Reversible polymerization copolymerization

In block copolymerizations with vinyl ethers, as seen in this example, a styrene derivative should be polymerized after the first-stage polymerization of a vinyl ether component, and an additional dose of a Lewis acid (activator) is usually needed to accelerate the second-phase polymerization. The reverse polymerization sequence (from a styrene derivative to a vinyl ether) often results in a mixture of block copolymers and homo-... 

Figure 22-7. Change in mole fraction of monomeric unit, of the reversibly polymerizing a-methyl styrene in the free radical copolymerization with methyl methacrylate at 60 C (above) or with acrylonitrile at 80 C (below) as a function of the overall monomer concentration. In the former case, reversibly depolymerizing disequences (—), and in the latter case, nondepolymerizing disequences (—), were found. (After data from P. Wittmer.)...

Entropy, enthalpy, and free energy of reversible polymerization Arrhenius relationship for rate constants Subcritical damped oscillations during thermal polymerization Polyrate of terpolymerization of AMS-AN-Sty Enthalpy of random copolymers Effect of chain sequence distribution Entropy and free energy of copolymerization Copolymer composition with and without ceiling temperature effect... 

Derives expressions for enthalpy and entropy of copolymerization, the chain sequence distribution of copolymers, and the Clapeyron equation for reversible polymerization... 

High molecular weight polymers or gums are made from cyclotrisdoxane monomer and base catalyst. In order to achieve a good peroxide-curable gum, vinyl groups are added at 0.1 to 0.6% by copolymerization with methylvinylcyclosiloxanes. Gum polymers have a degree of polymerization (DP) of about 5000 and are useful for manufacture of fluorosiUcone mbber. In order to achieve the gum state, the polymerization must be conducted in a kineticaHy controlled manner because of the rapid depolymerization rate of fluorosiUcone. The expected thermodynamic end point of such a process is the conversion of cyclotrisdoxane to polymer and then rapid reversion of the polymer to cyclotetrasdoxane [429-67 ]. Careful control of the monomer purity, reaction time, reaction temperature, and method for quenching the base catalyst are essential for rehable gum production. 

With most common monomers, the rate of the reverse reaction (depropagation) is negligible at typical polymerization temperatures. However, monomers with alkyl groups in the a-position have lower ceiling temperatures than monosubstituted monomers (Table 4.10). For MMA at temperatures <100 °C, the value of is <0.01 (Figure 4.4). AMS has a ceiling temperature of <30 °C and is not readily polymerizable by radical methods. This monomer can, however, be copolymerized successfully (Section 7.3.1.4). 

Polymerization equilibria frequently observed in the polymerization of cyclic monomers may become important in copolymerization systems. The four propagation reactions assumed to be irreversible in the derivation of the Mayo-Lewis equation must be modified to include reversible processes. Lowry114,11S first derived a copolymer composition equation for the case in which some of the propagation reactions are reversible and it was applied to ring-opening copalymerization systems1 16, m. In the case of equilibrium copolymerization with complete reversibility, the following reactions must be considered. 

As described in Section 9.1.2.2.3, several lanthanocene alkyls are known to be ethylene polymerization catalysts.221,226-229 Both (188) and (190) have been reported to catalyze the block copolymerization of ethylene with MMA (as well as with other polar monomers including MA, EA and lactones).229 The reaction is only successful if the olefin is polymerized first reversing the order of monomer addition, i.e., polymerizing MMA first, then adding ethylene only affords PMMA homopolymer. In order to keep the PE block soluble the Mn of the prepolymer is restricted to <12,000. Several other lanthanide complexes have also been reported to catalyze the preparation of PE-b-PMMA,474 76 as well as the copolymer of MMA with higher olefins such as 1-hexene.477... 

The molecular weight distribution of a polymer produced with a chain shuttling catalyst/CSA system is highly dependent on reaction conditions. The extent of reversibility with the catalyst/CSA pairs was therefore further explored through a series of polymerizations over a range of monomer conversions (i.e., yield). A representative example from this secondary screening process is described below for precatalyst 17. Several members from this well-studied bis(phenoxyimine)-based catalyst family [39] were identified as poor incorporators in the primary screen. A series of ethylene/octene copolymerizations using 17 was performed across a... 

The above theory can be extended to deal with other more complex cases. For example, the two ends of a biopolymer need not behave identically, and, as noted earher, MTs are helical polymers of asymmetric protomer units. Thus, two sets of on- and off-constants might be necessary. In other cases, such as in the polymerization of tubulin in the presence of tubulin-colchicine complex (Sternlicht et one may need to consider copolymerization. The kinetics of microtubule depolymerization are the reverse of elongation, and are gener-... 

