Polymerization with slow initiation

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]. 

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. 

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).

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]. 

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. 

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 MW - time profile closely resembled that of condensation polymerization a slow initial increase was followed by an exponential growth in MW with time. After 18 h the polymer exhibited Mw 250,000 g mol-1, and Mw/Mn=6. The... 

It is much easier to notice the effect of slow initiation by analysis of the evolution of molecular weights with conversion in Figs. 2 and 3. The small increase of the polymerization degree, in relation to the ideal case, disappears for the ratio / , = 0.1 at approximately 40% conversion. However, it is necessary to add subsequent portions of a monomer (conversions > 100%) for ratios / , = 0.03 and R, = 0.01 to asymptotically approach ideal M values as shown in Fig. 3. The polydispersities in systems with slow initiation depend on the ratio [M]0/[l]o and / , and are very low for R( = 1 and 10 (Af, /Af < 1.02) but approach MJMn 1.15 for the ratio... 

In the former case [Eq. (20)], the rate does depend on [M], Thus, kinetics alone may erroneously indicate that the covalent species D reacts directly with M (monomer), although it first ionizes to C. The latter case [Eq. (21)] results in zero-order kinetics in monomer. However, this may not happen in the polymerization process. If k-, < A 2[M], then once the ions C are generated, they can react many times with monomer before deactivation. Thus, the kinetics may be first order with respect to monomer and resemble a system with slow initiation and fast propagation. 

In 1969 Penczek and Kubisa [59] reported an exhaustive study of the kinetics and mechanism of BCMO polymerization initiated by the (i-C4H9 )j AI/H2O system and carried out in chlorobenzene solution at 55—95°C. This system produced homogeneous conditions for the polymerization and the whole process could be described as a non-stationary reaction with slow initiation, fast propagation, and slow degradative chain transfer to polymer. 

Fig. 14A-C. Isobutylene polymerization by TMPCl/TiCl4 and HX /TiCl4 at — 60 °C A,B determination of chain transfer constants in systems with slow initiation plus chain transfer to monomer C N vs conversion plot at various ED concentrations in the presence of initiator ([TEA] = (mol/l) =5-10 A = 1.5-HT3, = 3-l(T3, = 6-103) and in the absence of initiator (O, A, , V)...

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. 

Inhibitors slow or stop polymerization by reacting with the initiator or the growing polymer chain. The free radical formed from an inhibitor must be sufficiently unreactive that it does not function as a chain-transfer agent and begin another growing chain. Benzoquinone is a typical free-radical chain inhibitor. The resonance-stabilized free radical usually dimerizes or disproportionates to produce inert products and end the chain process. 

Chain transfer, the reaction of a propagating radical with a non-radical substrate to produce a dead polymer chain and a new radical capable of initiating a new polymer chain, is dealt with in Chapter 6. There are also situations intermediate between chain transfer and inhibition where the radical produced is less reactive than the propagating radical but still capable of reinitiating polymerization. In this case, polymerization is slowed and the process is termed retardation or degradative chain transfer. The process is mentioned in Section 5.3 and, when relevant, in Chapter 6. 

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. 

Quasiliving Polymerization of Methyl Vinyl Ether. Similarly to IBVE polymerization, MVE was polymerized with premixed p-DCC/AgSbF initiating systems in CH2CI2 solvent at -70°C by slow and continuous monomer addition. Polymer yields were vL00% at every reaction time. ... 

Kinetic curves relative to polymerization reactions in the solid state commonly show a sigmoidal shape with a slow initiation step followed by a steep increase, even by two orders of magnitude, of the reaction rate. A reaction with this kind of kinetic curve is said to have an autocatalytic behavior. 

There were several attempts to gain better control on the free radical polymerization process [18, 19], One of these methods was named the iniferter method. The compounds used in this technique can serve as m/tiator, trans/er agent and terminating agent [20-22], Another technique is based on the use of bulky organic compounds such as diaryl or triarylmethyl derivatives [23-25], The main disadvantages of these systems comprise slow initiation, slow exchange, direct reaction of counter radicals with monomers, and their thermal decomposition. Therefore, these techniques did not offer the desired level of control over the polymerization. 

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