Chapter 5 – Polymer Synthesis

This chapter was drafted by the 2007 class* and typed by Daniel Hulgan; it awaits editing.

Cornelia Rosu Brandon Cory Sr Curtis Mariham Yohannes Abraham Melissa Cormier Menard Xiaowei Tong Marsha Cole Brooke Frances Delhomme Melissa Marie Templet Krystal Renee Fontenot Amy Marie Pollard Timsy Kaur Uppal Javoris Vantre Hollingsworth Lucas James Veillon

Outline

I. Step polymerization

 General Aspects

 Molecular weight distribution

 Functional groups reactivity

 Equilibrium

II. Chain Growth polymerization

a) Free Radical Polymerization

b) Emulsion polymerization

c) Suspension polymerization

III. Living Polymerization

 ATRP

 SFRP

 RAFT IV. Ziegler-Natta Catalyzed polymerization

V. Ring Opening Polymerization

II. a) Free Radical Polymerization

One common way of making polymers from vinyl monomers is free radical polymerization. The process starts with the addition of an initiator like Z, Z1—azobis(isobutyronitrile) (AIBN) or benzoyl peroxide

--Peroxides like benzoyl peroxide are thermally unstable and decompose into radicals at a temperature and rate dependent on structure.

--Similarly, azo compounds like AIBN that have cyano groups on the carbons form radicals at relatively low temperatures (halflife 1-3 hours at 80°C). The driving force for decomposition is formation of nitrogen and the resonance-stabilized cyano propyl radical.

Initiation step

This step involves the formation of the free radical as shown above followed by addition of the initiator radical to a monomer.

The more and more monomer is added, the chain keeps growing because there is always a radical formed. This process is known as propagation.

Propagation continues until some reaction occurs to terminate it. The two principles ways of termination are coupling or combination and disproportionation.

Coupling

Two radicals each with unpaired electrons can react to form a bond.

Disproportionation

This involves transfer of an atom, usually hydrogen, from one chain end to another.

Another possible termination reaction involves combination of initiator radicals with chain end radicals known as primary radical termination. This process is significant only at relatively high initiator levels or when high viscosities limit the diffusion of high-molecular weight chain end radicals.

Reference: M.P. Stevens, Polymer Chemistry, 1999, 3rd ed.

II. b) and c)

”To control thermal and visocosity problems” Odian

Heterogeneous: 1. Precipitation (powder or granular polymerization)

2. Suspension (bead or pearl)

3. Emulsion

Precipitation

Ad 1) “The polymer formed is insoluble in the reaction medium” Odian

Example: solution polymerization of acrylonitrile in water. Initiators soluble in the initial reaction medium.

Suspension

Ad 2) monomer is suspended as droplets (50-500 µm in diameter) in water.

For: styrene, acrylic and methacrylic esther, vinyl chloride, vinyl acetate, tetrafluoroethylene.

Water: monomer from 1:1 to 4:1

Stabilizers (dispersant or surfactants) are added to prevent coalescence. The mixture is agitated from the same reason.

Example of stabilizers?

Usually less than 0.1 wt% of the aqueous phase (much lower than for emulsion polym)

The two phases cannot be maintained in suspension polymerization without agitation” Odian

(But they can for emulsion)

--“Dispersant seldom form colloidal micelles as in emulsion polymerization

--Initiator soluble in monomer droplets --Each droplet is a miniature bulk polymerization system

--Inverse suspension poly:

“Organic solvent as continuous phase with droplets of a water-soluble monomer (e.g. acrylamide)

3. Emulsion—for some radical chain polymerization (p. 350 Odian)

Similar to suspension poly different in mechanism and reaction chain

--Different type and smaller size of particles in which poly occurs

--Different initiator

--Dep. m on reaction parameters

--First employed during the Second World War to produce synthetic rubbers from 1,3--butadiene and styrene

Advantage

 Easy to control process

 Thermal and viscosity problems smaller than for bulk poly

 Product often does not need further separations

 Possible to increase the molecular weight of polymer without decreasing polymerization rate

4. Components

1) Monomer

2) Dispersing medium—usually water

3) Emulsifier (surfactant or soap)—have both hydrophilic and hydrophobic part

4) Water-soluble initiator

--When concentration of surfactant exceeds critical micelle concentration (cmd), “The excess surfactant molecules aggregate together to form small colloidal clusters = micelles”

--In most emulsion poly, the bulk surfactant is in micelles (2-1000 nm); each micelle contains 50-150 surfactant molecules

--Micelles spherical, but not always. 5.

