chem 7010 macromolecular synthesis 2 nd introduction to polymer synthesis: general aspects of...

CHEM 7010Macromolecular Synthesis

2nd Introduction to Polymer Synthesis: General Aspects of Polymer Structures

onPolymer Properties

2011

Multidimensional Order of Structure-Property Relationships in Polymers

Molecular

*Chemical Composition

*MW/MWD

*Branching and/or

Cross-linking

Microscopic

*Crystallinity

*Phases *Orientation

Synthesis and Processing

Macroscopic

*Strength

*Modulus

*Impact Resistance

Permeability

Clarity

Microscopic

Macroscopic

Molecular

IMPACT OF MOLECULAR WEIGHT ON MATERIAL PROPERTIES

Increasing Degree of Polymerization, DP

Oligomers

Tough

Wax

Brittle Wax

meltingpoint

density

crystallinity

Tensile

Strength

Elongation

Properties

Low Density PE

High Density PE

Molecular Weight

Vinyl Monomers, CH2=CH-X

• X Polymer Abbreviation• H Polyethylene PE • CH3 Polypropylene PP • Cl Poly(vinyl chloride) PVC • Phenyl Polystyrene PSt • CN Polyacrylonitrile PAN • COOCH3 Poly(methyl acrylate) PMA• O-COCH3 Poly(vinyl acetate) PVAc

Vinylidene Monomers, CH2=C(X)Y

• X Y Polymer Abbreviation• CH3 CH3 Polyisobutylene PIB

• Cl Cl Poly(vinylidene chloride) PVDC• F F Poly(vinylidene fluoride) PVDF

• Phenyl CH3 Poly(-methyl styrene)

• CH3 COOCH3 Poly(methyl methacrylate) PMMA

• CN COOR Poly(alkyl -cyanoacrylate

Structural Complexity of Polymers

• Homopolymers• Head to Tail vs. Head to Head Adducts e.g. a-olefins

• 1,2- vs 1,4 Adducts; e.g. butadiene

• Tacticity of Enchainments

• Branching

Tacticity• Isotactic

• All asymmetric carbons have same configuration• Methylene hydrogens are meso• Polymer forms helix to minimize substituent interaction•

Syndiotactic

• Asymmetric carbons have alternate configuration• Methylene hydrogens are racemic• Polymer stays in planar zig-zag conformation• Heterotactic (Atactic)• Asymmetric carbons have statistical variation of configuration

X X X X XH

H H

X

X XH X X XX


• Copolymers• Identity and Number of Comonomers

• Ratio and Distribution of Comonomers

• Statistical Alternating

Gradient

• Blocks Grafts


• Molecular Weight – Molecular Weight Distribution, MWD

– Polydispersity Index, PDI

– Mn, Mw, Mz, Mv Averages

• Crosslinking Density

– Length of Crosslinks


Time Dependent Changes

• Chemical Reactions• Hydrolysis• Dehydrohalogenation• Photodegradation• Oxidation

Structural Complexity of Polymers• Thermal Degradation• Processing• Aging• Crystallization • Changes in Polymorphism

• Weathering-- Combination of Above

• Plasticizer Loss -- Imbrittlement

Microscopic Properties (Intermolecular Interactions)

• Morphology

• Chain entanglement –amorphous

• Chain ordering--liquid crystalline

• Crystallinity

• Phase separations (microdomains)

Types of Intermolecular Forces • Type of Force Relative Strength Low Molecular Analog Polymer

• Dispersion or Weak Methane Polyethylene • Van der Waals Hexane Polypropylene

• Dipole-Dipole Medium CH3Cl PVCCH3CO2CH3

PMMA

Hydrogen bonding Strong H2O Cellulose

CH3CONH2 Proteins Electrostatic Very Strong CH3CO2

-Na+ Ionomers

GLASS TRANSITION, Tg

• Definition: The onset of seqmental motion of seqments with 40-50 carbons atoms

• Physical Change Expansion of volume • Free volume required to allow segmental motion• Tg is an approximation• Depends upon measurement technique• Depends upon molecular weight• Polystyrene MW = 4000 Tg = 40C• = 300,000 = 100

GLASS TRANSITION, Tg

• Properties Affected

• Specific Volume / Density

• Specific Heat, Cp

• Refractive Index

• Modulus

• Dielectric Constant

• Permeability

FACTORS INFLUENCING Tg

• Tg is proportional to Rotational Freedom

• For symmetrical polymers Tg, / Tm in K 1/2

• unsymmetical polymers 2/3

• 1. Chain flexibility

• Silicone Ether Hydrocarbon Cyclic HC Aromatics


– Tg = -55C 88C

•

2. Steric Bulk of Substituents

Tg = -120C 5C -24C -50C

•Long side chains may act as plasticizers (C 6)

O O


• 3. Molecular Symmetry

• Asymmetry increases chain stiffness.

