Monosubstituted vinyl polymers

As shown in Figure 2.5, there are three possible stereoregular forms for monosubstituted vinyl polymers. These are isotactic, all of the pendant groups are on one side of the chiral carbon syndiotactic, the pendant groups appear on alternate sides of the chiral carbon and atactic, some mixture of geometries about the chiral carbon. 

The second example is the stereoregularity displayed by monosubstituted vinyl polymers of olefins. As we saw earlier, these types of polymers can occur in three forms of tacticity isotactic, syndiotactic, and atactic. Isotactic and syndiotactic polymers possess stereoregular structures. Generally these polymers are rigid, crystallizable, high melting, and relatively insoluble. On the other hand, atactic polymers are soft, low melting, easily soluble, and amorphous. 

In monosubstituted vinyl polymers and at least some other classes of polymers, flexible pendant groups reduce the glass transition of the polymer by acting as internal diluents, lowering the frictional interaction between chains. The total effect is to reduce the rotational energy requirements of the backbone. 

Certain regularities have been observed in the T s of substituted vinyl polymers of the type 4CH2CXY-M9). Thus, if X=H (i.e. monosubstituted vinyl polymers) there is no effect of tacticity on the T s of the respective polymers. Examples are polystyrene (Y -(. ), polypropylene (-CH-), alkyl acrylates (-C00R) and in the longer branched o-olefTns (-(CH2) CH-). In contrast, in unsymmetrically disubstituted vinyl polymers there is a large effect of tacticity on T. The best documented examples are in the methacrylate polymers (X = -CH., Y = -COOR) and in the a-methyl styrenes (X - -CH, Y = -()>)." In the latter polymers, the T of the syndiotactic is invariably substantially above that of the isotactic isomer. This effect must ultimately be related to the conformational properties of the macromolecu-lar chains and as a first approximation may be regarded as an intra-molecular effect. 

A general example is the monosubstituted vinyl polymers (CH2—CHX) , which can exist in two possible stereoregular forms — isotactic (substitution on the alternate carbons on the same sides of the chain) and syndiotactic (substitution on the alternate carbons on the opposite sides of the chain). These ordered stereoregular forms can assume either planar or helical conformations. Each of these ordered structures has well-defined, unique selection rules for IR and Raman activity. These various structures can be distinguished on the basis of spectral properties without a detailed knowledge of the molecular motions or energies, that is, without normal coordinate analysis. 

The band classification for each type of polymer chain stereoconfiguration and conformation for monosubstituted vinyl polymers can be calculated, as shown in Fig. 5.4. 

Fig. 5.4. The IR and Raman polarizations for monosubstituted vinyl polymers. (Source Ref. [2].)...

A specific example is poly(vinyl chloride) (PVC), which is a monosubstituted vinyl polymer that has a syndiotactic-rich character and a conformation that can be either an extended all-trans structure or a folded syndiotactic structure. The vibrational modes of these conformational models obey different selection rules and have different dichroic properties that can be used to spectroscopically test these structures [7]. The folded syndiotactic model of PVC has the [p,0] classification that requires unique Raman lines (no coincident IR frequency) that are polarized. The extended syndiotactic model has the two unique classifications of [d,0] and [p,(r], which means that the unique Raman lines are depolarized, and the Raman lines that are polarized have perpendicular dichroism in the IR spectrum. In the Raman spectrum of PVC [8], polarized lines are observed at 363, 638, 694, 1172, 1335, 1430 and 1914 cm and IR bands are also observed at each of these frequencies. This result rejects the folded syndiotactic structure, because this structure requires the polarized lines to be unique. In addition, each of these frequencies is perpendicularly dichroic in the IR spectrum, a fact that supports the planar syndiotactic structure. 

The polymerization of monosubstituted vinyl compounds that give polymers like PS and PP produces polymer chains that possess chiral sites on every other carbon in the polymer backbone. Thus, the number of possible arrangements within a polymer chain is staggering since the number of possible isomers is 2" where n is the number of chiral sites. For a relatively short chain containing 50 propylene units the number of isomers is about 1 x lO. While the presence of such sites in smaller molecules can be the cause of optical activity, these polymers are not optically active since the combined interactions with light are negated by other similar, but not identical, sites contained on that particular and other polymer chains. Further, it is quite possible that no two polymer chains produced during a polymerization will be exactly identical because of chiral differences. 

Stereochemistry of polymers plays an important role in determining their properties. Different isomers of a polymer can be obtained starting with the same monomer but having the monomer molecules connected in different ways. For example, a monosubstituted vinyl monomer, in theory, can polymerize head-to-tail (H-T), head-to-head (H-H), or irregularly, as shown below ... 

A monosubstituted vinyl monomer yields polymers having three regioregular arrangements of configuration (Fig. 5), described by triad stereosequences. The isotactic structure has all R groups on the same side of the backbone (mm) the syndiotactic structure has R groups on alternate sides of the polymer s backbone (rr) and the heterotactic or atactic structure has R groups randomly oriented on either side of the polymer s backbone. 

Monosubstituted vinyl monomers can produce polymers with different configurations—two regular structures (isotactic and syndiotactic) and a random (atactic) form. For polymers with the same chemical makeup, those forms derived from regular structures exhibit greater rigidity, are higher melting, and are less soluble relative to the atactic form. 

