Living mode polymerization

Polymer Synthesis and Characterization. This topic has been extensively discussed in preceeding papers.(2,23,24) However, we will briefly outline the preparative route. The block copolymers were synthesized via the sequential addition method. "Living" anionic polymerization of butadiene, followed by isoprene and more butadiene, was conducted using sec-butyl lithium as the initiator in hydrocarbon solvents under high vacuum. Under these conditions, the mode of addition of butadiene is predominantly 1,4, with between 5-8 mole percent of 1,2 structure.(18) Exhaustive hydrogenation of polymers were carried out in the presence of p-toluenesulfonylhydrazide (19,25) in refluxing xylene. The relative block composition of the polymers were determined via NMR. 

The polymerization was found to proceed smoothly to high conversions. The time dependence of logarithmic initial-to-current monomer concentration ratio ln(mo/m) is linear (Figure 2, curve 1), thus indicating the absence of chain termination processes, as case inherent in polymerization proceeding in the living mode. MW of the obtained polymers increases linearly with the conversion (Figure 2). The polydispersity indexes somewhat decrease with the conversion, a fact that is also typical of controlled radical polymerization. GPC... 

Recently, Ishizone d al. conducted a successful living anionic polymerization of 2-(l-adamantyl)-l,3-butadiene with sec-BuU in both THF and cyclohexane [62, 63]. Interestingly, the microstructure of the resulting polymer was predominantly regulated in 1,4-addition mode (88%, cis/trans=S2/lS), even in polar THF. Thus, the bulky adamantyl substituent significantly affected the stereoselectivity of the resultant polymer. 

Cationic polymerization is a very important procedure that has been adopted to prepare block copolymers consisting of monomeric units that cannot be polymerized by other methods, such as isobutylene (IB) and alkyl vinyl ethers (VEs), thus enhancing the potential of macromolecular engineering. Cationic polymerization proceeds through carbenium (or oxonium) sites in a controlled/living mode if appropriate conditions such as initiation/coinitiation (Lewis acid), additives, solvent, and temperature have been chosen. 

Several approaches involving a combination of cationic and anionic polymerizations have been reported for the synthesis of block copolymers. These approaches are based on the anionic-to-cationic transformation mechanism and vice versa. Regardless of the mechanism, well-defined block polymeric architectures can be prepared by living modes of both polymerization techniques. 

The synthesis of block copolymers of controlled structures is most conventionally accomplished through the use of living anionic polymerization. One can easily imagine, however, desirable block copolymers derived from monomers which are inert to anionic polymerization conditions, or which do not share any common mode of polymerization. In a recent series of papers (24-34), Richards and coworkers have addressed this problem in a general way, and have developed methods which convert one kind of active center into another. Within the context of cyclic ether polymerizations, Richards has focused on the preparation of block copolymers of styrene and tetrahydrofuran (THF) several methods of accomplishing this copolymerization are described in the following paragraphs. 

The number average molar mass, Mn, and molar mass distribution (MMD) are controlled by interplay of kinetic parameters, which will be described in more detail in Chapter 5. In principle there are two ways of terminating a growing chain bimolecular termination and transfer. These two modes of termination have led to the two main categories of living radical polymerization (a) reversible termination and (b) reversible chain transfer. 

Lipase is an enzyme which catalyzes the hydrolysis of fatty acid esters normally in an aqueous environment in living systems. However, hpases are sometimes stable in organic solvents and can be used as catalyst for esterifications and transesterifications. By utihzing such catalytic specificities of lipase, functional aliphatic polyesters have been synthesized by various polymerization modes. Typical reaction types of hpase-catalyzed polymerization leading to polyesters are summarized in Scheme 1. Lipase-catalyzed polymerizations also produced polycarbonates and polyphosphates. 

Wiesbrock E, Hoogenboom R, Abeln CH et al. (2004) Single-mode microwave ovens as new reaction devices accelerating the living polymerization of 2-ethyl-2-OxazoUne. Macromol Rapid Commun 25 1895-1899... 

Termination occurs by transfer of a positive fragment, usually a proton, from the solvent or some transfer agent (often deliberately added), although other modes of termination are also known. Many anionic polymerizations are living polymerizations when the reaction components are appropriately chosen. 

Alternatively, we may look at the state of ultimate equilibrium of a system containing a monomer, the polymerization of which involves no termination and yields living polymers. In such a discussion, the modes of initiation of the polymerization and the concentration of the initiator or of the lowest living oligomer must be specified. 

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