constitution of most vinyl polymers because of the influence of resonance and ste- ric effects.
1-57
CH2 C CH2 CH2 CH2 CH2 C H
F
Head−to−Tail Head−to−Head Tail−to−Tail
C C C
H
F
H
F H
F
H
F
Vinyl monomers polymerize by attack of an active center (1-58) on the double bond.Equation (1-14)represents head-to-tail enchainment:
1-58 1-59
CH2 C C C
X Y
X Y
C X Y
+ CH2 CH2 CH2
X Y
(1-14)
while Eq. (1-15) shows the sequence of events in head-to-head, tail-to-tail polymerization:
CH2 C C C C
X Y
X Y
X Y
+ CH2 CH2 CH2
X Y
1-60
(1-15)
The active center may be a free-radical, ion, or metalcarbon bond (Chapter 8).
In any event the propagating species 1-59 will be more stable than its counterpart 1-60 if the unpaired electron or ionic charge can be delocalized across either or both substituents XandY. WhenXand/orY is bulky there will be more steric hin- drance to approach of the two substituted C atoms than in attack of the active cen- ter on the methylene C as in reaction (1-14). Poly(vinyl fluoride) contains some head-to-head linkages because the F atoms are relatively small and do not contrib- ute significantly to the resonance stabilization of the growing macroradical.
Positional isomerism is not generally an important issue in syntheses of poly- mers with backbones that do not consist exclusively of enchained carbons. This is because the monomers that form macromolecules such as poly(ethylene tere- phthalate) (1-5) or nylon-6,6 (1-6) are chosen so as to produce symmetrical poly- meric structures that facilitate the crystallization needed for many applications of these particular polymers. Positional isomerism can be introduced into such macromolecules by using unsymmetrical monomers like 1,2-propylene glycol
(1-61), for example. This is what is done in the synthesis of some film-forming polymers like alkyds (Section 7.4.2) in which crystallization is undesirable.
1-61 CH3 CH2OH
OH H C
It has been suggested that tail-to-tail linkages in vinyl polymers may constitute weak points at which thermal degradation may be initiated more readily than in the predominant head-to-tail structures.
Polymers of dienes (hydrocarbons containing two C—C double bonds) have the potential for head-to-tail and head-to-head isomerism and variations in double-bond position as well. The conjugated diene butadiene can polymerize to produce 1,4 and 1,2 products:
CH2
1 2 3 4
C H H
C CH2
C H
X
H C 1,2– polybutadiene
CH2 H H
C
(1-16)
The C atoms in the monomer are numbered in reaction (1-16) and the polymers are named according to the particular atoms involved in the enchainment. There is no 3,4-polybutadiene because carbons 1 and 4 are not distinguishable in the monomer structure. This is not the case with 2-substituted conjugated butadienes like isoprene:
CH2 C C CH2 CH2
CH2 CH3 C C
H CH2
C C CH3
CH3
CH2 C C CH2 H X
X
CH2 3,4 − polyisoprene 1,2 − polyisoprene
1,4 − polyisoprene H H
CH3
(1-17)
Each isomer shown in reaction (1-17) can conceivably also exist in head- to-tail or head-to-head, tail-to-tail forms, and thus there are six possible
35 1.11 Constitutional Isomerism
constitutional isomers of isoprene or chloroprene (structure of chloroprene is given inFig. 1.4), to say nothing of the potential for mixed structures.
The constitution of natural rubber is head-to-tail 1,4-polyisoprene. Some meth- ods for synthesis of such polymers are reviewed in Chapter 11.
Unconjugated dienes can produce an even more complicated range of macro- molecular structures. Homopolymers of such monomers are not of current commer- cial importance but small proportions of monomers like 1,5-cyclooctadiene are copo- lymerized with ethylene and propylene to produce so-called EPDM rubbers. Only one of the diene double bonds is enchained when this terpolymerization is carried out with ZieglerNatta catalysts (Section 11.5). The resulting small amount of unsa- turation permits the use of sulfur vulcanization, as described inSection 1.3.3.
1.11.2 Branching
Linear and branched polymer structures were defined in Section 1.6. Branched polymers differ from their linear counterparts in several important aspects.
Branches in crystallizable polymers limit the size of ordered domains because branch points cannot usually fit into the crystal lattice. Thus, branched polyethyl- ene is generally less rigid, dense, brittle, and crystalline than linear polyethylene, because the former polymer contains a significant number of relatively short branches. The branched, low-density polyethylenes are preferred for packaging at present because the smaller crystallized regions which they produce provide trans- parent, tough films. By contrast, the high-density, linear polyethylenes yield plas- tic bottles and containers more economically because their greater rigidity enables production of the required wall strengths with less polymer.
A branched macromolecule forms a more compact coil than a linear polymer with the same molecular weight, and the flow properties of the two types can dif- fer significantly in the melt as well as in solution. Controlled introduction of rela- tively long branches into diene rubbers increases the resistance of such materials to flow under low loads without impairing processability at commercial rates in calenders or extruders. The high-speed extrusion of linear polyethylene is simi- larly improved by the presence of a few long branches per average molecule.
Branching may be produced deliberately by copolymerizing the principal monomer with a suitable comonomer. Ethylene and 1-butene can be copolymer- ized with a diethylaluminum chloride/titanium chloride (Section 11.5) and other catalysts to produce a polyethylene with ethyl branches:
CH2 C CH2 CH2 CH2 C CH2 CH2
H
CH2 CH3
H
CH2 CH3 CH2 + CH2
(1-18)
The extent to which this polymer can crystallize under given conditions is controlled by the butene concentration.
Copolymerization of a bifunctional monomer with a polyfunctional comono- mer produces branches that can continue to grow by addition of more monomer.
An example is the use of divinylbenzene (1-62) in the butyl lithiuminitiated polymerization of butadiene (Section 12.2). The diene has a functionality of 2 under these conditions whereas the functionality of 1-62 is 4. The resulting
1-62 H C CH2
H C CH2
elastomeric macromolecule contains segments with structure 1-63. Long branches such as these can interconnect and form cross-linked network structures depend- ing on the concentration of polyfunctional comonomer and the fractions of total monomers which have been polymerized. The reaction conditions under which this undesirable occurrence can be prevented are outlined inSection 9.9.
1-63 CH2 C
H
C CH2 CH2 C CH2 CH2 H
C C
H H H
CH2 C CH2 CH2 H
C C H H
Another type of branching occurs in some free-radical polymerizations of monomers like ethylene, vinyl chloride, and vinyl acetate in which the macro- radicals are very reactive. So-called self-branching can occur in such polymeriza- tions because of atom transfer reactions between such radicals and polymer molecules. These reactions, which are inherent in the particular polymerization process, are described in Chapter 8.
Although the occurrence of constitutive isomerism can have a profound effect on polymer properties, the quantitative characterization of such structural varia- tions has been difficult. Recent research has shown that the13C chemical shifts of polymers are sensitive to the type, length, and distribution of branches as well as to positional isomerism and stereochemical isomerism (Section 1.12.2). This tech- nique has great potential when the bands in the polymer spectra can be assigned unequivocally.
37 1.11 Constitutional Isomerism