--When water-insoluble monomer is added, very small fraction dissolves in continuous aqueous phase

--Some portion of the monomer enters interior of the micelles

--Most monomer (>95%) is dispersed in monomer droplets. Size of droplets depends on rate of stirring. Droplets are stabilized by surfactant molecules absorbed on their surfaces.

Droplets 1-100nm

--Initiator in water while for suspension poly initiators are oil soluble

--In micelles meet organic soluble monomer and water-soluble initiator

--as polymerization proceeds, micelles grow

III. Living Radical Polymerizations

I. Atom Transfer Radical Polymerization, (ATRP)

Reaction scheme:

 Need a metal which can undergo one electron redox transfers

 Bromine is typically used due to its reactivity and reduced side reactions.  Polymerization rate and Xn are given by the following; (Odian)

 Rp can also be expressed in time.

+ 2+ ln([M]0/[M]) = ((kpK[I][Cu ])/([Cu ]))t

ATRP and other living polymerizations also follow a poisson distrubition of molecular weights  ATRP has performed with various monomers such as: styrene, acrylonitrile, (meth)acrylates, (meth)acrylamides, 1,3-dienes and 4-vinyl pyridine.

II. Stable Free Radical Polymerization  Occurs similar to ATRP except no organic halide is involved. Uses reversible deactivation of the propagating radical.

 Typical deactivators are nitroxides (Tempo), triazolinyl, trityl, and dithiocarbamates.

III. Reversible Addition Framentation Transfer.  Unlike ATRP and SFRP, which are reversible termination of the propagating radical, RAFT uses reversible chain transfer

 Needs a chain transfer reagent along with a typical radical initiator.

Note: all information for living radical polymerization was taken from Odian (4th ed.)

Ziegler-Natta Polymerization [Coordination Polymerization]

 Introduction

Ziegler-Natta polymerization is a method of vinyl-polymerization. It’s important because it allows one to make polymers of specific tacticity. It was by two scientists: 1953, Karl Ziegler of the Max Planck Institute, Germany, found that nickel in combination with triethylaluminum dimerized olefins. This prompted a survey of the effect of other transition metal. It was discovered that group IV metal, especially titanium, were effective polymerization catalysts for ethylene. Following Ziegler’s successful preparation of linear polyethylene in 1953, Giulio Natta prepared and isolated isotactic polypropylene at the Milan Polytechnic Institute. This was immediately recognized for its practical importance. Ziegler and Natta shared the Nobel Prize in Chemistry in 1963.

 Composition and Properties of Ziegler-Natta Catalyst

A Ziegler-Natta catalyst is composed of at least two parts: a transition metal component and a main group metal alkyl component. Typically, they are based on titanium compounds and organometallic aluminum

compounds, for example triethylaluminum, (C2H5)3Al. Another transition metal include V (V), Mo, Zr, and Cr (VI). The component of these

transition metal, e.g. MtXn (X=Cl, Br, I), MtOXn, Mt(aeac)n, Cp2TiX2, are used in the coordination polymerization of α-olifen. The main group metal alkyl complex also include LiR, MgR2, and ZnR2. But the aluminum complex is the most common one, e.g. AlHnR3-n, AlRnX3-n (n=0~1, X= Cl, Br, I).

Alkylation:

TiCl4 + AlR3  RTiCl3 + AlR2Cl

TiCl4 + AlR2Cl  RTiCl3 + AlRCl2

RTiCl4 + AlR3  R2TiCl2 + AlR2Cl

Decomposition and Reduction of Alkyl Aluminum:

RTiCl3  TiCl3 + R°

R2TiCl2  RTiCl2 + R°

TiCl4 + R°  TiCl3 + RCl

Termination of Free Radical:

2 R°  Dead Chain

The above reactions used TiCl4 – AlR3 as example and explained how these two components react with each other in the polymerization system.

 Mechanism

These are two sets of Ziegler-Natta catalyst/co-catalyst systems. Though the analyse of C-14 NMR, it was confirmed that the chain propagation occurs at the surface of aluminum.

Another mechanism was proposed, which called Cassee-Arlman mechanism In this mechanism, polymerization is thought to start with coordination of the olefin to the vacant site of an activated TiR species and subsequent insertion into the Ti-R bond.

Ring Opening Polymerization

Define: Ring-opening is a form of addition polymerization. The terminal end of a polymer acts as a reactive center, where further cyclic monomers join to form a larger polymer chain through ionic propagation.

