Polymerization

Polymerization: process of combining large number of monomers through chemical

reaction to form polymer (don’t end up with a unique molecule)

 Polymerization occurs by the sequential reactions of monomers, which means that a

successive series of reactions occurs as the repeating units (structural units) are linked
together by covalent bonds under appropriate reaction conditions.

 This can proceed by the reaction of monomers to form a dimer, which in turn reacts
with another monomer to form a trimer and so on.

 Reaction may also be between dimers, trimers, or any molecular species within the
reaction mixture to form a progressively larger molecule.

 In either case, a series of linkages is built between the repeating units, and the resulting
polymer molecule is often called a polymer chain.

 Low molecular-weight polymerization products such as dimers, trimers, tetramers, etc.,

are known as oligomers (generally possess undesirable thermal/mechanical properties).
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 Functionality
 Polymerization occur only if monomers in the reaction have the proper functionality.
 The functionality of a molecule is simply its interlinking capacity, or the number of
sites it has available for bonding with other molecules under the specific conditions of
polymerization reaction (Rudin, 1982).
 A molecule may be classified as monofunctional, bifunctional, or polyfunctional
depending on whether it has one, two, or greater than two sites available for linking
with other molecules.
 Functional groups on a monomer react during synthesis to form bonds with other
monomers. The number of these influences the structure of the synthesis product.

Effect of number of functional groups Resulting molecule

One (monofunctional) Dimer (formed from two monomers)
Two (bifunctional) Straight chain polymer
Three or more (polyfunctional) Branched or network polymer

 Thus, the minimum functionality required for polymerization is 2

 A bifunctional monomer, i.e., a monomer with functionality 2, can link with two other
molecules under suitable conditions e.g., styrene, C6H5CH=CH2 has functionality 2
because of the presence of a carbon-carbon double bond. 8
 The presence of two condensable groups in both hexamethylenediamine (–NH2) and
adipic acid (–COOH) makes each of these monomers bifunctional.
 A polyfunctional monomer can react with more than two molecules under suitable
conditions e.g. glycerol (3 OH groups) is trifunctional & divinyl benzene (2 double
bonds) is tetrafunctional in polyesterification & addition reactions, respectively.
 Functionality in polymerization reaction is, however, defined only for a given reaction
even though the interlinking capacity of a monomer is ordinarily apparent from its
structure.
 For example, a glycol, HOROH, has a functionality of 2 in esterification or ether-forming
reactions, but its functionality is zero in amidation reactions.
 A diamine like hexamethylenediamine has a functionality of 2 in amide-forming
reactions:

 However, in esterification reactions a diamine has a functionality of zero.

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 Degree of Polymerization
Degree of polymerization (DP): no. of repeating units strung together in the polymer
chain (molecule).
 A high DP is normally required for a material to develop useful properties and before it
can be appropriately described as a polymer.
 Polystyrene, with a DP of 7, is a viscous liquid (not of much use), whereas commercial
grade polystyrene is a solid and the DP is typically in excess of 1000.

 The degree of polymerization represents one way of quantifying the molecular length
or size of a polymer.
 It specifies the length or size of a polymer molecule.
 This also determine the molecular weight (MW) of Polymer.
 By definition, MW(Polymer) = DP × MW(Repeat Unit).

Example 1.1: What is the molecular weight of commercial grade polystyrene (PS)?

Solution: molecular weight of the repeating

unit is 104 (8 × 12 + 1 × 8).

