Normally, a Lewis acid (aprotonic acid) was used as catalyst, such as _______________________, etc.. (sometimes, bronsted acids or carboncation salts can also be used). They are all strong electron acceptors. Lewis acids, require a cocatalyst, usually a Lewis base or other proton donor, to initiate polymerization.
High rate of polymerization at low temperature is a characteristic of ionic polymerizations. The reaction normally occurs at low temperature (< r.t.)
Mechanism.
Initiation: BF3 + H2O --> H+ (BF3OH)-
H+ + (CH3)2C=CH2 --> (CH3)3C+
Propagation:
(CH3)3C+ + (CH3)2C=CH2 --> (CH3)3C-CH2-C+(CH3)2
(CH3)3C-CH2-C+(CH3)2 + (CH3)2C=CH2 --> (CH3)3C-CH2-C(CH3)2 -CH2-C+(CH3)2
Termination: reaction of the chain with a inhibitor, or the catalyst to forma a covalent bond, or by the rearrangement reaction or by chain transfer reaction.
Kinetics. See page 87-88
C. Anionic Polymerization
Mechanism:
Initiation: NH2 ¯+ CH2=CHX --> H2NCH2CHX¯
Propagation: H2NCH2CHX¯ + CH2=CHX --> H2NCH2CHX-CH2CHX¯
Termination: Usually involves transfer, such as H-.
"Living" polymers -- If all other species are eliminated, the growing chain remains active indefinitely. The polymer can have an extremely narrow distribution of molecular weight.
The polymer can be "killed" by addition of a termination agents, such as water (or other positive species or active hydrogen--a hydrogen that can fall of th eorganic molecule and act as an acid) at the end of the reaction.
Catalysts for anionic processes include NaNH2, BuLi, RMgX (Grignard), NaH, all of which are very strong basic catalysts.
D. Coordination Polymerization. An unique polymerization technique using Ziegler or Ziegler-Natta catalyst. The product is normally a highly linear chain and stereoregular polymer.
Organometallic catalyst - organometallic catalysts are used to promote the formation of carbanion species for anionic polymerization. Stereochemical consideration- For the free radical process, the intermediate could be attacked with an equal probability (recall racemization.) Now, the presence of the metal ions ion on the "back" side of the reaction site makes a "front" side attack more likely and an isotactic polymer (as opposed to an atactic polymer) is more likely.
Ziegler-type catalysts originally involved the formation of a complex precipitate of some unspecified color from aluminum triethyl and titanium
tetrachloride. Ziegler catalyst are complexes formed by the interaction of alkyls of metals of groups I-III in the periodic table with halides and other
derivatives of transition metals of group IV-VIII. Some typical complexes are listed in table 4-2.
Examples of Complex System Catalysis-
Al3R - Ti3Cl or Al3R - Ti4Cl are called Ziegler-Natta catalyst.
In these catalysts, at least one empty coordination site
accommodates the growing polymer chain.
Under the Ziegler-Natta (ZN) process, ethylene is bubbled into the suspended
catalyst at room temperature, and it rapidly polymerizes to a high molecular
weight, linear polyethylene. Linear polyethylene (as opposed to branched
polyethylene) has a higher crystallinity, so that the product differs
significantly from polyethylene prepared from a high pressure, free radical
process.
The initiation, propagation, and termination steps for ZN polymerization are
shown in the following:
A simple representation is as the following:
(metal alkyl = AR, the monomer = M, ▄▄ = transition-metal compound)
Initiation:
▄▄—AR à M—▄▄—AR
Propagation:
M—▄▄—AR à ▄▄—A—M—R à ▄▄—A—Mx—R
Termination: (through transfer)
▄▄—A—Mx—R + M à ▄▄—A—M—R + Mx
Diolefins (monomers with two double bonds) such as
butadiene (CH2=CH-CH=CH2) and isoprene yield both cis- and trans- 1,4 polymers
from Ziegler Natta catalyzed processes. Variations in the metal halide and/or
the ratio of alkyl to halide give different structures.
The essential feature of the Ziegler Natta process is that monomers are
____________, one after the other, into a polarized titanium carbon bond. The
polymer grows out of the active center, rather as a hair grows from the root.
The termination step: The propagating end of the polymer chain is negatively
charged and therefore, the reaction may be regarded as an anionic
polymerization. Chain growth may be terminated by several different processes:
notice the correspondence between one fragment picking up a hydrogen, and the other fragment picking up a pi bond.
·
A chain transfer type reaction may be brought about by the
addition of an active hydrogen compound such as an alcohol:
Cat-CH2CHR-[-CH2-CHR-]n-CH2CH3 + R'-OH --> Cat-OR' + CH3-CHR-[-CH2-CHR-]n-CH2CH3
The primed alkyl (R') could be the same or different from the unprimed alkyl.
The above chain transfer breaks a metal bond. In general, if an organometallic
bond is broken, a proton goes to the carbon where the organometal was previously
attached
A disadvantage of Zeigler
Natta catalysts is that the catalyst is dispersed in the final product.
Polymerization with Supported Metal Oxide Catalysts.
Metal oxides have been used to give linear polyolefins, but they have not been used for isotactic structures. Nickel, cobalt, chromic, or vanadium oxides supported on silica-alumina have been effective for ethylene and olefin copolymers at low temperatures.
E. Ring-Opening Polymerization
Similar to vinyl group, instead of break of double bond, the ring breaks.
Molecules include Lactone (ring containing COO group), lactam (ring containing CON group), cyclic ether, cyclic anhydride, etc.
Chapter 2. Step-Reaction polymerizations (condensation polymerization)
Definition of Condensation Polymer
A condensation polymer can be defined as a polymer in which the structural unit contains fewer atoms than the
monomer or monomers from which the polymer is derived. There must be an elimination of a by-product, usually
water. As an example, a polyester may be obtained from the reaction of a diol with a diacid. (Polymer chemists abbreviate
"dicarboxylic acid" to "diacid.") 
Poly(ethylene terephthalate), the most important polyester, is made from ethylene glycol and terephthalic acid.
PET is used for clothing (62%), home furnishings (17%),
and textile cord (10%) (is textile cord, a special kind of rope) No year listed
for when this data was taken.
Types of Condensation Polymer Functionalities
Applications for various condensation polymers
Polyester- clothing (dacron)
Polyamide- nylons
Polyurethane- foams
Polyurea- foams
Polyacetals- engineering plastics
Silicones- silicones, heat exchange fluids
Polyanhydride- not widely used
Phenolformaldehyde- plastics, coatings
Cellulose polymers- woods, plant fibers, cotton, flax, starches, cotton fibers
Homework:
Write chemical equations for the major steps in the polymerization of the following: