Structural units Polymerization

Monomer Type of polymerization Structural unit Approximate melting (Tm) or softening (Tg) temperature in Properties... 

Taking into consideration the Si—O bonds within the glass surface, the difference between a strongly reacted layer and a highly polymerized network is difficult to define. However, with the above model the siloxane layer also contains partially polymerized structural units and/or hydrolysed remnants of the three-dimensional layer which would be expected from the random deposition of the hydrolysed APS. Consequently, some fragments may arise from pendant chains. Thus, the actual struture of the deposit will consist of a poly-siloxane probably chemically bonded to the glass surface every third silicon atom. 

Molar mass of polymeric structural units and of structural groups... 

A significant key to these catalysts is the observation that systems of different composition produce polymers of widely different structures. In a vinyl polymerization, structural units are added to a growing... 

Furthermore, if the structural unit to be polymerized is a vinyl group, mixed reactions can be carried out with a second compound such as a vinyl derivative or maleic anhydride. 

Natural rubber is composed of polymerized isoprene units. When rubber is under tension, ozone attacks the carbon-carbon double bond, breaking the bond. The broken bond leaves adjacent C = C bonds under additional stress, eventually breaking and placing shll more stress on surrounding C = C bonds. This "domino" effect can be discerned from the structural formulas in Fig. 9-4. The number of cracks and the depth of the cracks in rubber under tension are related to ambient concentrations of ozone. 

Pure NI3 has not been isolated, but the structure of its well-known extremely shock-sensitive adduct with NH3 has been elucidated — a feat of considerable technical virtuosity.Unlike the volatile, soluble, molecular solid NCI3, the involatile, insoluble compound [Nl3.NH3] has a polymeric structure in which tetrahedral NI4 units are comer-linked into infinite chains of -N-I-N-I- (215 and 230 pm) which in turn are linked into sheets by I-I interactions (336 pm) in the c-direction in addition, one I of each NI4 unit is also loosely attached to an NH3 (253 pm) that projects into the space between the sheets of tetra-hedra. The stmcture resembles that of the linked Si04 units in chain metasilicates (p. 349). A further interesting feature is the presence of linear or almost linear N-I-N groupings which suggest the presence of 3-centre, 4-electron bonds (pp. 63, 64) characteristic of polyhalides and xenon halides (pp. 835-8, 897). 

Figure 13.4 (a) ITie cri-bridged polymeric structure of liquid SbFs (schematic) show-ing the three sorts of F alom. (b) Structure of the tetrameric molecular unit in crystalline (SbFs)4 show[Pg.562]

The advantage of the activated displacement polymerization is the facile incorporation of different and unconventional structural units in the polymer backbone. Most of the heteroarylene activated polyethers prepared by this route are soluble in many organic solvents. The solubility behavior of new polyethers is shown in Table 8. In contrast to many polyphenylenequi-noxalines, poly(aryl ether phenylquinoxalines) prepared by the quionoxaline activated displacement reaction are soluble in NMP. Solubility in NMP is important since it is frequently used for polymer processing in the microelectronics industry [27]. 

Radical polymerization of diallylamine derivatives produce water-soluble polymers of low molecular weight [22,55-57]. In order to increase the molecular weight, acrylamide has been copolymerized with these diallylamine derivatives to produce cationic polymers with variable charge density depending on the content of the structural units of pyrrolidinium rings and acrylamide in polymeric chains [22,55,58-61]. 

Later on, Ballard et al. [42, 43] developed an improved precursor route starting from 5,6-diacetoxycyclohexa-1,3-diene (18), the so-called 1C1 route. The soluble precursor polymer 19 is finally aromatized thermally into PPP 1 via elimination of two molecules of acetic acid per structural unit. Unfortunately, the polymerization of the monomer does not proceed as a uniform 1,4-polymerization in addition to the regular 1,4-linkages ca. 10% of 1,2-linkages are also formed as result of a 1,2-polymerization of the monomer. 

In this early work, both initiation and termination were seen to lead to formation of structural units different from those that make up the bulk of the chain. However, the quantity of these groups, when expressed as a weight fraction of the total material, appeared insignificant. In a polymer of molecular weight 100,000 they represent only ca 0.2% of units Thus, polymers formed by radical polymerization came to be represented by, and their physical properties and chemistry interpreted in terms of, the simple formula 1. 

