Polymers, living block copolymer

Disulfide derivatives and hexasubstituted ethanes2,15 may also be used in this context to make cnd-functional polymers and block copolymers. The use of dilhiuram disulfides as thermal initiators was explored by Clouet, Nair and coworkers.206 Chain ends are formed by primary radical termination and by transfer to the dilhiuram disulfide. The chain ends formed are thermally stable under normal polymerization conditions. The use of similar compounds as photoin iferters, when some living characteristics may be achieved, is described in Section 9.3.2.1.1. 

Living anionic polymers and block copolymers can be linked by coupling reactions. A living AB block copolymer can be linked by 1,6-dibromohexane to yield an ABA triblock copolymer... 

The first example of Iiving polyolefin with a uniform chain length was found in the low-temperature polymerization of propylene with the soluble catalyst composed of V(acac)3 and Al(C1Hi)2Cl. The mechanism of the living coordination polymerization is discussed on the basis of the kinetic and stereochemical data. Subsequently, some applications of living polypropylene are introduced to prepare tailor-made polymers such as terminally functionalized polymers and block copolymers which exhibit new characteristic properties. Finally, new types of soluble Ziegler-Natta catalysts are briefly surveyed in connection with the synthesis of living polyolefins. 

As in the case of the ring-opening metathesis polymerisation of cycloolefins, an important matter is the control of polymerisation to prepare acetylenic polymers having precise structures. A living polymerisation is of practical importance in the synthesis of monodisperse polymers, such as terminally functionalised polymers and block copolymers. The metathesis catalysts that promote the living polymerisation of acetylene [42] and acetylenic monomers include M0OCI4 SnBu EtOFkNbCls and Ta, Mo and W alkylidenes [84, 133, 152, 153]. 

A series of at least 14 papers [200-208] have been published dealing with the synthesis of telechelic polymers or block copolymers from the radical polymerization of various vinyl monomers with substituted 1,1,2,2-tetraphenyl ethanes. These aromatic compounds, known for over a century [209], are efficient in radical polymerization [201,210], They behave as both initiators and terminating agents [200] that can be involved in living radical polymerization as illustrated in the following reaction ... 

The remarkable contrd fliat can be exercised on certain processes. Predetermined DP s, narrow DP distributions, and the synthesis of telechelic polymers and block copolymers are now possible in some instances, and living cationic systems are beii perfected. These achievements close tire gap between anionic and cationic pol5meri-sation. 

The inclusion of apparently dead polymer into block copolymer is of interest (Table II). Certainly less of the first PMDS polymer was incorporated in this system than in the corresponding living system, because all the... 

MAO or MMAO also effectively activates late-transition metal halide a-olefin polymerization precatalysts such as 2,6- Pr-substituted aryl bulky di-imine Ni(II) dibromides for branched polymers and block copolymers by living polymerization at low temperatures,22 para- and unsubstituted aryl di-imine Ni(II) dibromides for production of linear... 

Living Polymers and Block Copolymers Made by ROMP... 

Living polymerization using titanacyclobutane initiators enabled also the preparation of block copolmers by sequential addition of different monomers [114-116] and synthesis of highly conjugated polymers and block copolymers of 3,4-diisopropylidene-cyclobutene [116]. 

A modification of the Thom-Csanyi ROMP route to PPV makes use of a silyl-substituted paracyclophane derivative, which, under ROMP conditions, reacted to give a soluble precursor polymer. fransformation of the PPV could be achieved by treating the precursor polymer with acid or by hydrolysis of the silyl group followed by thermal treatment (Fig. 73). Because the pofymerization is living, polymers and block copolymers of well-defined molecular weight can be prepared [916]. 

Albagli, D., G. Bazan, M. S. Wrighton, and R. R. Schrock. 1992. Well-defined redox-active polymers and block copolymers prepared by living ring-opening metathesis polymerization. (11) 4150 158. 

The results of the 2G and 3G dendrimer-like star-branched polymers and block copolymers, after fractional precipitation, are summarized in Table 5.2. The resulting polymer all possessed the observed Mn values in good agreement with those calculated, and narrow molecular-weight distributions (M /Mn 1.1). Since the off-center living polymers and the 2G living dendrons used as subunits are sampled during the synthesis and well characterized prior to the synthesis and then reacted with a multifunctional core to synthesize the 2G and 3G polymers, this procedure corresponds to an example of an arm-first process. 

Matsuo, A., Watanabe, T., and Hirao, A. (2004) Synthesis of well-defined dendrimer-like branched polymers and block copolymer by the iterative approach involving coupling reaction of living anionic polymer and functionalization. Macromolecules, 37,6283-6290. 

Active species of CROP of THF are long-living, which makes possible their quantitative conversion to a variety of functional polymers that may be used for synthesis of polymers with complex architectures, such as star polymers or block copolymers. 

A critical feature of ROMP is that the polymerization can be living, such that chain termination and transfer events are much slower than chain propagation. In this way, ROMP affords polymers or block copolymers with defined lengths and low polydispersity indexes (PDIs). By exploiting these mechanistic features, ROMP can be used to rapidly synthesize defined bioactive polymers with structural features optimized for the desired application. 

As we have already reported, sequential living cationic polymerizations of functionalized vinyl ethers (8) readily give amphiphilic block copolymers (17). Such sequential living polymers are equally applicable to the star polymer synthesis. A typical example utilizes an AB living block copolymer ( ) that consists of 2-(acetoxy)ethyl vinyl ether and IBVE (10 and 30 units per chain, respectively). The... 

Block copolymers are closer to blends of homopolymers in properties, but without the latter s tendency to undergo phase separation. As a matter of fact, diblock copolymers can be used as surfactants to bind immiscible homopolymer blends together and thus improve their mechanical properties. Block copolymers are generally prepared by sequential addition of monomers to living polymers, rather than by depending on the improbable rjr2 > 1 criterion in monomers. 

A brief review has appeared covering the use of metal-free initiators in living anionic polymerizations of acrylates and a comparison with Du Font s group-transfer polymerization method (149). Tetrabutylammonium thiolates mn room temperature polymerizations to quantitative conversions yielding polymers of narrow molecular weight distributions in dipolar aprotic solvents. Block copolymers are accessible through sequential monomer additions (149—151) and interfacial polymerizations (152,153). 

When the initial monomer supply is exhausted, the anionic chain ends retain their activity. Thus, these anionic chains have been termed living polymers. If more monomer is added, they resume propagation. If it is a second monomer, the result is a block copolymer. 

Anionic polymerization, if carried out properly, can be truly a living polymerization (160). Addition of a second monomer to polystyryl anion results in the formation of a block polymer with no detectable free PS. This technique is of considerable importance in the commercial preparation of styrene—butadiene block copolymers, which are used either alone or blended with PS as thermoplastics. 

The second front originates in the polymer synthesis community. Efforts are mainly directed toward production of monodisperse block copolymers by living polymerizations. These stmctures typically result in microphase separated systems if one block is a high T material and the other is elastomeric in... 

In the absence of impurities there is frequently no termination step in anionic polymerisations. Hence the monomer will continue to grow until all the monomer is consumed. Under certain conditions addition of further monomer, even after an interval of several weeks, will eause the dormant polymerisation process to proceed. The process is known as living polymerisation and the products as living polymers. Of particular interest is the fact that the follow-up monomer may be of a different species and this enables block copolymers to be produced. This technique is important with certain types of thermoplastic elastomer and some rather specialised styrene-based plastics. 

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