Reference: Living ring-opening polymerization of N-Sulfonylazinidine. Synthesis of high molecular weight linear polyamines Ian C. Stewart, Cameron C. Lee, Robert G. Bergman, and F. Dean Toste : J. Am. Chem. Soc.; 2005; 127 (50) pp 17616-17617; (Communication)

The Process The treatment of some cyclic compounds with catalysts brings about cleavage of the ring followed by polymerization to yield high-molecular weight polymers.

Anionic Ring Opening Polymerization

Define: When the reactive center of the propagating chain is a carbanion, the reaction is called an anionic ring-opening polymerization.

Cationic Ring Opening Polymerization When the reactive center of the propagating chain is a carbocation, the polymerization is called cationic ring-opening polymerization.

These two reactions nicely exemplify the ionic ring opening mechanisms. Other more complicated systems work by analogous mechanisms.

Chapter 5 Condensation or Step Growth Polymerization

A. General Aspects

B. Molecular Weights Distribution

C. Equilibrium Considerations

D. Reactivity of functional groups

E. Step-Growth Monomers

F. Step-Growth Polymers

 Polyesters

 Polyamides

 Polyurethanes and Polyureas

 Formaldehyde Thermosets

A. General Aspects The step-growth polymerization is one of the most use way to synthesize polymers. This type of polymerization involves a bifunctional monomer (a “monomer with two functional groups per molecule)

The General scheme for step-growth polymerization:

These kind of reactions are known as condensation polymerization. Also, the polymer community find step-growth polymerization a better definition because for some reactions when low molecular weights are achieved the mechanism is very likely with chain growth.

B. Molecular Weight Distribution

In step-growth polymerization, the average chain length is observed to increase very slowly during the majority of the reaction.

Insert Figure (the paper says there is a figure here, but it is not on the paper. Will discuss next time I come in.)

Only at the very beginning, the molecular weight obey to the most probable distribution. In the medium each monomer can couple with other monomer. After the number of dimers and trimers increases, the high polymer molecular weight is achieved very slow because of the reactions between monomers and dimers, dimers and trimers, etc.

Another figure but under same conditions as above.

Monomer + monomer = dimer

Dimer + monomer = trimer

Dimer + dimer = tetramer

Trimer + monomer = tetramer

Trimer + dimer = pentamer Trimer + trimer = hexamer

Tetramer + monomer = pentamer

Tetramer + dimer = hexamer

Tetramer + trimer = heptamer

Tetramer + tetramer = octamer

Pentamer + monomer = hexamer

Etc.

Etc.

Which can be expressed as the general reaction

n-mer + m-mer = (n + m)-mer

The Average degree of polymerization Xn is obtained as the ratio of the number of X-R-Y monomers initially present, N0, to the number of molecules present after the extent of reaction reaches p or Np = N0(1-p).

Thus, Xn = N0/[N0(1-p)] = 1/(1-p)

Xn = 1/(1-p)  the Carother’s Equation

We consider the expression for Xn and Xw based on a probability approach developed by Flory (1953).

Let’s consider a step-growth polymer containing a structural units. This polymer have been occurred through the reaction of a-1 functional groups A and has one unreacted A group remaining, where either A-R-A-, B-R-B-, or A-R-B type monomers were used.

The probability that an A group has reacted is of course p, the extent of the reaction, so the probability that (a-1) A groups have reacted is p^(a-1) and (1-p) is the probability of finding an unreacted A group. The product of these probabilities is then the probability of finding a polymer with a structural units, Pa = pa-1(1-p). If there are the N total polymers in our sample, then clearly the number of polymers containing a structural units is given by Na = PaN. If our step-growth polymerization began with N0 structural units (monomers), then N = N0(1-p) , 2 a-1 and the expression for Na becomes Na = N0(1-p) p . Then, Xn is given by

So, Xn summed over a which supon evaluation of the summation yields Xn = 1/(1-p)

The weight-average degree of polymerization Xw is developed: the sample of polymer contains a units

The breadth of molecular weight distribution for the step-growth polymer is represented by the ratio

(Xw/Xn) = [1/(1+p)]/[(1+p)/(1-p)] = 1+p

This is usually called the polydispersity index PDI

PDI (Xw/Xn) = 2.0 as p1.0

c. Reactivity of functional groups

The practical synthesis of high polymers requires knowledge of the kinetics of the polymerization reaction. The differences between step and chain polymerizations reside in large part in kinetic features. Many step polymerizations are assumed having the reaction rate constants independent of the reaction time or polymer molecular weight. The independence of the reactivity of a functional group on molecular size can be observed from the reaction rates in a homologous series of compounds differing from each other only in molecular weight in table A are shown. The data for the esterification of a series of homologous carboxylic acids.