Monomer Different functionality

The polymer repeat unit arises from reacting together two different functional groups which usually originate on different monomers. In this case the repeat unit is different from either of the monomers. In addition, small molecules are often eliminated during the condensation reaction. Note the words usual and often in the previous statements exceptions to both statements are easily found. 

A monomer is a reactive molecule that has at least one functional group (e.g. -OH, -COOH, -NH2, -C=C-). Monomers may add to themselves as in the case of ethylene or may react with other monomers having different functionalities. A monomer initiated or catalyzed with a specific catalyst polymerizes and forms a macromolecule—a polymer. For example, ethylene polymerized in presence of a coordination catalyst produces a linear homopolymer (linear polyethylene) ... 

Reactions between one monomer species with two different functional groups. One functional group of one molecule reacts with the other functional group of the second molecule. For example, polymerization of an amino acid starts with condensation of two monomer molecules ... 

Although no small molecule gets eliminated, the reaction can be considered a condensation polymerization. Monomers suitable for polymerization by ring opening condensation normally possess two different functional groups within the ring. Examples of suitable monomers are lactams (such as caprolactam), which produce polyamides, and lactons, which produce polyesters. 

When the same monomer is used to build up a polymer, it is termed a homopolymer. Copolymers have two different monomers or functional groups, and terpolymers have three groups. There are even tetrapolymers available. These polymers may be further classified, for example, as random, graft, or block copolymers. Block copolymers, which have alternating sections of specific molecular chains, are used in BW formulations (but are more commonly found in CW programs). 

Modification of monomers is fundamentally different than postreactions in that it can allow better control of the molecular structure. Both diphenols and dihalides can be modified to incorporate functional groups or new monomers containing functional groups can be synthesized with similar structures as their counterparts. 

On the other hand, nylon 6 is prepared from caprolactam, which behaves as a bifunctional monomer bearing two different functional groups, and hence the polymer is made up of just one type of unit along the backbone it is therefore a homopolymer. 

As previously indicated, both condensation and addition polymers may be prepared from monomers of functionality exceeding two, with resulting formation of nonlinear polymers. Hence the distinction between linear and nonlinear polymers subdivides both the condensation and the addition polymers, and four types of polymers are at once differentiable linear condensation, nonlinear condensation, linear addition, and nonlinear addition. The distinction between linear and nonlinear polymers is clearly warranted not only by the marked differences in their structural patterns but also by the sharp divergence of their properties. 

Table 6-3. Comparison of the dipoles of the isolated individual monomers (dipole M) compare to the dipole moments of molecules within the dimer (dipole D) via the interaction, calculated with different functionals. Units are atomic units, and we give as well the difference in length and orientation ...

The type of copolymer formed during step growth polymerization depends on the reactivity of the functional groups and the time of introduction of the comonomer. A random copolymer forms when equal concentrations of equally reactive monomers polymerize. The composition of the copolymer, then, will be the same as the composition of the reactants prior to polymerization. When the reactivities of the monomers-differ, the more highly reactive monomer reacts first, creating a block consisting predominandy of one monomer in the chain the lower reactivity monomer is added later. This assumes that there is no chain transfer and no monofunctional monomer present. If either of these conditions were to exist,... 

Step polymerization proceeds by the stepwise reaction between the two different functional groups of the monomers. One proceeds slowly from monomer to dimer and larger-sized species ... 

The size of the polymer molecules increases at a relatively slow rate in step polymerization compared to chain polymerization due to the lower rate constants in the former since k is of the order of 10 3-10 liter/mole-sec. One proceeds slowly from monomer to dimer, trimer, tetramer, pentamer, and so on until eventually large polymer molecules have been formed. Any two molecular species containing respectively the two different functional groups can react with each other throughout the polymerization. The average size of the molecules increases slowly with time and high molecular weight polymer is not obtained until near the very end of the reaction (i.e., above 95% conversion). ... 

The most widely used polymeric resist materials are random copolymers, in which monomers with different functionalities are randomly distributed in the polymer matrix so that the final properties are based on the average level... 

In nature, polycondensations of trifunctional monomers having two different functional groups occur under enzymatic control, resulting in tree-shaped, highly branched, but still soluble, macromolecules. 

Condensation polymers generally result from simple reactions involving two different monomers, each containing different functional groups. The usual example is terephthalic acid and ethylene glycol to make polyester. The two monomers will react in such a way that a small molecule like water or methanol is given off a co-product... 

Statistical copolymers containing repeating units each with a different functional group can be obtained using appropriate mixture of monomers. For example, a polyestermide can be synthesized from a ternary mixture of a diol, diamine, and diacid or a binary mixture of a diacd and amine-alcohol [East et al., 1989]. 

The sequential addition method also allows the synthesis of many different block copolymers in which the two monomers have different functional groups, such as epoxide with lactone, lactide or cyclic anhydride, cyclic ether with 2-methyl-2-oxazoline, imine or episul-Hde, lactone with lactide or cyclic carbonate, cycloalkene with acetylene, and ferrocenophane with cyclosiloxane [Aida et al., 1985 Barakat et al., 2001 Dreyfuss and Dreyfuss, 1989 Farren et al., 1989 Inoue and Aida, 1989 Keul et al., 1988 Kobayashi et al., 1990a,b,c Massey et al., 1998 Yasuda et al., 1984]. 

A very promising variant on this type of condensation polymerisation is to use monomers that possess groups X and Y which can be eliminated from the same molecule (Scheme 8.1, Route C). This circumvents the need for careful control of reaction stoichiometry as an equal number of the different functional groups are built in to the monomer. Furthermore, in certain cases, polymerisation of monomers of this type can follow a chain-growth polycondensation type of... 

FIGURE 19-11 Cytochrome be, complex (Complex III). The complex is a dimer of identical monomers, each with 11 different subunits. (a) Structure of a monomer. The functional core is three subunits cytochrome b (green) with its two hemes (bH and foL, light red) the Rieske iron-sulfur protein (purple) with its 2Fe-2S centers (yellow) and cytochrome ci (blue) with its heme (red) (PDB ID 1BGY). (b) The dimeric functional unit. Cytochrome c, and the Rieske iron-sulfur protein project from the P surface and can interact with cytochrome c (not part of the functional complex) in the intermembrane space. The complex has two distinct binding sites for ubiquinone, QN and QP, which correspond to the sites of inhibition by two drugs that block oxidative phosphorylation. Antimycin A, which blocks electron flow from heme bH to Q, binds at QN, close to heme bH on the N (matrix) side of the membrane. Myxothiazol, which prevents electron flow from... 

The calculation of the gel point or the molecular weight distribution becomes difficult if the functional groups in monomers differ in reactivity (26, 27). Polycondensation of glycerol with symmetrical dibasic acid or of an unsymmetrical dibasic acid with symmetrical tribasic alcohol may serve as examples. The reaction probabilities of various functional group , necessary for the calculation of molecular weight distributions and gel points, have to be determined either from complete chemical analysis or from the kinetics of the respective reactions. 

The monomers from which proteins are synthesised are the a L-amino-acids. The amino-acids are differentiated by the various radical groups attached to the or carbon (or C 2). Since there are generally four different functional groups attached to the C2 atom, it is asymmetrical and thus will have two possible optical isomers. However, with few exceptions, only the L-amino acids are found in proteins. 

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