 Consequently, the molecular weight of this

type of polystyrene is 104,000.
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II. Historical Development
(Reading Assignment)
III. Classifications of Polymers

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Classification of Polymers

Based on
Thermal
behavior

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A. Origin: Natural vs Synthetic
 Natural Polymers (Biopolymers): found in plants or animals, and
produced by living organisms or biological action/origin
Ex: natural rubber (wood), silk (leather), wool (cotton), cellulose, starch, enzymes,
proteins, polypeptides (amino acid), polynucleotide (nucleic acids—RNA/DNA),
polysaccharides (sugars)
 Synthetic Polymers: artificially manufactured (man-made) or industrial polymers
consisting of various families: fibers, elastomers, plastics, adhesives, etc. and subgroups
Ex: polyethylene (LDPE/HDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride
(PVC), nylon, Bakelite, Teflon (PTFE), polyurethane, Polyethylene terephthalate (polyester)

 Semi-synthetic polymers: natural polymer which are chemically modified

Ex: cellulose derivatives such as, cellulose acetate (rayon) and cellulose nitrate etc.

 Inorganic polymers: with a skeletal structure that does not include carbon atoms in
the backbone
Ex: siloxanes, silanes, phosphazenes

 Hybrid polymers: combination of two different types of polymers like synthetic and
biopolymeric constituents, inorganic and organic polymeric components
Ex: gelatin-methacryloyl (GelMA), hyaluronic acid (HA)-PEG, Epoxy-polysulphides 14
15
B. Polymer structure (molecular architecture)
1. Linear, Branched, Cross Linked or Network Polymer
Closely related to material properties
Linear chain polymer: consist of a single continuous chain i.e. uninterrupted long
straight chain of repeat units (weak secondary forces between chains - closely packed)

Branched chain Polymer: includes side chains of repeat units connecting onto the
main chain of repeat units (occasional branches off longer chain)

Cross linked polymer: includes interconnections between chains (liner/branched

chains connected by covalent bonds)

Network polymer: a cross linked polymer that includes many interconnected linear
chains (numerous interconnections between chains; one giant molecule – 3D network)

(1) (2) (3) (4)

Linear Branched Cross-linked Network

Direction of increasing strength How density and melting point will vary?
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2. Tacticity or Stereochemistry Linkages or configuration
 based on the stereoregularity (tacticity) of the side group orientations on the
backbone - control the crystallinity
 tacticity of a polymer has important implications for its degree of long-range order.

ISOTACTIC : same orientation of side groups

i.e. R groups on same side of backbone

 chain has ordered (symmetric) arrangement of the R groups which leads to close
molecular-chain packing and crystallinity (very strong and less flexible)
 used as fabrics for carpets, automobile parts, battery casings, medicine bottles

All cis-polyisoprene
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 SYNDIOTACTIC : alternating arrangements of
side-groups i.e. R groups on alternating sides
of backbone)

 close packed alternating (symmetric) arrangement of R groups on either side of

backbone of the chain, which lead toward a tougher, more dense crystalline
structure (very strong, dense and less flexible)
 new applications are emerging

 Ziegler-Natta catalysts used for iso- and syndio-tactic

ATACTIC : random orientations of side-groups

along the chain (most common)

 random (asymmetric) arrangement of R groups which prevents close packing and a

noncrystalline polymer is formed
 soft, weak and less cloudy than the more crystalline isotactic and syndiotactic forms
 has low density and high flexibility (weak intermolecular forces)

