Chain transfer reversible

For very active transfer agents, the transfer agcnt-dcrivcd radical (T ) may partition between adding to monomer and reacting with the polymeric transfer agent (Pn ) even at low conversions. The transfer constant measured aecording to the Mayo or related methods will appear to be dependent on the transfer agent concentration (and on the monomer conversion). A reverse transfer constant can be defined as follows (eq. 20)  

Systems that give reversible chain transfer can display the characteristics of living polymerization. Such systems are discussed in Section 9.5. 

The reactivity of RCTA towards the active species has a great influence on the M and MMD evolution with monomer conversion (x), and is controlled by the value of Cex (Cex = kex/fcp) where kp is the rate constant for propagation. Analytical equations for the evolution of Xn and PDI with x, are given as follows (Muller et al, 1995)  

1 Methyl methacrylate (60° C) CH2 CH2 CH3(C02Me)CH3 C02Me 0.013 Cacioli etal. (1986) 

9 Methyl methacrylate (60° C) s s Poly(methyl methacrylate) Ph 140 Goto et al. (2001) 

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Microemulsion and miniemulsion polymerization processes differ from emulsion polymerization in that the particle sizes are smaller (10-30 and 30-100 nm respectively vs 50-300 ran)77 and there is no discrete monomer droplet phase. All monomer is in solution or in the particle phase. Initiation usually takes place by the same process as conventional emulsion polymerization. As particle sizes reduce, the probability of particle entry is lowered and so is the probability of radical-radical termination. This knowledge has been used to advantage in designing living polymerizations based on reversible chain transfer (e.g. RAFT, Section 9.5.2)." 2... 

It can also be noted that reversible chain transfer, in RAFT and similar polymerizations, and reversible activation-deactivation, in NMP and ATRP,... 

Certain alkyl iodides give reversible chain transfer with S and some fluoro-olefins (Section 9.5.4). In these cases, the polymerization can show some living characteristics. 

The reversible chain transfer process (c) is different in that ideally radicals are neither destroyed nor formed in the activation-deactivation equilibrium. This is simply a process for equilibrating living and dormant species. Radicals to maintain the process must be generated by an added initiator. 

It is also known that alkyl cobaloximes are subject to radical-induced decomposition.2 7 This suggests an alternative to the mechanism shown in Scheme 9.28 involving reversible chain transfer (Section 9.5). 

Analytical expressions have been derived for calculating dispcrsitics of polymers formed by polymerization with reversible chain transfer. The expression (eq. 17) applies in circumstances where the contributions to the molecular weight distribution by termination between propagating radicals, external initiation, and differential activity of the initial transfer agent are negligible.21384... 

ORl OX w di-Miutyl peroxyoxalalc deactivation by reversible chain transfer and bioinolecular aclivaiion 456 atom transfer radical polymerization 7, 250, 456,457, 458,461.486-98 deactivation by reversible coupling and untmolecular activation 455-6, 457-86 carbon-centered radical-mediated poly nierizaiion 467-70 initiators, inferlers and iriiters 457-8 metal complex-mediated radical polymerization 484... 

The Zr-FI catalyst selectively forms PE even in the presence of ethylene and 1-octene, while the Hf complex affords amorphous copolymers, resulting in the catalytic generation of PE- and poly(ethylene-c6>-l-octene)-based multiblock copolymers through a reversible chain transfer reaction mediated by R2Zn. The development of an FI catalyst with extremely high ethylene selectivity as well as a reversible chain transfer nature has made it possible to produce these unique polymers. Therefore, both Ti- and Zr-FI catalysts are at the forefront of the commercial production of polyolefinic block copolymers. 

Independent of the ligand system, two different activation methods have been used in performing the propylene polymerization experiments. In both cases, the catalytic activities and molecular weights of the polymers are a sensitive function of the aluminum content provided by the activators. This dependence suggested an additional reversible chain transfer to aluminum when activating with MAO. As lower contents of A1 are provided in the polymerization system in the case of in situ activation with TIBA/borate, the only mechanism occurring is the chain back-skip. Furthermore, the differences in the polymer microstructures prepared with MAO and borate as cocatalysts are reflected. They sustain the proposed reversible chain transfer. 

Hild S, Rieger B, Troll C, Cobzaru C (2006) Elastomeric poly(propylene) from dual-side metallocenes reversible chain transfer and its influence on polymer microstructure. Macromol Chem Phys 207 665-683... 

Reversible Chain Transfer in Dual-Catalyst Systems. 71... 

For these processes to work, fast and reversible chain transfer between the catalysts and the CSA is necessary. Such reversible chain transfer has been called variously... 

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. 

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

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. 

Rieger et al. described a heteroatom-containing C, symmetric metallocene 13 whose stereoselectivity depended on the activator [28, 29], The resulting PP contained fewer stereoerrors when activated with a combination of TiBA and trityl tetrakis(pentafluorophenyl)borate than with MAO. In addition, the molecular weight was lower with MAO. To explain this, it was proposed that some of the stereoerrors arise by reversible chain transfer to aluminum. 

Later Rytter et al. reported possible polymer chain exchange with polypropylene produced with a combination of 8 and 11 with TMA [32], The number of stereoerrors increased in the binary system at higher TMA levels. As discussed in the case of Przybyla and Fink (vida supra), pentad analysis is less compelling evidence for reversible chain transfer. In addition, the gel permeation chromatography (GPC) data showed bimodal peaks, indicating very limited reversible transfer. 

As an opposite extreme, cases of fully reversible chain transfer (Ca° = C) were simulated. The effect of conversion on MJMn is plotted in Fig. 4 as a function of Ca with Aeq = 50. Early in the reaction, or low Xp the A1JMn quickly plunges below 2.0, followed by a steady convergence back to 2.0 at higher Xf. Faster reversible chain... 

Fig. 4 Simulated M IM vs. conversion as a function of chain shuttling constant for reversible chain transfer, with C° = C,...

The plot in Fig. 6 depicts the response of both Mn and MJMn as a function of conversion for a realistic case of fast reversible chain shuttling, with Ca° = Ca = 50 and Aeq = 200. In the fast reversible chain transfer regime, these reactions have some... 

Fig. 6 Simulated Mn and MJMn vs. conversion for polymerization with reversible chain transfer when C° = C =50 and A = 200...

The situation becomes more complex for semi-reversible chain transfer, where kcl and /cRT are both positive, but kCT > kRT As demonstrated in Fig. 7, MJMn can be greater than, less than, or equal to 2.0, depending on the conversion and the magnitudes of the chain transfer constants. The Mn of the polymer is simply a function of C ° the value of C has no effect on M up to C = C °. However, M is dramatically affected by lower values of Ct. If C° Ca > 0, then the initial increase in M IM is dramatic, and M IM does not dip below 2 until high conver-sion. However, as C approaches C 0, the initial increase in MJMn is negligible, and MJMn drops below 2.0 at low conversion. In any case, if Ca > 0, then MJMn asymptotically approaches two from the low side. 

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