Nucleophilicity of monomers

The importance of termination can be ex nessed by the ratio of k /kp, depending on the difference of nucleophilicities of monomer and ankins as well as on the inherent stability of the anion. 

Cationic polymerization of both cyclic amines and sulfides is combined in this section because of the similarities in their polymerization. This similarity is due to the high nucleophilicities of monomers that belong to these groups, the high stability of the active centers and the relatively higher basicity of heteroatoms in the polymer units than in the corresponding monomers. This combination of features leads to very similar synthetic methods used for polymer preparations. 

Knowledge of the order of basicities of cyclic and linear ethers is important for understanding certain phenomena in cyclic ether polymerization. As indicated earlier, chain transfer to polymer is a general feature of the cationic polymerization of cyclic ethers because the nucleophilic site of the monomer molecule (oxygen atom) is transferred to the polymer unit. To what extent chain transfer to polymer competes with propagation depends on the relative nucleophilicity of monomer and polymer unit. Thus, for five-membered THF, the polymer unit is a weaker base than the monomer. This makes the polymer less reactive than the monomer in nucleophilic substitution type reactions. Consequently, for this monomer, chain transfer to polymer is slow as compared to propagation. In contrast, in the polymerization of three-membered EO, the polymer unit is more basic than monomer. Therefore, reactions involving the polymer chain are important in this system. Practical consequences will be discussed in the subsequent sections devoted to polymerization of different classes of cyclic ethers. 

Polymerization using the Stille coupling, the cross-coupling of aryl-alkenyl halides with organotins in the presence of palladium catalysts (Scheme 9.10),13 appeared in 1989 (Scheme 9.11).14 The low nucleophilicity of organotins makes it possible to use functionalized monomers for the polymerization.15... 

The relative rate of cationic homopolymerization is governed by three factors, ie. the concentration of the propagating species, the ring-opening reactivity of the growing species and the nucleophilic reactivity of the monomer. From kinetic studies196 197 of the polymerization of oxazolines and oxazines it was found that the second factor was the most important. On the other hand, the relative reactivity in the cationic copolymerization is mainly determined by the nucleophilicity of the monomer and for 2-substituted 2-oxazolines this is in the order of benzyl > methyl > > isopropyl > H > phenyl195. ... 

For the system in which transfer to monomer is molecular weight controlling an increase in nucleophilicity of counterions increases the PIB molecular weights. This is clearly shown by the decrease in Mv as 7-irradiation (absence of Gs) >... 

In sum, a relation of counteranion nucleophilicity and the molecular weight in isobutylene polymerization is discovered, according to which an increase in G nucleophility leads to an increase in the rate of termination but a decrease in the rate of chain transfer to monomer. Thus, an increase in G6 nucleophilicity leads to increased termination and hence decreased molecular weight for systems in which termination is molecular weight governing. Similarly, it leads to a decrease in rate of transfer and hence to an increase in molecular weights for systems in which chain transfer controls molecular weight. The nucleophilicity of G is determined by the... 

The initiator efficiency has to be considered jointly with the monomers involved The nucleophilicity of the initiator should be matched to the electron affinity of the monomer, as initiation should be fast and quantitative 7). If it is too small, initiation may be slow (and/or incomplete), which implies broadening of the molecular weight distribution and possibly loss of molecular weight control. If the nucleophilicity of the initiator is too high, side reactions may occur, as in the case of methyl methacrylate, where the ester carbonyl is attacked15). 

Table 3. Examples of systems in which nucleophilicity of initiator matches electron affinity of monomer...

Sequential addition of monomers 6 7-26-27-114) is the most obvious procedure. Once the first monomer has been polymerized, the resulting living species is used as a polymeric initiator for the polymerization of the second one. The monomers are to be added in the order of increasing electron affinity to provide efficient and fast initiation 26 U4). This condition is rather restrictive, and the number of monomer systems that can be used is limited (Table 5). Moreover, when the second monomer contains an electrophilic function (e.g. ester) which could lead to side reactions, it is necessary to first lower the nucleophilicity of the living site. This is best done by intermediate addition of 1.1-diphenylethylene25). The stabilized diphenylmethyl anions do not get involved in side reactions with ester functions, while initiation is still quantitative and fast. 

If the nucleophilicity of the anion is decreased, then an increase of its stability proceeds the excessive olefine can compete with the anion as a donor for the carbenium ion, and therefore the formation of chain molecules can be induced. The increase of stability named above is made possible by specific interactions with the solvent as well as complex formations with a suitable acceptor 112). Especially suitable acceptors are Lewis acids. These acids have a double function during cationic polymerizations in an environment which is not entirely water-free. They react with the remaining water to build a complex acid, which due to its increased acidity can form the important first monomer cation by protonation of the monomer. The Lewis acids stabilize the strong nucleophilic anion OH by forming the complex anion (MtXn(OH))- so that the chain propagation dominates rather than the chain termination. 

