Chain transfer with polymer

The kinetic treatment corresponds to that given above for the case of chain transfer with polymer. We have merely to write for the rate of generation of cross-linked units... 

The main topic of interest is the properties of molecules of finite size, having no large rings, and in general having trifunctional branch-points. These are typically produced by chain-transfer with polymer in free-radical polymerizations, though they can of course be made in other ways. Molecules with branch-points of higher functionality are also of interest, especially star-shaped molecules with several arms, as these are both easy to synthesize and relatively easy to discuss theoretically. 

Several general disadvantages of bulk polymerization (removal of the reaction heat, shrinkage, nonsolubility of the resulting polymer in the monomer, side reactions in highly viscous systems such as the Trommsdorff effect or chain transfer with polymer) are responsible for the fact that many polymerization processes are carried out in the presence of a solvent. A homogeneous polymerization occurs when both monomer and polymer are soluble in the solvent. When the polymer is insoluble in the solvent, the process is defined as solution precipitation polymerization. Other heterogeneous polymerization reactions in liquid-solid or liquid-liquid systems such as suspension or emulsion polymerizations are described later. Conventional solution polymerization is compared with solution precipitation polymerization for the synthesis of acrylic resins in Ref. [34]. 

As noted above, chain transfer to polymer does not interfere with the determination of other transfer constants, since the latter are evaluated at low conversions. In polymer synthesis, however, high conversions are desirable and extensive chain transfer can have a dramatic effect on the properties of the product. This comes about since chain transfer to polymer introduces branching into the product ... 

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

Nonlinear structures may arise in vinyl polymerizations through chain transfer with monomer or with previously formed polymer molecules, but such processes usually occur to an extent which is scarcely significant. A more common source of nonlinearity in the polymerization of a 1,3-diene is the incorporation in a growing chain of one of the units of a previously formed polymer molecule. The importance of both branching by chain transfer and cross-linking by addition of a polymer unit increases with the degree of conversion of monomer to polymer. 

As stated above, a necessary precondition for CCTP and chain shuttling is reversible chain transfer with a chain transfer agent. We distinguish CCTP from chain shuttling in that the former is reversible exchange of polymer chains between like catalysts whereas the latter is reversible exchange of polymer chains between two or more different kinds of catalysts. 

The authors conducted a similar investigation of precatalysts 7 and 11 using TiBA and trityl tetrakis(pentafluorophenyl)borate as the cocatalyst. They concluded that this material contained no fraction that could be characterized as blocky. It was therefore proposed that reversible chain transfer occurred only with MAO or TMA and not with TiBA. This stands in contrast to the work of Chien et al. [20] and Przybyla and Fink [22] (vida supra), who claim reversible chain transfer with TiBA in similar catalyst systems. Lieber and Brintzinger also investigated a mixture of isospecific 11 and syndiospecific 12 in attempts to prepare iPP/sPP block copolymers. Extraction of such similar polymers was acknowledged to be difficult and even preparative temperature rising elution fractionation (TREF) [26, 27] was only partially successful. 

Reactivity Is typical of an acrylamide. For example, compound 1 shows essentially 1 1 copolymerizablllty with butyl acrylate. Copolymerizablllty has also been demonstrated with styrene, other acrylates and methacrylates, vinyl acetate (VAc), VAc/ethylene and vinyl chlorlde/ethylene. High molecular weight polymers and copolymers remain soluble. Indicating any chain transfer to polymer, e.g. through abstraction of the acetal hydrogen. Is minor. 

Polymer production proceeds as described in structure 5.25. An initiator, such as sulfuric acid, produces an oxonium ion and a gegenion. The oxonium ion then adds to the oxirane, ethylene oxide, producing a macrooxonium ion with growth eventually terminated by chain transfer with water. 

Using the methods described, the values of Cm and Ci in the benzoyl peroxide polymerization of styrene have been found to be 0.00006 and 0.055 respectively [Mayo et al., 1951]. The amount of chain transfer to monomer that occurs is negligible in this polymerization. The chain-transfer constant for benzoyl peroxide is appreciable, and chain transfer with initiator becomes increasingly important as the initiator concentration increases. These effects are shown in Fig. 3-7, where the contributions of the various sources of chain ends are indicated. The topmost plot shows the total number of polymer molecules per 105 styrene monomer units. The difference between successive plots gives the number of polymer molecules terminated by normal coupling termination, transfer to benzoyl peroxide, and transfer to styrene. 

The low reactivity of a-olefins such as propylene or of 1,1-dialkyl olefins such as isobutylene toward radical polymerization is probably a consequence of degradative chain transfer with the allylic hydrogens. It should be pointed out, however, that other monomers such as methyl methacrylate and methacrylonitrile, which also contain allylic C—H bonds, do not undergo extensive degradative chain transfer. This is due to the lowered reactivity of the propagating radicals in these monomers. The ester and nitrile substituents stabilize the radicals and decrease their reactivity toward transfer. Simultaneously the reactivity of the monomer toward propagation is enhanced. These monomers, unlike the a-olefins and 1,1-dialkyl olefins, yield high polymers in radical polymerizations. 

Several chain transfer to polymer reactions are possible in cationic polymerization. Transfer of the cationic propagating center can occur either by electrophilic aromatic substituation or hydride transfer. Intramolecular electrophilic aromatic substituation (or backbiting) occurs in the polymerization of styrene as well as other aromatic monomers with the formation of... 

Termination may also occur by chain transfer with the initiator (e.g., water or alcohol) or a deliberately added chain-transfer agent. Deliberate termination of growth is carried out to produce polymers with specific molecular weights or, more often, telechelic polymers with specific end groups. Hydroxyl and amine end groups are obtained by using water and ammonia as chain-transfer agents. Carboxyl-ended telechelics can be obtained by termination with ketene silyl acetal followed by hydrolysis with base [Kobayashi et al., 1989]. 

In radical template polymerization, when only weak interaction exists between monomer and template and pick-up mechanism is commonly accepted, the reaction partially proceeds outside the template. If macroradical terminates by recombination with another macroradical or primary radical, some macromolecules are produced without any contact with the template. In fact, such process can be treated as a secondary reaction. Another very common process - chain transfer - proceeds simultaneously with many template polymerizations. As a result of chain transfer to polymer (both daughter and template) branched polymers appear in the product. The existence of such secondary reactions is indicated by the difficulty in separating the daughter polymer from the template as described in many papers. For instance, template polymerization of N-4-vi-nyl pyridine is followed, according to Kabanov et aZ., by the reaction of poly(4-vinylpyridine) with proper ions. The reaction leads to the branched structure of the product ... 

The termination of the growing polymeric chain may occur through several different processes, mostly by chain transfer. Either the process of chain transfer with the monomer, or the reaction of dissociation to hydride, leads to the formation of terminal vinylidenic groups, whose presence was noticed in the olefin polymers, obtained with the previously described catalysts (22). 

The tris-allyl complex, in each case, produced a 1.2 growth step of the butadiene molecule. With the more anionic (or less cationic) cobalt salt, the growth occured to only the dimer before it underwent anionic hydride chain transfer. With less anionic chromium the 1.2 chain growth continued on the produce polymer. 

Chain transfer reactions in THF polymerizations have not been considered until rather recently. Compounds known to be effective chain transfer agents include dialkyl ethers, orthoesters, and water. In addition, chain transfer to polymer and with gegenion is possible. 

The similarity between polymers and homologous low molecular weight compounds may be sometimes disturbed on account of the presence of some highly reactive sites, e.g. unsaturated end groups due to chain termination by disproportionation (a) or to chain transfer with monomer (6) ... 

---