Catalyst chain transfer polymerization

Controlled/ Living radical polymerization (CRP) of vinyl acetate (VAc) via nitroxide-mediated polymerization (NMP), organocobalt-mediated polymerization, iodine degenerative transfer polymerization (DT), reversible radical addition-fragmentation chain transfer polymerization (RAFT), and atom transfer radical polymerization (ATRP) is summarized and compared with the ATRP of VAc catalyzed by copper halide/2,2 6 ,2 -terpyridine. The new copper catalyst provides the first example of ATRP of VAc with clear mechanism and the facile synthesis of poly(vinyl acetate) and its block copolymers. 

VAc has been successfully polymerized via controlled/ living radical polymerization techniques including nitroxide-mediated polymerization, organometallic-mediated polymerization, iodine-degenerative transfer polymerization, reversible radical addition-fragmentation chain transfer polymerization, and atom transfer radical polymerization. These methods can be used to prepare well-defined various polymer architectures based on PVAc and poly(vinyl alcohol). The copper halide/t is an active ATRP catalyst for VAc, providing a facile synthesis of PVAc and its block copolymers. Further developments of this catalyst will be the improvements of catalytic efficiency and polymerization control. 

Bulk and solution polymerizations refer to polymerization systems where the polymer produced is soluble in the monomer. This is in contrast to heterogeneous polymerization where the polymer phase is insoluble in the reaction medium. In bulk polymerization, only monomer provides the liquid portion of the reactor contents, whereas in solution processes, additional solvent can be added to control viscosity and temperature. In both processes, small amounts of additional ingredients such as initiators, catalysts, chain transfer agents, and stabilizers can be added to the process, but in all cases, these are also soluble in the reactor medium. As described in more detail below, the viscosity of the reaction medium and managing the energetics of the polymerization pose the most significant challenges to operation of bulk and solution processes. 

Three events are involved with chain-growth polymerization catalytic initiation, propagation, and termination [3], Monomers with double bonds (—C=C—R1R2—) or sometimes triple bonds, and Rj and R2 additive groups, initiate propagation. The sites can be anionic or cationic active, free-radical. Free-radical catalysts allow the chain to grow when the double (or triple) bonds break. Types of free-radical polymerization are solution free-radical polymerization, emulsion free-radical polymerization, bulk free-radical polymerization, and free-radical copolymerization. Free-radical polymerization consists of initiation, termination, and chain transfer. Polymerization is initiated by the attack of free radicals that are formed by thermal or photochemical decomposition by initiators. When an organic peroxide or azo compound free-radical initiator is used, such as i-butyl peroxide, benzoyl peroxide, azo(bis)isobutylonitrile, or diazo- compounds, the monomer s double bonds break and form reactive free-radical sites with free electrons. Free radicals are also created by UV exposure, irradiation, or redox initiation in aqueous solution, which break the double bonds [3]. 

COORDINATIVE CHAIN TRANSFER POLYMERIZATION AND COPOLYMERIZATION BY MEANS OF RARE EARTH ORGANOMETALLIC CATALYSTS FOR THE SYNTHESIS OF TAILOR-MADE POLYMERS... 

Coordinative chain transfer polymerization (CCTP, also called catalytic chain transfer polymerization) is typically a process that comprises a chain transfer step that must be i) reversible and ii) much faster than propagation. The growing polymer chain is exchanged between a CTA and the catalyst when attached to the CTA, it is just a dormant chain, whereas propagation takes place on the catalyst (Scheme 27.1). As a consequence, if the CTA is in excess, several macromolecular chains can be produced per catalyst molecule, and ideally (if the transfer rate is not determining), all chains will have the same length. 

Scheme 27.4 Coordinative chain transfer polymerization (CCTP) of myrcene with neodymium borohydride/dialkyl magnesium catalysts.

The versatility of chain transfer polymerization together with its easiness of use relative to the synthetic effort that would require a one catalyst, one material approach or to the difficulties encountered by other routes explored for fine-tuning polymerization selectivities makes this approach undoubtedly an attractive and promising field for the future of coordination polymerization. CCTP renders the emergence of the one catalyst, several materials paradigm a reality. 

Scheme 12 Ternary living coordlnatlve chain transfer polymerization using a Hf/AI/Zn catalyst system. Reprinted with permission from Zhang, W. Wei, J. SIta, L.R. Angew. Chem., Int. Ed. 2010, 49,1786. °...

A bewildering array of names are used to describe the various controlled/living radial polymerization techniques currently in use. These include stable free radical polymerization (SFRP) [35-38], nitroxide mediated polymerization (NMP) [39], atom transfer radical polymerization (ATRP) [40-42 ] and degenerate transfer processes (DT) which include radical addition-fragmentation transfer (RAFT) [43, 44] and catalyst chain transfer (CCT). These techniques have been used to polymerize many monomers, including styrene (both linear and star polymers) acrylates, dienes, acrylamides, methacrylates, and ethylene oxide. Research activity in this field is currently expanding at a very high rate, as is indicated by the many papers published and patents issued. 

Molecular Weight. PE mol wt (melt index) is usually controlled by reaction temperature or chain-transfer agents. Reaction temperature is the principal control method in polymerization processes with Phillips catalysts. On the other hand, special chemical agents for chain transfer are requited for... 

Typical heterogeneous Ziegler catalysts operate at temperatures of 70— 100°C and pressures of 0.1—2 MPa (15—300 psi). The polymerization reactions are carried out ia an iaert Hquid medium (eg, hexane, isobutane) or ia the gas phase. Molecular weights of LLDPE resias are coatroUed by usiag hydrogea as a chain-transfer ageat. 

Tetrabutylammonium benzoate has been used as a catalyst for the polymerization of PO over the temperature range 40—108°C and the yield of polymer was typically low (2—78%) a large amount of unsaturation was present due to chain transfer (95). When synthetic hydrotalcite,... 

If a vinyl monomer is polymerized in the presence of cellulose by a free radical process, a hydrogen atom may be abstracted from the cellulose by a growing chain radical (chain transfer) or by a radical formed by the polymerization catalyst (initiator). This leaves an unshared electron on the cellulose chain that is capable of initiating grafting. As cellulose is a very poor transfer agent [10], very little copolymer results from the abstraction of hydrogen atoms by a growing chain radical. The... 

---