Cationic polymerization molecular weight distribution

Cationic polymerization of phosphoranimines initiated by small amounts of PCI5 in dichloromethane at ambient temperature offers a new route for the preparation of polyphosphazenes. Initiation of Me3SiNPCl3 gives poly(dichloro-phosphazene) with a narrow molecular weight distribution. The polymerization can be characterized as a living cationic polymerization. ... 

In Chap. 8, we have learned that a flow microreactor system incorporating a micromixer is useful in controlling competitive consecutive reactions. The ultimate reaction system in which reactions occur in chains, or consecutively, is polymerization. This chapter describes how we can exploit the advantages of the flow microreactor system in controlling the molecular weight or molecular weight distribution in polymerization reactions, including cationic polymerizalion and anionic polymerization. 

Cationic polymerization was considered for many years to be the less appropriate polymerization method for the synthesis of polymers with controlled molecular weights and narrow molecular weight distributions. This behavior was attributed to the inherent instability of the carbocations, which are susceptible to chain transfer, isomerization, and termination reactions [48— 52], The most frequent procedure is the elimination of the cation s /1-proton, which is acidic due to the vicinal positive charge. However, during the last twenty years novel initiation systems have been developed to promote the living cationic polymerization of a wide variety of monomers. 

The cationic ring opening polymerization of e-caprolactone, CL, and 8-valerolactone, VL, was investigated using n-Bu0H/HCl-Et20 as the initiation system [56]. It was observed that narrow molecular weight distribution samples were obtained. These results were combined with those previously... 

Symmetric triblock copolymers of the ABA type, where B was PTHF and A poly(2-methyl-2-oxazoline), PMeOx, were prepared by cationic polymerization with trifluoromethanesulfonic anhydride as a difunctional initiator [58]. Subsequent hydrolysis of the PMeOx blocks with HC1 in a methanol/ water mixture resulted in the formation of the corresponding polyethylen-imine blocks (Scheme 20). Samples with relatively low molecular weight distributions were obtained. 

M), one observes a bimodal molecular weight distribution. This indicates the simultaneous occurrence of cationic and radical polymerizations since the two are known to yield different-molecular-weight polymers. With decreasing water concentration ( [Pg.225]

The theoretical molecular weight distributions for cationic chain polymerizations are the same as those described in Sec. 3-11 for radical chain polymerizations terminating by reactions in which each propagating chain is converted to one dead polymer molecule, that is, not including the formation of a dead polymer molecule by bimolecular coupling of two propagating chains. Equations 2-86 through 2-89, 2-27, 2-96, and 2-97 withp defined by Eq. 3-185... 

For polymerizations carried out to high conversions where the concentrations of propagating centers, monomer, and transfer agent as well as rate constants change, the polydispersity index increases considerably. Relatively broad molecular-weight distributions are generally encountered in cationic polymerizations. 

Since carbocations are involved in cationic polymerization, a possible side reaction is their isomerization through hydride (alkyde) migration to more stable (less reactive) carbocations. This can lead to a polymer of broad molecular weight distribution or, if the isomerization is irreversible, to termination. 

Yoshida and coworkers also developed a microreaction system for cation pool-initiated polymerization [62]. Significant control of the molecular weight distribution (Mw/Mn) was achieved when N-acyliminium ion-initiated polymerization of butyl vinyl ether was carried out in a microflow system (an IMM micromixer and a microtube reactor). Initiator and monomer were mixed using a micromixer, which was connected to a microtube reactor for the propagation step. The polymerization reaction was quenched by an amine in a second micromixer. The tighter molecular weight distribution (Mw/M = 1.14) in the microflow system compared with that of the batch system (Mw/M > 2) was attributed to the very rapid mixing and precise control of the polymerization temperature in the microflow system. 

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