The intermediate product S3 (see above) will also take part in this polymerization. Infrared spectra of the polysulfuroxide show a strong absorption at 1123 cm which was assigned to the structural unit -S-S(=0)-S- by comparison with model compounds like trisulfane-2-oxides (RS)2S=0 [31] and SsO (see above). In chloroform solution S2O reacts at —60 °C to S5O and SO2 (see above). 

For the synthesis of the target structures, it is absolutely necessary to introduce solubilizing substituents in the positions peripheral to the benzoyl substituents. The primary coupling product, 117, a poly(2,5-dibenzoyl-l,4-phenyl-ene) derivative - a poly(para-phenylene) with two benzoyl substituents in each structural unit - is, as expected, very poorly soluble. Highly substituted monomers (2,5-dibromo-l,4-bis(3,4-dihexyloxy-benzoyl)benzene), containing four solubilizing alkoxy groups per monomer unit, allow the synthesis of polymeric materials with M of about 12,000 and M, of about 22,000 [139]. 

The polymers listed above, and all other linear polymers as well, are formed from monomers which enter into two, and only two, linkages with other structural units. This statement corresponds to the previous remark that the structural units of linear polymers necessarily are bivalent. The interlinking capacity of a monomer ordinarily is apparent from its structure it is clearly prescribed by the presence of two condensable functional groups in each monomer in the third and fourth examples above. The ability of the extra electron pair of the ethylenic linkage to enter into the formation of two bonds endows styrene with the same interlinking capacity. In accordance with the functionality concept introduced by Carothers, all monomers which when polymerized may join with two, and only two, other monomers are termed bifunctional. Similarly, a hifunctional unit is one which is attached to two other units. It follows that linear polymers are composed exclusively (aside from terminal units) of bifunctional units. ... 

Since Carothers introduced the distinction between condensation and addition polymers based on the composition relationship between monomer and structural unit, a number of examples of polymerization processes have been found which formally resemble the condensation type but which proceed without evolution of a by-product. For example, a glycol and a diisocyanate react intermolecularly as follows ... 

The process proceeds through the reaction of pairs of functional groups which combine to yield the urethane interunit linkage. From the standpoint of both the mechanism and the structure type produced, inclusion of this example with the condensation class clearly is desirable. Later in this chapter other examples will be cited of polymers formed by processes which must be regarded as addition polymerizations, but which possess within the polymer chain recurrent functional groups susceptible to hydrolysis. This situation arises most frequently where a cyclic compound consisting of one or more structural units may be converted to a polymer which is nominally identical with one obtained by intermolecular condensation of a bifunctional monomer e.g., lactide may be converted to a linear polymer... 

Representative condensation polymers are listed in Table I. The list is by no means exhaustive, but it serves to indicate the variety of condensation reactions which may be employed in the synthesis of polymers. Cellulose and proteins, although their syntheses have not been accomplished by condensation polymerization in the laboratory, nevertheless are included within the definition of condensation polymers on the ground that they can be degraded, hydrolytically, to monomers differing from the structural units by the addition of the elements of a molecule of water. This is denoted by the direction of the arrows in the table, indicating depolymerization. 

In Section 3 of this chapter it was mentioned that polymers obtained by intermolecular condensation of bifunctional monomers may often be prepared alternatively by an addition polymerization of a cyclic compound having the same composition as the structural unit. Typical examples are shown in Table III. The processes indicated are appropriately regarded as addition polymerizations. Each of these polymers may also be prepared through the condensation of suitable bifunctional monomers. The dimethylsiloxane polymer, for example, may be prepared, as indicated in Table I (p. 45), through the condensation of dimethyl dihydroxysilane formed by hydrolysis of the di-chlorosilane... 

The chemical and physical properties of the polymers obtained by these alternate methods are identical, except insofar as they are affected by differences in molecular weight. In order to avoid the confusion which would result if classification of the products were to be based on the method of synthesis actually employed in each case, it has been proposed that the substance be referred to as a condensation polymer in such instances, irrespective of whether a condensation or an addition polymerization process was used in its preparation. The cyclic compound is after all a condensation product of one or more bifunctional compounds, and in this sense the linear polymer obtained from the cyclic intermediate can be regarded as the polymeric derivative of the bifunctional monomer(s). Furthermore, each of the polymers listed in Table III may be degraded to bifunctional monomers differing in composition from the structural unit, although such degradation of polyethylene oxide and the polythioether may be difficult. Apart from the demands of any particular definition, it is clearly desirable to include all of these substances among the condensation... 

---