Polymerization dead end

If the initiator concentration used in a free-radical polymerization system is low and insuf dent, leading to a large depletion or complete consumption of the initiator before maximum conversion of monomer to polymer is accomplished, it is quite likely to observe a limiting conversion poo which is less than the maximum possible conversion pc, as shown in Fig. 6.2. This is known as the dead-end effect and it occurs when the initiator concentration decreases to such a low value 

Recalling Eq. (6.31) for radical chain polymerization initiated by thermal homolysis of an initiator  

A plot of the left side of Eq. (6.78), which is equivalent to the expression ln[(ln[M]oo - ln[M])/(ln[M]oo - ln[M]o)], versus time (/) permits evaluation of kd (see Problem 6.11). Once kd is determined, / can be obtained from either Eq. (6.24) or (6.77) using the value of kplk available from other studies. The thermal dissociation rate constants and activation energy values for several commonly used initiators are listed in Table 6.3. 

Problem 6.11 Isoprene was polymerized in bulk at several temperatures using AIBN at an initial concentration of 0.0488 mol/L in dead-end polymerization experiments (Gobran et al., 1960). In every case, the conversion increased with time until a limiting value was obtained beyond which no further polymerization was observed. No autoacceleration effect was observed in this system. The data of fractional degree of conversion (p) with time, including the limiting value of conversion (poo), determined at each temperature are shown in the table below  

Determine the kinetic parameters kd and for isoprene-AIBN system and the activation energy 

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The synthesis of telechelics by what Tobo]sky,9> termed dead-end polymerization is described in several review s.191,191 In dead-end polymerization very high initiator concentrations and (usually) high reaction temperatures are used. Conversion ceases before complete utilization of the monomer because of depletion of the initiator. Target molecular weights are low (1000-5000) and termination may be mainly by primary radical termination.. The first use of this methodology to prepare lelechelic polystyrene was reported by Guth and Heitz.177... 

The hindered carbon-centered radicals are most suited as mediators in the polymerization of 1,1-disubstituled monomers e.g. MMA,78,95 other methacrylates and MAA,06 and AMS97). Polymerizations of monosubstituted monomers are not thought to be living. Dead end polymerization is observed with S at polymerization temperatures <100°C.98 Monosubstituted monomers may be used in the second stage of AB block copolymer synthesis (formation of the B block).95 However the non-living nature of the polymerization limits the length of the B block that can be formed. Low dispersities are generally not achieved. 

Increasing the temperature of polymerization does not always lead to higher rates of polymerzation. Higher temperature leads to faster dissociation of the initiator and complete depletion of the initiator resulting in a "dead end" polymerization (Bohme and Tobol-sky (1966)). Dead-end polymerization refers to one in which initiator concentration decreases to such a low value that the polymerization stops short of completion and a limiting conversion of monomer to polymer is observed (Odian (1970)). 

A closer look at the nonisothermal and isothermal policy results reveals some additional interesting features with regard to optimization. As mentioned earlier, isothermal policies were determined by two factors. One was the M, value and the other was the dead end polymerization caused by depletion of initiator. It was also observed that the minimum time from a nonisothermal policy was considerably less than the minimum time due to the isothermal policy whenever H>, was the controlling factor in the isothermal policy when the isothermal policy was controlled by initiator depletion, a nonisothermal policy did not show significant improvement in minimum time relative to the isothermal one. 

In this paper we formulated and solved the time optimal problem for a batch reactor in its final stage for isothermal and nonisothermal policies. The effect of initiator concentration, initiator half-life and activation energy on optimum temperature and optimum time was studied. It was shown that the optimum isothermal policy was influenced by two factors the equilibrium monomer concentration, and the dead end polymerization caused by the depletion of the initiator. When values determine optimum temperature, a faster initiator or higher initiator concentration should be used to reduce reaction time. 

Comparison of isothermal and nonisothermal policies revealed some interesting features of the polymer system. When M , values determine the isothermal policy, a nonisothermal operation reduces the minimum time compared to isothermal operation (by about 15%). However, when dead-end polymerization influences isothermal operation, a nonisothermal operation does not offer significant improvement. 

SPEC was essentially able to market their Zr02-based ultrafiltration membranes to an already existing market in the sense that these membranes replaced polymeric UF membranes in a number of applications. They also developed a certain number of new applications. For Ceraver, the situation was different. When the Membralox membranes were first developed, microfiltration was performed exclusively with dead-end polymeric cartridge filters. In parallel to the development of inorganic MF membranes, Ceraver initiated the development of cross-flow MF with backflushing as a new industrial process. 

Fig. 3-4 Dead-end polymerization of isoprene initiated by azobisisobutyronitrile. After Gobran et al. [1960] (by permission of Wiley-Interscience, New York).

This reaction has the characteristics of a dead end polymerization, and the conversion of monomeric MMA to polymer can be controlled via the azo content of the polystyrene and the reaction temperature. The separation of the reaction products into homopolymer and block copolymer was achieved by selective solvent extraction thus, cyclohexane was used to dissolve the homopolystyrene, acetonitrile the homo-poly-MMA and the copolymer was completely soluble in benzene. The compositition of the crude product as a function of the ratio of MMA/prepolymer is shown in Fig. 4.5 58> ... 

Dead-end polymerization is a useful technique for assessing k. Equation (6-35) can be written... 

Figure 6.3 Test of Eq. (6.99) for dead-end polymerization of isoprene-AIBN system at different temperatures (Problem 6.15).

Three different types of rate constants are of concern in ideal polymerization kinetics described by Eq. (6.26) —those for initiation (kd), propagation (kp), and termination (kt). The use of polymerization data under steady-state conditions allows the evaluation of only kd (see Dead-end polymerization). The ratio kp/kf ox k /kt can be obtained from Eq. (6.25) since Rp, Ri, and [M] are measurable. However, steady-state data do not allow... 

Controlled radical polymerization Chain transfer agent l,4-Diazabicydo[2,2,2]octane Dead-end polymerization D im ethylform amide Dimethyl sulfoxide 4,4,-Di-(5-nonyl)-2,2 bipyridine... 

Scheme 6 Synthesis of carboxy-telechelic polystyrene by dead-end polymerization (DEP)...

Table 4 Acid functionality /cooh of oligostyrene obtained by dead-end polymerization (DEP) with 4,4 -azobis(4-cyanovaleric acid) (ACVA)...

The effect of oxygen on the polymerization characteristics of the monomer is quite dramatic. In bulk polymerization of reasonably purified vinyl acetate, the process is autoaccelerated from the start and goes nearly to completion with a residual monomer content of 2-4%. When the monomer distillation is carried out in contact with air, inhibiting impurities form quite rapidly. These lead to dead-end polymerizations with 30-40% unreacted monomer left in the product [17]. 

In devising experimental procedures for the polymerization of vinyl esters, the elimination of oxygen is extremely important. Joshi [17] has shown that the bulk polymerization of vinyl acetate and vinyl propionate exhibited autoacceleration from the start and proceeded nearly to completion with only 2-4% unreacted monomer within 200 min in one case. When, however, the monomer had come in contact with air, inhibiting impurities developed and even after hours of heating, dead-end polymerization had taken place with 30-40% of unreacted monomer remaining. 

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