Macroanion polymerization

Steady State Population Density Distributions. Representative experimental population density distri-butions are presented by Figure 1 for two different levels of media viscosity. An excellent degree of theoretical (Equation 8) / experimental correlation is observed. Inasmuch as the slope of population density distribution at a specific degree of polymerization is proportional to the rate of propagation for that size macroanion, propagation rates are also observed to be independent of molecular weight. 

It appears that a combination of polymeric ions with difunctional initiators should be of some advantage over the transformation processes described above [241]. We have found that the combination of a macroanion with a dicationic initiator and of a macrocation with a dianionic initiator proceeds with high efficiency [242]. Macroions can therefore be transformed by difunctional initiators... 

This kind of transformation is mentioned in the patent literature [250]. After mixing with RLi or ROLi, 1-alkenes polymerizing on ZN catalysts yield macroanions of the poly(l-alkene). In the presence of a suitable monomer, a block copolymer is formed... 

Anionic polymerizations are generally much faster than free-radical reactions although the A p values are of the same order of magnitude for addition reactions of radicals and solvated anionic ion pairs (free macroanions react much faster). The concentration of radicals in free-radical polymerizations is usually about 10 -10 M while that of propagating ion pairs is 10 -10 M. As a result, anionic polymerizations are lO -lO times as fast as free-radical reactions at the same temperature. 

A variation of the sequential monomer addition technique described in Section 9.2.6(i) is used to make styrene-diene-styrene iriblock thermoplastic rubbers. Styrene is polymerized first, using butyl lithium initiator in a nonpolar solvent. Then, a mixture of styrene and the diene is added to the living polystyryl macroanion. The diene will polymerize first, because styrene anions initiate diene polymerization much faster than the reverse process. After the diene monomer is consumed, polystyrene forms the third block. The combination of Li initiation and a nonpolar solvent produces a high cis-1,4 content in the central polydiene block, as required for thermoplastic elastomer behavior. 

Two typra of methods were u d in the cationic polymerization of hetraocycles to determine the concentration of the Rowing species. The first type is based on methods used in free-radical polymerization (radical trapping) and in anionic polymerization when the macroanions are quantitatively converted into the stable end groups whose concentrations can then be measured. 

Quantitative determination of the concentration of macroanions in the anionic polymerization of heterocyclics is based on the same approach of end-capping with P-contain-ing compounds. 

Problem 8.3 Account for the fact that anionic polymerizations are generally much faster than free-radical reactions although the kp values are of the same order of magnitude for addition reactions of radicals and solvated ion pairs (free macroanions react much faster). 

Synthesis and characterization of block copolymers. Polyisobutylene b-polyiteru butyl methacrylate) (PIB-b-PtBMA) block copolymers were synthesized by using the PIB-DPE" macroanion as initiator for the polymerization of tBMA. 

DPV-capped PIB chains in a mixture yield the same macroanion (Scheme 2) and lead to the same block copolymer by anionic polymerization. 

Ionic polymerizations are characterized by successive monomer additions to a growing macroion. Here, anionic polymerizations, with macroanions as active growth centers, are distinguished from cationic polymerizations which have macrocations as active growing centers ... 

A new monomer molecule is added onto the growing anionic end of the polymer chain in every propagation step in the case of a classic anionic polymerization. In this case, it is immaterial how the macroanion is formed or how many anions are available per polymer chain. 

A great many anionic polymerizations are "living a chain propagation without termination or transfer follows a very fast start reaction, i.e., there is no destruction of the individual chain carriers. In this case, the rate of polymerization, Vt t, is simply given by the rate of the propagation, which, in turn, is a first-order reaction with respect to the concentrations of macroanions and monomer ... 

Living polymerizations do not have either transfer or termination reactions, that is, the active chain carrier remains bound to an individual polymer chain up to the yield determined by the monomer-polymer equilibrium. The ionic ends of the living polymer can thus be used to produce block polymers of defined structure. This ability, however, depends on the polarity of the growing macroanion and the monomer to be added on. To a first approximation, the polarity can be described in terms of what is known as the e values of the two monomers. Electron-poor monomers have high e values and electron-rich monomers have strongly negative e values (see also Section 22.2.5). For example, the poly(methyl methacrylic anion) (monomer e = 0.40) starts the polymerization of acrylonitrile e = 1.20), but not that of styrene e = —O.SO). Conversely, however, the poly(styryl anion) can start the polymerization of methyl methacrylate. 

