Living polymer chains

FIG. 12 Segment density profile as function of the distance from the wall Z for flexible (empty symbols) and semi-rigid (full symbols) living polymer chains at T = 0.4 [28]. The fractional occupancy of lattice sites by polymer segments is shown for the layers in the left half of the box. Dashed lines are guides for the eyes. 

AT is intended to include any and all of the effects of the sorption rate of monomer on the surface, steric arrangement of active species, the addition of the monomer to the live polymer chain, and any desorption needed to permit the chain to continue growing. We assume a steady state in which every mole of propylene that polymerizes is replaced by another mole entering the shell from the gas, so that all of the fluxes are equal to Ny gmol propylene reacted per second per liter of total reactor volume. The following set of equations relates the molar flux to each of the concentration driving forces. 

In this system, a living PE chain-end reacts with Et2Zn only after all ethylene has been consumed, leading to a Zn-terminated PE chain and a Ti species capable of growing another living chain upon addition of monomer. This is the first example of the production of multiple living polymer chains per catalyst. 

There are many electrophiles which not only terminate living polymer chains but also produce end-group substitution. For example, macromolecules with hydroxyl, carboxyl, thiol, or chlorine termini can be prepared by reacting living polymers with such compounds as epoxides, aldehydes, ketones, carbon dioxide, anhydrides, cyclic sulfides, disulfides, or chlorine (15-23). However, primary and secondary amino-substituted polymers are not available by terminations with 1° or 2° amines because living polymers react with such functionalities (1.). Yet, tert-amines can be introduced to chain ends by use of -N-N-di-methylamino-benzaldehyde as the terminating agent (24). 

In principle, mono-, di-, and polyfunctional terminal polymers are all available by electrophilic termination of living polymer chains. For example, difunctional polymers can be prepared by the use of dilithium initiation, followed by ditermination (25-35). However, the strict end-use requirements (e.g., linear chain extension) for difhnctional materials are especially demanding. 

Eq. (21) has been interpreted in terms of a mechanism 84) where the rate-determining step of the propagation of a living polymer chain is the insertion of the coordinated propylene monomer into a vanadium-polymer bond, V -P. 

The relations (23) and (24) seem to indicate that a strong interaction between vanadium and the propylene monomer is unfavorable for the insertion of the coordinated propylene into a living polymer chain. 

The recent development of living cationic polymerization systems has opened the way to the preparation of rather well defined star homopolymers and miktoarm star polymers [19 and see the chapter in this volume]. Divinyl ether compounds were used as linking agents in a manner similar to the DVB method for anionic polymerization. Typically the method involves the reaction of living polymer chains with a small amount of the divinyl compound. A star polymer is formed carrying at the core active sites capable of initiating the polymerization of a new monomer. Consequently a miktoarm star copolymer of the type AnBn is produced. 

Individual methods have also been devised for the preparation of miktoarm stars. One of these approaches involves the preparation of macromonomers possessing either central or end vinyl groups which can be used to produce miktoarm stars either by copolymerization of the double bonds or by reacting the double bonds with living polymer chains, thus creating active centers able to initiate the polymerization of another monomer. All these methods are limited to specific synthetic problems and cannot be used for the preparation of a wide range of different structures. 

Fig. 28. Modifications of the IR spectrum of living polymer chains on stoichiometric a-Cr203 induced by CO solid curve, before 12CO adsorption dashed curve, after 12CO adsorption (p = 2.66 kPa) (Inset) Expanded view of the

Alternatively, a living polymer chain can be deactivated by reacting its active site with an unsaturated nucleophile. Here again the double bond should not undergo side reactions. 

The structure and chemical properties of metal-allyl compounds (ir-allylic, dynamic and a-allylic) which can be considered as models of a living polymer chain in butadiene polymerization have been studied. The polymerization of dienes proceeds only in dynamic allylic systems through the metal-ligand ir-bond in a-isomers. 

The kinetic scheme with constant reaction of the polymer/monomer droplet increases fairly quickly with conversion, and the mobility of the polymer chains rapidly falls below the mobility of the monomer. The reduced diffusion of live polymer chains in the droplet will reduce the rate of termination of polymerization. The associated increase in the number of radicals will cause a rapid increase in the polymerization rate. This phenomenon is well known as the Trommsdorf or gel effect [8,9]. The gel effect causes a growth of the polymer chain length and widening of the molecular weight distribution (Figure 9.5). 

Polyacrylates and polymethacrylates can be end-capped by a functional group at one chain-end according to two strategies, by which either the initiator bears the envisioned functional group (protected or not), or living polymer chains are reacted with a duly substituted electrophile. 

Here a live polymer chain of j monomer units tranfers its free radical to the monomer to form the radical R, and a dead polymer chain of 7 monomer units. 

Net rate of In general, the net rate of disappearance of live polymer chains with ... 

Figure E7-5.I Moments of live polymer chain lengths (a) number-average chain length (b> weight-average chain length (c) polydispersity.

More recently it has been shown that in the polymerization with TT-crotylnickel iodide the order in monomer falls from a value close to unity at [M] below 0.5 mole 1" to below 0.5 at [M] > 4 mole 1 . These observations have been interpreted in terms of scheme (c) on p. 162, namely coordination of two monomer molecules with the catalyst and with most of the catalyst existing in the complex (inactive) state. The molecular weights of the polymers are double those calculated from the kinetic scheme put forward [61] and this is attributed to coupling of live polymer chains on termination [251]. Molecular weight distributions are binodal consistent with slow propagation and transfer. 

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