• 4. Polar Interactions increase Tg

• Hydrogen bonding

• 5. Molecular Weight up to Critical Limit

• 6. Crosslinking

• Reduces Segment Mobility

FACTORS INFLUENCING Tm

• 1. Chain flexibility

• Silicone Ether Hydrocarbon Cyclic HC Aromatics

• 2. Substituents Producing Lateral Dipoles

• Hydrogen bonding

• 3. Molecular Symmetry

• Symmetry allows close packing

FACTORS INFLUENCING Tm

• 4. No Bulky Substituents to Disrupt Lattice if placement is Random

• 5. Structural Regularity• monomer placement• head to tail • 1,2- vs 1,4- • 1,2- vs 1,3- vs 1,4- aromatic substitution• geometric isomers of enchainments• cis or trans -C=C-; cyclic ring• tacticity

FACTORS REQUIRED TO PROMOTE CRYSTALLIZATION

• Thermodynamic

• 1. Symmetrical chains which allow regular close packing in crystallite

• 2. Functional groups which encourage strong intermolecular attraction to stabilize ordered alignment.

FACTORS REQUIRED TO PROMOTE CRYSTALLIZATION

• Kinetic• 1. Sufficient mobility to allow chain

disentanglement and ultimate alignment• Optimum range for mobility• Tm -10 Tg + 30• at Tm segmental motion too high• at Tg viscosity too high• 2. Concentration of nuclei• concentration of nucleating agents• thermal history of sample

Macroscopic Properties (Physical Behavior)

• Tensile and/or Compressive Strength• Elasticity • Toughness• Thermal Stability• Flammability and Flame Resistance• Degradability• Solvent Resistance• Permeability• Ductility (Melt Flow)

Step Polymerization (Polycondensation)

After many repetitions:

Polyethyleneterephthalate (PET)

Step Polymerization

Two Routes: A-A + B-B; or A-B

1 on 39

2 on 39

1 on 40

2 on 40

Either way, need:• high conversion• stoichiometric amount

Step vs. Chain Polymerization

Step Polymerization

• Any two molecular species can react. • Monomer disappears early. • Polymer MW rises throughout. • Growth of chains is usually slow (minutes to days). • Long reaction times increase MW, but yield hardly changes. • All molecular species are present throughout. •Usually (but not always) polymer repeat unit has fewer atoms than had the monomer.

Step vs. Chain, cont.Chain Polymerization

• Growth occurs only by addition of monomer to active chain end. • Monomer is present throughout, but its concentration decreases. • High polymer forms immediately. • MW and yield depend on mechanism details. •Chain growth is usually very rapid (seconds to microseconds). • Only monomer and polymer are present during reaction. • Usually (but not always) polymer repeat unit has the same atoms as had the monomer.

Step Polymerization

Concepts

Comparison of Step and Chain Polymerization

1. Step: any 2 molecules in the system can react with each other Chain: chain growth occurs on end of growing polymer

2. Step: loss of monomer at early stage (dimers, tetramers, etc.) Chain: monomer concentration decreases steadily

3. Step: broad molecular weight distribution in late stages Chain: narrower distribution; just polymer and monomer

Step Polymerization

Basis for Kinetics (2-1a):

3 on 40

4 on 40

5 on 40

6 on 40

Step Polymerization

Basis for Kinetics:

1on 41

2 on 41

Step Polymerization

Basis for Kinetics (2-1b): NOTE: Assume equal reactivity of functional groups

table 2.1on 42

Thus, assumption looks ok

Step Polymerization

Kinetics (2.2)

Example: diacid + diol

HO

O O

OH HO OH+

-n H2O

HO

O O

O OH

Mechanism (acid catalyzed):

2 on 44

1 on 45

2 on 45

Chain Polymerization

Concepts

General Mechanism

Initiation

I R* * could be radical, cation, or anion

Propagation

Chain PolymerizationConcepts: Nature of Chain Polymerization; 3-1

3-1a. Comparison of Chain and Step Polymerizations

Chain: a. Rapid high mw b. Snapshot: monomer, high poly, growing chains c. MW does not change with time

Step: a. Monomer gone fast, get dimer, trimer, etc. b. MW increases with time c. No high mw until end

0

2

4

6

8

10

12

0 2 4 6 8 10

chain

step


Concepts: Nature of Chain Polymerization; 3-1

3-1b-1. General Considerations of Polymerizability

Thermodynamics

(Chain t-fer)



3-1b-2. Effects of Substituents on C=C monomers for Ionic Polymerization

A. Initiation opportunities: 2 types of bond cleavage/resonance

B. Radical, Anionic, Cationic: Depends on: i. Inductance

ii. Resonance




i. Inductanceii. Resonancee.g. Polym of vinyl ether

Example of resonance stabilization of cation byDelocalization of of positive charge.

If oxygen not there, no stabilization

C. Donating GroupsGood for cationic




ii. Resonance

e.g. Polym of Styrene

C. Donating Groups



D. Withdrawing Groups


i. inductance

ii. resonance

Good for anionic


3-1b-2. Effects of Substituents on C=C monomers for radical polymerization


A. Radicals not affected by charge, but can have resonance stabilization

B. Also stabilized by primary<secondary<tertiary

Draw your own pic


Concepts: Structural Arrangement of Monomer Units; 3-2

3-1a. Possible Modes of Propagation

A. Two possible points of attachmenti. On carbon 1:

ii. On carbon 2:




B. If attachment is regular, get head-to-tail (H-T)

C. If attachment is irregular, get head-to-head (H-H) [and T-T]




Usually only 1-2% H-H. How do we know?

A. Chemical Methods

B. NMR Methods (we’ll check this out in the Spectroscopy section)

chem 7010 macromolecular synthesis 2 nd introduction to polymer synthesis: general aspects of...

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