In the conventional emulsion polymerization, a hydrophobic monomer is emulsified in water and polymerization initiated with a water-soluble initiator. Emulson polymerization can also be carried out as an inverse emulsion polymerization [Poehlein, 1986]. Here, an aqueous solution of a hydrophilic monomer is emulsified in a nonpolar organic solvent such as xylene or paraffin and polymerization initiated with an oil-soluble initiator. The two types of emulsion polymerizations are referred to as oil-in-water (o/w) and water-in-oil (w/o) emulsions, respectively. Inverse emulsion polymerization is used in various commerical polymerizations and copolymerizations of acrylamide as well as other water-soluble monomers. The end use of the reverse latices often involves their addition to water at the point of application. The polymer dissolves readily in water, and the aqueous solution is used in applications such as secondary oil recovery and flocculation (clarification of wastewater, metal recovery). 

Since the reversal of activity of butadiene with respect to styrene in alkyllithium system has been observed (12), it would be of interest to find out whether the inversion phenomenon still holds in the case of the lithium morgholinide system. Four temperatures, namely 30, 40, 50 and 60 C were chosen for this study. At 30°C polymerization temperature the curve is characteristic of block copolymerization when one plots percent bound styrene vs percent conversion (Fig. 1). Initially, a small amount (/>/3%) of styrene is polymerized. This is followed by a block of butadiene. The remaining styrene is then polymerized after all the butadiene is consumed. This result is identical to the alkyllithium initiated copolymerization. 

The copolymerization equation is valid if all propagation steps are irreversible. If reversibility occurs, a more complex equation can be derived. If the equilibrium constants depend on the length of the monomer sequence (penultimate effect), further changes must be introduced into the equations. Where the polymerization is subjected to an equilibrium, a-methylstyrene was chosen as monomer. The polymerization was carried out by radical initiation. With methyl methacrylate as comonomer the equilibrium constants are found to be independent of the sequence length. Between 100° and 150°C the reversibilities of the homopolymerization step of methyl methacrylate and of the alternating steps are taken into account. With acrylonitrile as comonomer the dependence of equilibrium constants on the length of sequence must be considered. 

In the following the reversibility of the polymerization of a-methyl-styrene has been taken into account. The copolymerization curves are calculated via Equation 33 together with Equation 34. The equilibrium constants necessary for the calculation are taken from Table II. The depolymerization of methyl methacrylate (I) can be neglected in the temperature range investigated since the equilibrium constants for this monomer (fC2) are extremely small compared with the value for -methyl-styrene—e.g., (21) at 100°C, Ki = 22.9 mole/liter, K2 — 0.12 mole/liter at 80 °C, Ki = 12.9 mole/liter, fC2 = = 0 049 mole/liter. 

In the following the reversibility of the polymerization of a-methyl-styrene has been considered. The copolymerization curves were calcu-... 

It is not possible to predict which mechanism is involved in a certain copolymerization. In the system a-methylstyrene-methyl methacrylate depolymerization of sequences of two monomer units seemed to occur as well as depolymerization of a-methylstyrene from longer sequences. In the system a-methylstyrene -acrylonitrile the sequence of two monomer units of a-methylstyrene is stable and does not depolymerize. The reversibility of the polymerizations of a-methylstyrene and methyl methacrylate can be explained by sterically induced strain in the polymer chain (13). In the copolymer a-methylstyrene-methyl methacrylate this strain involves the whole polymer chain whereas in the a-methylstyrene— acrylonitrile system the strain is broken by the acrylonitrile sequences and is built up again in the a-methylstyrene. This explains the difference in the depolymerization tendencies of sequences of two units of a-methylstyrene and longer sequences in this system. 

N-isopropylacrylamide 1 is added to the polymerization mixture to increase hydro-phobicity of the monolith required for the separations in reversed phase mode. Vinylsulfonic acid 12 provides the chargeable functionalities that afford electroosmo-tic flow. Since the gelation occurs rapidly already at the room temperature, the filling of the channel must proceed immediately after the complete polymerization mixture is prepared. The methacryloyl moieties attached to the wall copolymerize with the monomers in the liquid mixture. Therefore, the continuous bed fills the channel volume completely and does not shrink even after all solvents are removed. Fig. 6.8 also shows scanning electron micrograph of the dry monolithic structure that exhibits features typical of macroporous polymers [34],... 

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