The monomers used most commonly in chain-growth polymerization are ethylene (ethene) and substituted ethylenes. In the chemical industry, monosubstituted ethylenes are known as alpha olefins. Polymers formed from ethylene or substituted ethylenes are called vinyl polymers. Some of the many vinyl polymers synthesized by chain-growth polymerization are listed in Table 28.1. 

Monosubstituted vinyl monomers form polymers containing a series of asymmetric carbon atoms along the molecule. The precise arrangement of these asymmetric carbons gives rise to three different possible stereochemical arrangements. Firstly,... 

Recurrence regularity refers to the regularity with which the repeating unit occurs along the polymer chain. This may be illustrated by examining the polymers resulting from monosubstituted vinyl monomers. Here there are three possible arrangements ... 

Abstract Novel vinyl polymers were synthesized from 1,1- or 1,2-disubstituted and 1,1,2-trisubstituted ethylenes by radical polymerization. The polymers obtained consist of a rigid chain structure on account of the bulky side groups compared with flexible poly(monosubstituted ethylene)s. The substituted polymethylenes obtained from 1,2-disubstituted ethylenes such as fumaric and maleic derivatives were revealed to have new properties different from ordinary vinyl polymers. Radical polymerization behaviors of these multi-substituted ethylenes and some properties of the resulting polymers were investigated. 

Polymers of vinylidene monomers (1,1-disubstituted ethylenes) have lower Fg s than the corresponding vinyl polymers. Polyisobutene and polypropylene comprise such a pair and so do poly(vinylidene chloride) and poly(vinyl chloride). Symmetrical disubstituted polymers have lower Tg s than the monosubstituted macromolecules because no conformation is an appreciably lower energy form than any other (cf. the discussion of polyisobutene in Section 4.3). 

The addition of gem-disubstituted olefins, CH2=CXY, on polysilane 2 also worked well [23,24], For example, the addition of 2-methoxypropene and methylenecyclohexane afforded the expected adducts with 73% and 77% degrees of substitution, although a higher loss of molecular weight with respect to the hydrosilylation of monosubstituted olefins is observed. Copolymer 21, containing both mono- and disubstituted olefins, was made from 2 in a single reaction by adding 50 mol% vinyl acetic acid and an excess of 2-methoxypro-pene to the THF-polymer solution [24],... 

The rates of radical-monomer reactions are also dependent on considerations of steric effects. It is observed that most common 1,1-disubstituted monomers — for example, isobutylene, methyl methacrylate and methacrylo-nitrile—react quite readily in both homo- and copolymerizations. On the other hand, 1,2-disubstituted vinyl monomers exhibit a reluctance to ho-mopolymerize, but they do, however, add quite readily to monosubstituted, and perhaps 1,1-disubstituted monomers. A well-known example is styrene (Ml) and maleic anhydride (M2), which copolymerize with r — 0.01 and T2 = 0 at 60°C, forming a 50/50 alternating copolymer over a wide range of monomer feed compositions. This behavior seems to be a consequence of steric hindrance. Calculation of A i2 values for the reactions of various chloroethylenes with radicals of monosubstituted monomers such as styrene, acrylonitrile, and vinyl acetate shows that the effect of a second substituent on monomer reactivity is approximately additive when both substituents are in the 1- or cr-position, but a second substituent when in the 2- or /3-position of the monomer results in a decrease in reactivity due to steric hindrance between it and the polymer radical to which it is adding. 

This means that a monosubstituted alkene (a vinyl group) will have characteristic signals for each of the three protons on the double bond. Here is the example of ethyl acrylate (ethyl propenoate, a monomer for the formation of acrylic polymers). The spectrum looks rather complex at first, but it is easy to sort out using the coupling constants. 

A multitude of monomers can be used in radical polymerization and it is impossible to list them all. The fundamental feature of the vast majority of monomers in question is the vinylic double bond. Thus the simplest monomer is ethylene, which, however, can only be polymerized imder high pressnre and high temperature to the commercially very important low density polyethylene. Common monomers are monosubstituted or uns5unmetrically (1,1-) disnbsti-tuted ethylenes, CH2=CHR or CH2=CRR. The substitnents R and R determine the properties of resulting polymers and also the kinetic and thermodynamic... 

The polymerization of a monosubstituted ethylene, such as a vinyl compound, leads to polymers in which every other carbon atom is a chiral center. This is often marked with an asterisk for emphasis ... 

Furthermore, monosubstituted and disubstituted derivatives in the positions 9 and 10 of dimethyleneocta-hydronaphthalene have been polymerized using the above catalytic systems to vinylic or ring-opened polymers [Eqs. (101) and (102), Rj and R2 = alkyl and aryl groups]. 

Production of a cation of allylleneimine by electron bombarding is well known in studies of mass spectrometry on pyrrole [32]. A polymer, from a vinyl pyrrole, which is produced from uncleavaged pyrrole and acetylene consists of monosubstituted pyrrole rings. This is made evident by the discrepancy in IR spectra between polypyrrole and poly(pyrrole-2,5-diyl)-like polypyrrole [33], and by the thermal decomposition... 

---