD. ... 

Rettig H, Krause E, Bomer HG (2004) Atom transfer radical polymerization with polypeptide-initiators a general approach to block copolymers of sequence-defined polypeptides and synthetic polymers. Macromol Rapid Commim 25 1251... 

Water-soluble peroxide salts, such as ammonium or sodium persulfate, are the usual initiators. The initiating species is the sulfate radical anion generated from either the thermal or redox cleavage of the persulfate anion. The thermal dissociation of the persulfate anion, which is a first-order process at constant temperature, can be gready accelerated by the addition of certain reducing agents and small amounts of polyvalent metal salts, or both. By using redox initiator systems, rapid polymerizations are possible at much lower temperatures (25—60°C) than are practical with thermally initiated systems (75-90°C). 

Formation of block polymers is not limited to hydrocarbon monomers only. For example, living polystyrene initiates polymerization of methyl methacrylate and a block polymer of polystyrene and of polymethyl methacrylate results.34 However, methyl methacrylate represents a class of monomers which may be named a suicide monomer. Its polymerization can be initiated by carbanions or by an electron transfer process, the propagation reaction is rapid but eventually termination takes place. Presumably, the reactive carbanion interacts with the methyl group of the ester according to the following reaction... 

The styrene conversion versus reaction time results for runs in the laminar flow regime are plotted in Figure 8. Both the rate of polymerization and the styrene conversion increase with increasing flow rate as noted previously (7). The conversion profile for the batch experimental run (B-3) is presented as a dashed line for comparison. It can be seen that the polymerization rates for runs with (Nj e e 2850 are greater than the corresponding batch polymerization with a conversion plateau being reached after about thirty minutes of reaction. This behavior is similar to the results obtained in a closed loop tubular reactor (7J) and is probably due to an excessively rapid consumption of initiator in a... 

Ethylene oxide polymerization may be initiated similarly by substances (alcohols, amines, mercaptans) capable of generating a hydroxyl group through reaction with the monomer. In the presence of strongly acidic or basic catalysts, successive addition of ethylene oxide molecules proceeds rapidly in the following manner ... 

While subtle differences between gels synthesized with different initiators may exist, they are not easy to anticipate. The most important concern is usually the polymerization rate induced by a given initiator concentration [7], Polymerization reactions are highly exothermic, so a fast initiation rate can lead to a rapid temperature increase since the initiation and polymerization rate both increase rapidly with temperature, this process becomes autocatalytic. Poor quality, irre-producible gels result on a production scale such a runaway reaction could be-... 

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