Polydispersity polymerization method

A potential drawback of all the routes discussed thus far is that there is little control over polydispersity and molecular weight of the resultant polymer. Ringopening metathesis polymerization (ROMP) is a living polymerization method and, in theory, affords materials with low polydispersities and predictable molecular weights. This methodology has been applied to the synthesis of polyacctylcne by Feast [23], and has recently been exploited in the synthesis of PPV. Bicyclic monomer 12 [24] and cyclophane 13 [25) afford well-defined precursor polymers which may be converted into PPV 1 by thermal elimination as described in Scheme 1-4. 

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

A disadvantage of traditional acrylamide polymerization reactions is the heterogeneity of the products that result. A radical polymerization method that produces polymers of similar structure but that are much more homogeneous is atom-transfer radical polymerization (ATRP) [155,156]. ATRP has been used to synthesize carbohydrate-substituted polymers with low polydispersities [157,158,159,160,161]. Materials that display sugar residues such as glu-cofuranose [160], glucopyranose [161], and A-acetyl-D-glucosamines [159]. 

Controlled living radical polymerization methods were developed to produce polymers with predetermined molecular weights, low polydispersity index, specific... 

The major drawback of suspension polymerization method is formation of polydispersed MIP particles. Also, porogenic solvents such as chloroform and toluene cannot be used as these are miscible with mineral oil [Haginaka et al., 2008). 

The most important achievement of living /controlled polymerization methods and techniques is the synthesis of tailor-made block copolymers with specific macromolecular architecture, chemical composition/functionality, low molecular polydispersity, and corresponding minimized heterogeneity. 

Poly(ferrocenylsilane)s are high-molecular-weight polymers. The molecular weights of polymers prepared by the thermal ROP range from 10 to 10 with polydispersity indices of 1.5-2.5 [3, 7, 44-46], The molecular weights of the polymers prepared by the anionic polymerization method can be controlled by varying the amount of initiator. Further, polymers with narrow PDFs (less than 1.3) can be obtained by the anionic polymerization [3]. 

In this article, block copolymers prepared by controlled polymerization methods only are considered, primarily di- and triblock copolymers. Multiblock copolymers such as poly(urethanes) and poly(urethane-ureas) prepared by condensation polymerization are not discussed (see Polyurethanes (PUR)). Although these materials do exhibit microphase separation, it is only short range in spatial extent due to the high polydispersity of the pol5uners. 

The complexation of DNA and polycations is a function of the intrinsic properties of the two components. For instance, from the use of synthetic polycations for complexing DNA also arises the problem of polydispersity of polymers (a polymer sample is usually composed of macromolecular species of differing molar masses) compared with DNA, which is monodisperse. Because the polydispersity of the polycation could be an issue in studies of IPECs, sugar-based polymers (usually polydisperse except if fractionated), conjugated polymers (polydispersity, Mw/Mn > 2), branched PEI derivatives, and hyperbranched polymers are out of the scope of this review, as already mentioned. Only polymers synthesized via controlled or living polymerization methods will be discussed [55-57]. 

The molecular weight distributions ordinarily obtained by the more important polymerization methods are shown in Table 3.9. Natural proteins, nature s best, are both monodisperse and 100% stereospecific. The Ziegler-Natta polymerization route, while highly stereospecific, has a broad molecular weight distribution. In general, the polydispersity index can be determined from an analysis of the kinetics of the reaction in practice, various phenomena cause the products to be much broader in molecular weight distributions (67). 

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