Living polymerization with slow initiation

When living centres are slowly generated, the polymerization accelerates with time. Older centres have time to grow to larger dimensions than the fresh centres. Several authors [22-24] have paid attention to kinetic analysis of living polymerizations with slow initiation, the most recent of these studies being that of Pepper [25]. 

Let us consider the simplest case where initiation and propagation are of 

Termination does not occur in living systems, therefore [M°] = [l]0 — [I], and the rate of monomer consumption is described by the equations 

The polymerization will accelerate with time until all initiator molecules are transformed to living centres or until complete monomer consumption. The 

When all initiator is consumed in the course of the polymerization, the reaction rate becomes dependent only on the monomer concentration, and ln[M] will be directly proportional to /. Under these conditions, kp can be calculated from eqn. (84) because 

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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. 

To prove that under these conditions, the IB polymerization is living, a monofunctional analogue of 1,2-p-methoxyphenyl-2-methoxypropane, was used to study the kinetics by incremental monomer addition technique. Results of this study indicated Hving polymerization with slow initiation [61,62]. 

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).

There are two most-often occurring non-steady-state polymerizations The initiation is slow and finally steady state is achieved this is the case for a typical radical polymerization and, in fact, for any steady-state process having inevitably a period of building invariable concentration of the active species. It was analyzed for radical polymerization and this case will be described first. A similar situation may arise in, for example, living anionic polymerization, with slow initiation-fast propagation, although, depending on the fep/fet ratio, the behavior of the systems may differ substantially. 

The Slow Initiation curves shown in Figure 2.1 were constructed as Tn =p[M]o/(p]o PI) vs. p, with p[M]o defined by eqn (2.27), and with [M]o = ImolL and P]o = 0.01 mol L. Examination of Figure 2.1 shows that as kpjki approaches unity, the Slow Initiation curves approach the Theoretical curve. Indeed for k jk-i = 1, eqn (2.26) reduces to the classical rate equation for a living polymerization with instantaneous initiation. Thus, instantaneous initiation should be understood to simply mean k >kp. 

Eq. (17) predicts that, when Pn is 100, the polydispersity is equal to 1.01, so that the polymer is virtually monodisperse. However, such ideal monodisperse polymers have scarcely been synthesized. The lowest values of polydispersity (Mw/Mn = 1.05-1.10) have been attained in homogeneous anionic polymerization 43). Gold 41) calculated the polydispersity of a polymer in the living polymerization with a slow initiation reaction and showed that the value of Pw/Pn increases slightly to a maximum (1.33) with an increase in polymerization time, followed by a decrease toward 1.00. Other factors affecting the molecular weight distribution of living polymer have been discussed in several papers 5S 60). 

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. 

Various side reactions that are likely to lead to a slow loss of "living" ends have been described. With disulfide initiators, one (initiation by the dithiocarbamyl radical) is unavoidable since the experiment relies on the same radical species to both initiate polymerization and terminate chains. 

A radical initiator based on the oxidation adduct of an alkyl-9-BBN (47) has been utilized to produce poly(methylmethacrylate) (48) (Fig. 31) from methylmethacrylate monomer by a living anionic polymerization route that does not require the mediation of a metal catalyst. The relatively broad molecular weight distribution (PDI = (MJM ) 2.5) compared with those in living anionic polymerization cases was attributed to the slow initiation of the polymerization.69 A similar radical polymerization route aided by 47 was utilized in the synthesis of functionalized syndiotactic polystyrene (PS) polymers by the copolymerization of styrene.70 The borane groups in the functionalized syndiotactic polystyrenes were transformed into free-radical initiators for the in situ free-radical graft polymerization to prepare s-PS-g-PMMA graft copolymers. 

In the early 1980s, Kennedy and his co-workers reported quasiliving polymerizations, which are phenomenologically akin to living polymerizations [57]. These processes involved slow and continuous monomer addition to a stirred initiator solution kept at a relatively low temperature. The monomers used therein included a-methylstyrene, isobutene, styrene, and alkyl vinyl ethers. In most cases, the number-average molecular weights steadily increased with the weight of the added monomer and the formed polymers had relatively narrow MWDs. 

Metal-catalyzed living or controlled radical polymerizations can generally be achieved with initiating systems consisting of an organic halide as an initiator and a metal complex as a catalyst or an activator as described above. However, these polymerizations are slow in most cases due to low concentration of the radical species, as required by the general principle, the dormant-active species equilibria, for living radical polymerization (see the Introduction). 

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