Exercise 1: Draw the structure of atactic polystyrene and syndiotactic PMMA 18

 3. Amorphous or Crystalline
When polymers are cooled from the molten state or concentrated from the solution,
molecules are often attracted to each other and tend to aggregate as closely as possible
into a solid with the least possible potential energy.
 For some polymers, in the process of forming a solid, individual chains are folded and
packed regularly in an orderly fashion. The resulting solid is a crystalline polymer with a
long-range, three-dimensional, ordered arrangement.
 Crystallinity: degree of long-range order i.e. periodical arrangement of molecules
 However, since the polymer chains are very long, it is impossible for the chains to fit into
a perfect arrangement equivalent to that observed in low-molecular-weight materials.
 A measure of imperfection always exists. The degree of crystallinity, i.e., the fraction of
the total polymer in the crystalline regions, may vary (from a few percentage to about
90%) depending on the crystallization conditions.
a) Semi-crystalline – ordered structure composed of microscopic crystallites i.e. domains
of crystalline structure and characterized by their melting temperature (Tm) at which the
ordered regions break up and become disordered.
Ex: Polyethylene, Polypropylene, Polyvinyl chloride, Polyamide (Nylon), Polyacrylonitrile,
Polyester (fiber), Polyethylene terephthalate (PET), Polytetrafluoroethylene (PTFE).
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In contrast to crystallizable polymers, amorphous polymers possess chains that are
incapable of ordered arrangement in the solid state.
 These polymers vitrify, forming an amorphous glassy solid in which the molecular chains
are arranged at random and even entangled.
b) Amorphous
– short-range order of repeating units or no ordered structure and characterized by their
glass transition temperature (Tg) at which they transform abruptly from glassy state (hard)
to the rubbery state (soft) due to chain motion (soften above Tg and then liquid above Tm)
Ex: Acrylonitrile Butadiene Styrene (ABS), polystyrene, polycarbonate, polyetherimide (PEI),
polysulfone, polymethylmethacrylate (PMMA), silk, cellulose (proteins), polylactic acid

Liquid Crystalline Polymers

 They have phases characterized by structures intermediate between the ordered
crystalline structure and the disordered fluid state.
 Solids of liquid crystalline polymers melt to form fluids in which much of the molecular
order is retained within a certain range of temperature.
 The ordering is sufficient to impart some solidlike properties on the fluid, but the forces
of attraction between molecules are not strong enough to prevent flow.
 Ex: Polybenzamide
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 Amorphous: no periodic arrangement of
chains or molecules

Semicrystalline: consists of small

crystalline regions (crystallites)
surrounded by amorphous regions

 Degree of crystallinity strongly affects material properties, such as tensile strength

increases with % of crystallinity but flexibility increases with Amorphous content
Factors favouring crystallinity - more ordered/regular (higher degree of crystallinity)
 Fewer short branches – allowing molecules to pack closely together
 Higher degree of stereoregularity - syndio-/iso-tactic are more ordered than atactic
HDPE: consist of linear chains with little branching which pack closely together, leading to a
high degree of order - makes it stiffer (rigid) & opaque (white) e.g. milk bottles, drainpipes
LDPE: consist of numerous short branches which interfere close packing of molecules,
leading to low ordered (amorphous) structure with lower density & stiffness i.e. more
flexible & transparent - suitable for use in films e.g. plastic carrier bags, food wrapping.
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4. Monomer Composition & Arrangement:
Homopolymer or Copolymer
Homopolymer: made up of identical monomers (composed of only one repeating unit)
• Consist of only one type of monomer or constitutional repeating unit (A)
AAAAAAAAAAAAAAA
 The most produced/used polymers are homopolymers of terminal alkenes
 Produced by radical polymerization

ethylene polyethylene
methylmethacrylate PMMA
Nylon 6,6 and PET (same repeating units but different structural unit)
Copolymer: made up of different monomers (composed of different repeating unit)
• Consists of two or more constitutional repeating units (A.B)
ABABAABBABBBAAAB

Ex: Copolymer of styrene and acrylonitrile (polymerized in the same reactor) 22

Several classes of copolymer are possible
Statistical copolymer (Random copolymer):
two or more different repeating unit are
distributed randomly on the chain molecule
-ABAABABBBAABAABB-
Alternating copolymer:
alternating (ordered) sequences of the different
repeating units along the polymer chain
-ABABABABABABABAB-
Block copolymer:
chain consists of relatively long sequences (blocks) AB diblock copolymer
of each repeating unit chemically bound together
-AAAAAAAAA-BBBBBBBBB-AAAAA-BBBB-

Graft copolymer:
Sequences of one repeating unit are “grafted” onto a backbone of the
another type
-AAAAAAAAAA-AAAAAAAA- AB graft copolymer
B B B
B B B
B B B
B B B 23