The block copolymer preparative techniques reviewed in this report include sequential addition to macroanions, "pseudoliving" carbenium ion systems, used of coordination catalysts, coupling of polymer termini, and various preparations employing free radical polymerization. The review covers 133 papers and patents. 

It is obvious that anion chain transfer must be avoided to prepare good yields of block copol37mers with "living" macroanions. Because of susceptibility to chain transfer many monomers capable of anionic polymerization will not readily produce block copolymers. 

For example, methyl methacrylate block copolymers are much less studied than those of styrene. Anion chain transfer occurs at the pendent ester group, drastically reducing the yield of block copolymers. Poly(methyl methacrylate-b-isoprene) has been prepared, however, by using an ingenious chain cap of l,l -diphenylethyl-ene(27,28). i l diphenylethylene will not anionically homopolymerize, therefore it adds only one mer to the macroanion. This anion is more stable in the presence of methyl methacrylate, but will initiate further polymerization. Other workers have reported the preparation of isoprene-methyl methacrylate block copolymers by sequential addition to "living" polyisoprene anions(29,30),... 

Anionic polymerization of lactams offers the best approach to the preparation of polyamide containing block copolymers. Styrene-nylon 6 block copolymers were prepared by adding e-caprolactam to polystyrene macroanions terminated with bisphenol A bis(chlorofor-mate)(31). Yamashita prepared ABA block copolymers of styrene-a-pyrrolidone and styrene- -caprolactam by sequential addition to styrene macroanions( ). Similarly Stehlik and Sebenda prepared N-acrylamide containing block copolymers(33). Block copolymers of isoprene-pivalolactam have also been reported( . In these cases the lactam was added to "living" polyisoprene anions. 

In the anionic polymerization of a-methylstyrene with sodium naphthalene the reaction proceeds to an equilibrium and it is possible to observe the temperature dependence of the equilibrium between monomer and polymer. After addition of the monomer, the deep green color of the initiator solution is transformed into the red color of the carbanions. At low temperatures (—70 to —40°C) the living polymer is formed and the solution becomes viscous. After warming, the macroanions... 

Similarly, stable macroanions obtained by the subsequent metalation of the proper end groups such as diphenylvinyl and diphenylmethoxy were used in the living polymerization of tBMA yielding PIB-l7-PtBMA block copolymers with almost quantitative efficiency (Scheme 35). Moreover, amphiphilic polymeric materials can be prepared by hydrolysis of ester moieties of the polymers obtained by this method for instance, amphiphilic PIB-I7-PMAA diblock was prepared by the hydrolysis of the acrylate segment of the suitable precursor copolymer. A series of linear and star copolymers consisting of PIB and PMMA were also prepared. ... 

In anionic polymerizations, two types of growth are possible via growing macroanions (Section 18.2.3) or via monomer anions (Section 18.2.4). 

The classic anionic polymerization is an anionic polymerization with growth via macroanions. A new monomer molecule is added onto the growing polymer chain end in every propagation step, as the general form of (18-1) shows. It is immaterial how the macroanion has been produced, that is, if the start reaction was a two-electron reaction with formation of a monomer anion or zwitterion or whether it was a one-electron transfer. 

The polymerization rate depends very much on the proportion of free macroanions present. In the anionic polymerization of styrene with sodium as gegenion in THF, kinetic measurements (see below) gave the rate constants for polymerization via free macroanions as /C(-> = 65000 dm mol s" and fc(+) = 80 dm mol s for the polymerization via ion pairs (Table 18-2). According to the relation Vp = kp[P ] [M], the rate of the propagation reaction depends on the active species concentration [P ] as well as on the rate constant kp. But in this system, the dissociation equilibrium constant Kjy for ion pairs into free ions is only 10" mol/dm. If the polymerization is carried out in a solution with 10 mol/dm, then the proportion of free ions is consequently only (10 10 ) = 0.01, that is, 1 %, with 99% of the active species being ion pairs. Thus, despite a much lower rate constant, the polymerization rate is also determined to a considerable extent by the ion pairs. 

The anionic polymerization of monomers with NH groups with strong bases does not occur via the addition of monomers to a growing macroanion, but via the addition of the monomer anion on a macromolecule. In the first step, a proton is abstracted by the strong base, for example, as in the reaction of an AT-carboxy anhydride with an alkoxide ion... 

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