Homopolymeric blocks

Depolymerization of Alginic Acid. Alginic acid depolymerization is a process that removes the MG blocks by cleaving the MG glycosidic bond which is more susceptible to acid hydrolysis than the glycosidic bonds in homopolymeric blocks. The resulting solid is a mixture of the two homopolymeric GG blocks and MM blocks. The method used was a modification of the procedure described by Penman and Sanderson (10). 

Preliminary studies of depolymerized alginic acid indicate that the GG-preference of the divalent alkaline earths remains in mixtures of homopolymeric blocks. One conclusion from our data might be that there is a minimum chain length required for maximum metal ion binding. Until further studies are completed, the extent of binding for the divalent ions and depolymerized alginate cannot be determined. 

Macromolecular properties of alginates are highly dependent on G/M ratio and sequential distribution of G and M residues within homopolymeric blocks. Poly-guluronate blocks are stiffer than polymannuronate and alternating blocks, which in turn are more soluble at low pH [6]. 

In this notation A and B can represent different stereo configurations, or microstructures of the same mer. Likewise, either A and/or B may be random or tapered copolymer blocks rather than homopolymeric blocks. A tapered A-B block is one that begins with a section of pure A mers and gradually incorporates B mers until at the end it becomes a section of pure B mers. The A-B-C designation represents the incorporation of a third monomer segment and would most probably comprise a three-phase system. An A-B-A block polymer based on styrene and butadiene would mean in this notation a polystyrene block-polybutadiene block-polystyrene block structure, that is, S-B-S. 

Alginic acid is a linear copolymer with homopolymeric blocks of (l-4)-linked p-D-mannuronate (M) and its C-5 epi-mer a-L-glucuronate (G) residues, respectively, covalently linked together in different sequences or blocks. The different forms of alginic acid are represented in Figure 1.29. The monomers can appear in homopolymeric blocks of consecutive G-residues (G-blocks), consecutive M-residues (M-blocks), or alternating M- and G-residues (MG-blocks). 

The stress-strain curves recorded for the two triblock copolymer samples during cold drawing of films with a constant rate of 1 mm/min are shown in Figure 11. The gradient copolymer has much higher tensile elongation (1000% vs. 300%) but much lower tensile strength (4.5 MPa vs. 7.3 MPa) than the copolymer with pure homopolymeric blocks. 

Alginates are polysaccharides that are linear copolymers of (3-D-mannuronic acid and a-L-guluronic acid residues with (1 4) linkages, as shown in Figure 14.13 [24]. The molecular chain is formed by homopolymeric blocks or heteropolymeric blocks of the two monomers, (3-D-mannuronic acid or a-L-guluronic acid. Alginates exhibit molecular weights between 20 and 600 kDa [3]. 

Alginate is a polysaccharide widely obtained from the cell walls of certain brown seaweeds. Alginate absorbs water quickly and is capable of absorbing 200-300 times of its own weight to form a viscous gel. Alginate is a linear anionic copolymer with homopolymeric blocks of (l )-linked y9-D-mannuronate (M) and a-L-guluronate (G) residues (Fig. 2), respectively, covalently linked together in... 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

Graft or block copolymers are expected to be formed via process (28) (i) and (ii), respectively, depending on whether the polymer Ap is of a crosslinking type or a degrading type. No homopolymerization occurs in... 

Homopolymerization of macroazoinimers and co-polymerization of macroinimers with a vinyl monomer yield crosslinked polyethyleneglycol or polyethyleneglycol-vinyl polymer-crosslinked block copolymer, respectively. The homopolymers and block copolymers having PEG units with molecular weights of 1000 and 1500 still showed crystallinity of the PEG units in the network structure [48] and the second heating thermograms of polymers having PEG-1000 and PEG-1500 units showed that the recrystallization rates were very fast (Fig. 3). 

A number of other chemistries which involve C-O bond cleavage have been reported.226 22 Druliner226 has reported on systems where NCO, 112, 113 or related species is the persistent radical. Homolysis rates for these systems were stated to he suitable for MMA polymerization at ambient temperature. The use of NCO has also been studied by Grande et al. z most recently for AA polymerization.2 0 Although control during AA homopolymerization was poor the process yielded NCO- terminated PAA that could be used to make PAA-block-PMMA.230... 

Polymers conjugated with 1-adamantyl moieties as lipophilic pendent groups can be utilized to design nanoparticulate dmg delivery systems. Polymer (1) in Fig. 26, which is synthesized by homopolymerization of ethyladamantyl malolactonate, can be employed as highly hydrophobic blocks to construct... 

Chien JCW, Rausch MD, Mallin DT, Lin YG, Dong SH (1990) rac-Ethylidene(l-r 5-tetram-ethylcyclopentadienyl)(l-ri5-indenyl).dichlorotitanium and its homopolymerization of propylene to crystalline-amorphous block thermoplastic elastomers. J Am Chem Soc 112 2030-2031... 

NMP is as successful as RAFT polymerization for the construction of block copolymers. A small library of block copolymers comprised of poly(styrene) (PSt) and poly(ferf-butyl acrylate) (FYBA) was designed and the schematic representation of the reaction is depicted in Scheme 10 [49]. Prior to the block copolymerization, the optimization reactions for the homopolymerization of St and f-BA were performed as discussed in this chapter (e.g., see Sect. 2.1.2). Based on these results,... 

The potential of transformation reactions for synthesizing a wider range of block copolymers has not been realized because either the reactions are not quantitiative or deterimental side reactions occur. Thus coupling of two propagating carbanions by one phosgene competes with the 1 1 transformation in Eq. 5-123. The anionic-to-radical transformation in Eq. 5-124 involves the formation of trimethyllead radical, which initiates homopolymerization of monomer B. 

If the macroradicals are stabilized by transfer to monomer, the tendency of the monomer to homopolymerize or to block copolymerize depends on the reactivity ratio of the monomer towards the two radicals. 

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