Block copolymers comprised of PS and polymethacrylate blocks with aliphatic stearyl or decyl side groups were prepared by the sequential addition of monomers, as shown in Scheme 1. Styrene was polymerized in THF at - 78 °C using s-BuLi as the initiator [11,12]. The nucleophilicity of the living polystyryllithium was reduced by reaction with DPE (in order to avoid reactions with the carbonyl groups), followed by the polymerization of the methacrylate monomer. Stearyl methacrylate, SMA is associated with... 

Triblock terpolymers PS-b-PBd-b-P2VP and PBd-b-PS-b-P2VP, where PBd is polybutadiene (mostly 1,2-PBd), were prepared in order to study the microphase separation by transmission electron microscopy, TEM and SAXS. In the first case the triblocks were synthesized by the sequential addition of monomers in THF using s-BuLi as the initiator [26]. For the second type of copolymers, living PBd-b-PS diblocks were prepared in benzene at 40 °C in the presence of a small quantity of THF in order to obtain the desired 1,2-content and to accelerate the crossover reaction as well. DPE was then added to decrease the nucleophilicity of the active centers in order to avoid side reactions with the THF, which in combination with benzene was the solvent of the final step. 

The additional complexity present in block copolymer synthesis is the order of monomer polymerization and/or the requirement in some cases to modify the reactivity of the propagating center during the transition from one block to the next block. This is due to the requirement that the nucleophilicity of the initiating block be equal or greater than the resulting propagating chain end of the second block. Therefore the synthesis of block copolymers by sequential polymerization generally follows the order dienes/styrenics before vinylpyridines before meth(acrylates) before oxiranes/siloxanes. As a consequence, styrene-MMA block copolymers should be prepared by initial polymerization of styrene followed by MMA, while PEO-MMA block copolymers should be prepared by... 

Monomer/Oligomer Synthesis. The first two steps in the four step reaction sequence of Figure 1 are capable of producing both monomer and oligomer. The first step, aromatic nucleophilic substitution, is a polymer forming reaction under the correct stoichiometric conditions. In order to favor the formation of monomer with a small amount of oligomer, the substitution was carried out at a 4 1 ratio of diol to dichlorodiphenyl sulfone. This led to a predominantly monomeric product (IV) with only the requirement that the excess diol be removed from the product to eliminate the potential presence of low molecular weight species in later reactions. 

Several particularities of phase transfer catalyzed polyetherification are as follows. Stoichiometric phase transfer catalyzed pol)rmerizations do not take place between stoichiometric ratio of monomers, since the nucleophilic monomer is always transferred in a small amount into the organic phase. Consequently, because their reaction is a non-stoichiometric one there is no need for an equimolar ratio between the two monomers to get polymers with high molecular weights. High molecular weight polymers are usually obtained also at low conversions. In several cases, even at 100 percent conversion the polydispersity of the obtained polymers is low, i.e., "Hw/Hh 1.3. At any conversion, the organic phase contains only pol3rmers with electrophilic chain ends, even when the nucleophilic monomer was used in excess. 

Dynamic polymeric systems utilizing the C=N exchange reaction have been reported by Lehn s group. They have suggested a polymerization system consisting of a fluorene-based dialdehyde monomer 4, cyclohexane diamine 5, and fluorene-based diamine 6 as a comonomer (Scheme 8.2) [20,21]. In principle, a 1 1 1 mixture of all monomers in ethanol was expected to yield the two-component polymers 7 and 8 together with all component-mixed polymers. However, polymer 7 was dominantly yielded (80%) due to the nucleophilicity of diamines. The nucleophilic-ity of aliphatic diaminocyclohexane is much higher than that of aromatic... 

Stronger Lewis acids such as SnCLi, TiCLt, and CH3AICI2 yield fast but uncontrolled polymerization with broad PDI. LCP of vinyl ethers can be achieved if the other components and reaction parameters are appropriately adjusted by various combinations of lower reaction temperature, added nucleophile, added common salt, and solvent prolarity. For example, polymerization of isobutyl vinyl ether using HC1 as the initiator (or one can use the preformed adduct of monomer and HC1) with SnCLt or TiCLj in CH2CI2 is non-LCP... 

The initiator required to polymerize a monomer depends on the reactivity of the monomer toward nucleophilic attack. Monomer reactivity increases with increasing ability to stabilize the carbanion charge. Very strong nucleophiles such as amide ion or alkyl carbanion are needed to polymerize monomers, such as styrene and 1,3-butadiene, with relatively weak electron-withdrawing substituents. Weaker nucleophiles, such as alkoxide and hydroxide... 

The stability of polystyryl carbanions is greatly decreased in polar solvents such as ethers. In addition to hydride elimination, termination in ether solvents proceeds by nucleophilic displacement at the C—O bond of the ether. The decomposition rate of polystyryllithium in THF at 20°C is a few percent per minute, but stability is significantly enhanced by using temperatures below 0°C [Quirk, 2002], Keep in mind that the stability of polymeric carbanions in the presence of monomers is usually sufficient to synthesize block copolymers because propagation rates are high. The living polymers of 1,3-butadiene and isoprene decay faster than do polystyryl carbanions. 

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