Block graft polymers

Synthesis of the block-graft polymers also started with the dlfunctlonal initiator described above. The o,oi-dilithlo-cls-l.A-polylsoprene was then lithiated further by reaction with sec-butyl lithium in the presence of tetramethylethylenedlamine. This resulted In the formation of llthlo sites randomly spaced along the polymer chains. The complete reaction sequence Is given below. 

W. J. Budant and A. S. Hoffman, Block and Graft Polymers, Reinhold Puhhshing Corp., New York, 1960. 

The anionic polymerization of methacrylates using a silyl ketene acetal initiator has been termed group-transfer polymerization (GTP). First reported by Du Pont researchers in 1983 (100), group-transfer polymerization allows the control of methacrylate molecular stmcture typical of living polymers, but can be conveniendy mn at room temperature and above. The use of GTP to prepare block polymers, comb-graft polymers, loop polymers, star polymers, and functional polymers has been reported (100,101). 

In addition to the primary appHcation of PTMEG ia polyurethanes, polyureas, and polyesters, a considerable number of reports of other block and graft polymers highlighting PTME units have appeared. Methods have been developed that allow the conversion of a cationicaHy polymerizing system to an anionic one or vice versa (6,182). 

Other copolymer forms are alternating copolymers, block copolymers and graft polymers. 

The most important practical application of the organometallic complex photoinitiators is the possibility of using these types of initiators in modifying the pre-existing polymer chain, e.g., block, graft, and crosslinked copolymers preparation. 

R. J. Ceresa, Block and Graft Polymers, Ch. 5, Butter- 110. worths, London (1962). 

There are additional factors that may reduce functionality which are specific to the various polymerization processes and the particular chemistries used for end group transformation. These are mentioned in the following sections. This section also details methods for removing dormant chain ends from polymers formed by NMP, ATRP and RAFT. This is sometimes necessary since the dormant chain-end often constitutes a weak link that can lead to impaired thermal or photochemical stability (Sections 8.2.1 and 8.2.2). Block copolymers, which may be considered as a form of end-functional polymer, and the use of end-functional polymers in the synthesis of block copolymers are considered in Section 9.8. The use of end functional polymers in forming star and graft polymers is dealt with in Sections 9.9.2 and 9.10.3 respectively. 

Aggarwal SLet al. (1973) in Burke JJ, Weiss V (eds) Block and Graft Polymers University Press Syracuse, p 157... 

Several excellent books and review articles have been published covering this particular area of polymer science [1-3]. Nevertheless, this review will highlight recent (2000-2004) advances and developments regarding the synthesis of block copolymers with both linear (AB diblocks, ABA and ABC triblocks, ABCD tetrablocks, (AB)n multiblocks etc.) and non-linear structures (star-block, graft, miktoarm star, H-shaped, dendrimer-like, and cyclic copolymers). Attention will be given only to those synthetic methodologies which lead to well-defined and well-characterized macromolecules. 

Generally, however, mixtures of two polymers arc insoluble in one another and form two-phase systems (91-104). Block and graft polymers, in... 

The mechanical properties of two-phase polymeric systems, such as block and graft polymers and polyblends, are discussed in detail in Chapter 7. However, the creep and stress-relaxation behavior of these materials will be examined at this point. Most of the systems of practical interest consist of a combination of a rubbery phase and a rigid phase. In many cases the rigid phase is polystyrene since such materials are tough, yet low in price. 

Several attempts have been made to superimpose creep and stress-relaxation data obtained at different temperatures on styrcne-butadiene-styrene block polymers. Shen and Kaelble (258) found that Williams-Landel-Ferry (WLF) (27) shift factors held around each of the glass transition temperatures of the polystyrene and the poly butadiene, but at intermediate temperatures a different type of shift factor had to be used to make a master curve. However, on very similar block polymers, Lim et ai. (25 )) found that a WLF shift factor held only below 15°C in the region between the glass transitions, and at higher temperatures an Arrhenius type of shift factor held. The reason for this difference in the shift factors is not known. Master curves have been made from creep and stress-relaxation data on partially miscible graft polymers of poly(ethyl acrylate) and poly(mcthyl methacrylate) (260). WLF shift factors held approximately, but the master curves covered 20 to 25 decades of time rather than the 10 to 15 decades for normal one-phase polymers. 

N. Polyblends, block, and graft polymers IJ. Brittle Fracture and Stress Concentrators... 

Copolymerrzation and plasticization E- Block and graft polymers and polyh.lcn4 Problems References... 

Design of Block, Star, and Graft Polymer Syntheses... 

While much of the current and near past research has emphasized modification of synthetic polymers, Increasing efforts will undoubtedly focus on the modification of regenerable polymers and the blending of natural polymers and natural polymers with synthetic polymers through block, graft, etc. approaches. 

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